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Curso completo de Desenvolvimento Web Crie 6 projetosHTML JAVASCRIPT CSS PHP 17. PHP & MySQL - Projeto aplicado Twitter Clone 18.1 arquivos da aula.zip

Curso completo de Desenvolvimento Web Crie 6 projetosHTML JAVASCRIPT CSS PHP 7. CSS3 4. Normalize css

Curso completo de Desenvolvimento Web Crie 6 projetosHTML JAVASCRIPT CSS PHP 9. JavaScript 31. Projeto final - Game estourando balões - arquivos do projeto

Curso completo de Desenvolvimento Web Crie 6 projetosHTML JAVASCRIPT CSS PHP 17. PHP & MySQL - Projeto aplicado Twitter Clone 31. Exibindo posts de quem estamos seguido em nossa timeline

Curso completo de Desenvolvimento Web Crie 6 projetosHTML JAVASCRIPT CSS PHP 9. JavaScript 32. Projeto final - Game estourando balões - parte 1

Curso completo de Desenvolvimento Web Crie 6 projetosHTML JAVASCRIPT CSS PHP 9. JavaScript 25. Eventos - parte 4 de 4 formulário

Curso completo de Desenvolvimento Web Crie 6 projetosHTML JAVASCRIPT CSS PHP 9. JavaScript 18. Estruturas de repetição - parte 3 de 4 do while

Curso completo de Desenvolvimento Web Crie 6 projetosHTML JAVASCRIPT CSS PHP 7. CSS3 7. Degradês & Transparências

Curso completo de Desenvolvimento Web Crie 6 projetosHTML JAVASCRIPT CSS PHP 17. PHP & MySQL - Projeto aplicado Twitter Clone 27. Deixando de seguir pessoas

Curso completo de Desenvolvimento Web Crie 6 projetosHTML JAVASCRIPT CSS PHP 12. PHP 23. Estruturas de repetição - parte 3 de 5 do while

Curso completo de Desenvolvimento Web Crie 6 projetosHTML JAVASCRIPT CSS PHP 14. Orientação a objetos PHP 1. Introdução

Curso completo de Desenvolvimento Web Crie 6 projetosHTML JAVASCRIPT CSS PHP 14. Orientação a objetos PHP 5. Construtor e destrutor

Curso completo de Desenvolvimento Web Crie 6 projetosHTML JAVASCRIPT CSS PHP 3. HTML 2. O que é HTML

Curso completo de Desenvolvimento Web Crie 6 projetosHTML JAVASCRIPT CSS PHP 17. PHP & MySQL - Projeto aplicado Twitter Clone 15. Cadastro de usuário - Bloquear inclusão de usuários já cadastrados parte 1

Curso completo de Desenvolvimento Web Crie 6 projetosHTML JAVASCRIPT CSS PHP 11. Iniciando com servidor Apache, PHP e MySQL 4. Configurando o servidor web - um pouco mais de XAMPP

Curso completo de Desenvolvimento Web Crie 6 projetosHTML JAVASCRIPT CSS PHP 12. PHP 10. Download - Projeto auxiliar catálogo de produtos

Curso completo de Desenvolvimento Web Crie 6 projetosHTML JAVASCRIPT CSS PHP 8. Bootstrap - Criando Sites Responsivos 1. O que irei aprender nesse módulo

Curso completo de Desenvolvimento Web Crie 6 projetosHTML JAVASCRIPT CSS PHP 17. PHP & MySQL - Projeto aplicado Twitter Clone 4.1 arquivos da aula.zip

Curso completo de Desenvolvimento Web Crie 6 projetosHTML JAVASCRIPT CSS PHP 9. JavaScript 12. Ifelse - parte 2 de 2

Curso completo de Desenvolvimento Web Crie 6 projetosHTML JAVASCRIPT CSS PHP 9. JavaScript 14. Operadores aritméticos

Curso completo de Desenvolvimento Web Crie 6 projetosHTML JAVASCRIPT CSS PHP 15. MySQL 14. Selecionando registros

Curso completo de Desenvolvimento Web Crie 6 projetosHTML JAVASCRIPT CSS PHP 10. JQuery 7. Eventos via JQuery - parte 2 de 3 teclado

Curso completo de Desenvolvimento Web Crie 6 projetosHTML JAVASCRIPT CSS PHP 8. Bootstrap - Criando Sites Responsivos 44. Spotify - Media queries (Arquivos da aula)

Curso completo de Desenvolvimento Web Crie 6 projetosHTML JAVASCRIPT CSS PHP 12. PHP 17. Ifelse - parte 2 de 2

Curso completo de Desenvolvimento Web Crie 6 projetosHTML JAVASCRIPT CSS PHP 9. JavaScript 27. HTML DOM - Manipulando elemento (node)

Today in this all panelist episode, we talk about JS features you should avoid using. However opinions don't always align, and some come with much debate! Although we couldn't cover them all, today we discuss:eval withargumentsdo while for I++continue classesprototypesthisvar with letdeleteSponsorsTop End DevsRaygun | Click here to get started on your free 14-day trialCoaching | Top End DevsLinksJavaScript Remote Conference 2022Twitter: ?@cmaxwPicksAJ- Virginia Ctenucha Ctenucha virginica (Esper, 1794) | Butterflies and Moths of North AmericaAJ- The Mistborn Trilogy by Brandon SandersonAJ- Thread pitch gauge at Lowes.com: Search ResultsCharles- AntidoteCharles- Conferences | Top End DevsCharles- 1883 - Yellowstone Prequel (Official Site) Watch on Paramount+Dan- Can I use... Support tables for HTML5, CSS3, etcDan - War in UkraineDan- Webb Space Telescope GSFC/NASASteve- Coworker Standing At Desk Obviously Just Hasn't Learned About Chairs YetSteve - Dad Jokes

Video content can be found here: https://www.youtube.com/channel/UC0BAd8tPlDqFvDYBemHcQPQ/

Ben finalizes Formkeep's Ember removal, toys with the idea of manual on boarding, and positions himself to begin experimenting with plans and pricing. Meanwhile, Chris deals with credit card fraud, too many inodes on Upcase's server, and finishes the changes required to offer free videos.UpcaseFormkeepBarbell Investment StrategyDesigning Forms That ConvertFree Gitsh Video from The Weekly IterationWhat is 2.5 * 26?Support Giant Robots Smashing Into Other Giant Robots

Curso completo de Desenvolvimento Web Crie 6 projetosHTML JAVASCRIPT CSS PHP 3. HTML 21. Projeto final site

While at TXJS - Adam and Wynn caught up with Douglas Crockford, author of both JavaScript: The Good Parts and the JSON spec, and a global namespace unto himself.

Mikeal Rogers, Alex Sexton, and special guest Juan Pablo Buritica discuss all things JavaScript in Latin America. The conferences, the communities, the meetups, JavaScript tooling, and more.

📝 Algorithms and data structures implemented in JavaScript with explanations and links to further readings JavaScript Algorithms and Data Structures This repository contains JavaScript based examples of manypopular algorithms and data structures. Each algorithm and data structure has its own separate READMEwith related explanations and links for further reading (including onesto YouTube videos). Read this in other languages: 简体中文 , 繁體中文 , 한국어 , 日本語 , Polski , Français , Español , Português , Русский , Türk , Italiana , Bahasa Indonesia , Українська , Arabic ☝ Note that this project is meant to be used for learning and researching purposesonly, and it is not meant to be used for production. Data Structures A data structure is a particular way of organizing and storing data in a computer so that it canbe accessed and modified efficiently. More precisely, a data structure is a collection of datavalues, the relationships among them, and the functions or operations that can be applied tothe data. B - Beginner, A - Advanced B Linked List B Doubly Linked List B Queue B Stack B Hash Table B Heap - max and min heap versions B Priority Queue A Trie A Tree A Binary Search Tree A AVL Tree A Red-Black Tree A Segment Tree - with min/max/sum range queries examples A Fenwick Tree (Binary Indexed Tree) A Graph (both directed and undirected) A Disjoint Set A Bloom Filter Algorithms An algorithm is an unambiguous specification of how to solve a class of problems. It isa set of rules that precisely define a sequence of operations. B - Beginner, A - Advanced Algorithms by Topic Math B Bit Manipulation - set/get/update/clear bits, multiplication/division by two, make negative etc. B Factorial B Fibonacci Number - classic and closed-form versions B Prime Factors - finding prime factors and counting them using Hardy-Ramanujan's theorem B Primality Test (trial division method) B Euclidean Algorithm - calculate the Greatest Common Divisor (GCD) B Least Common Multiple (LCM) B Sieve of Eratosthenes - finding all prime numbers up to any given limit B Is Power of Two - check if the number is power of two (naive and bitwise algorithms) B Pascal's Triangle B Complex Number - complex numbers and basic operations with them B Radian & Degree - radians to degree and backwards conversion B Fast Powering B Horner's method - polynomial evaluation B Matrices - matrices and basic matrix operations (multiplication, transposition, etc.) B Euclidean Distance - distance between two points/vectors/matrices A Integer Partition A Square Root - Newton's method A Liu Hui π Algorithm - approximate π calculations based on N-gons A Discrete Fourier Transform - decompose a function of time (a signal) into the frequencies that make it up Sets B Cartesian Product - product of multiple sets B Fisher–Yates Shuffle - random permutation of a finite sequence A Power Set - all subsets of a set (bitwise and backtracking solutions) A Permutations (with and without repetitions) A Combinations (with and without repetitions) A Longest Common Subsequence (LCS) A Longest Increasing Subsequence A Shortest Common Supersequence (SCS) A Knapsack Problem - "0/1" and "Unbound" ones A Maximum Subarray - "Brute Force" and "Dynamic Programming" (Kadane's) versions A Combination Sum - find all combinations that form specific sum Strings B Hamming Distance - number of positions at which the symbols are different A Levenshtein Distance - minimum edit distance between two sequences A Knuth–Morris–Pratt Algorithm (KMP Algorithm) - substring search (pattern matching) A Z Algorithm - substring search (pattern matching) A Rabin Karp Algorithm - substring search A Longest Common Substring A Regular Expression Matching Searches B Linear Search B Jump Search (or Block Search) - search in sorted array B Binary Search - search in sorted array B Interpolation Search - search in uniformly distributed sorted array Sorting B Bubble Sort B Selection Sort B Insertion Sort B Heap Sort B Merge Sort B Quicksort - in-place and non-in-place implementations B Shellsort B Counting Sort B Radix Sort Linked Lists B Straight Traversal B Reverse Traversal Trees B Depth-First Search (DFS) B Breadth-First Search (BFS) Graphs B Depth-First Search (DFS) B Breadth-First Search (BFS) B Kruskal’s Algorithm - finding Minimum Spanning Tree (MST) for weighted undirected graph A Dijkstra Algorithm - finding the shortest paths to all graph vertices from single vertex A Bellman-Ford Algorithm - finding the shortest paths to all graph vertices from single vertex A Floyd-Warshall Algorithm - find the shortest paths between all pairs of vertices A Detect Cycle - for both directed and undirected graphs (DFS and Disjoint Set based versions) A Prim’s Algorithm - finding Minimum Spanning Tree (MST) for weighted undirected graph A Topological Sorting - DFS method A Articulation Points - Tarjan's algorithm (DFS based) A Bridges - DFS based algorithm A Eulerian Path and Eulerian Circuit - Fleury's algorithm - Visit every edge exactly once A Hamiltonian Cycle - Visit every vertex exactly once A Strongly Connected Components - Kosaraju's algorithm A Travelling Salesman Problem - shortest possible route that visits each city and returns to the origin city Cryptography B Polynomial Hash - rolling hash function based on polynomial B Rail Fence Cipher - a transposition cipher algorithm for encoding messages B Caesar Cipher - simple substitution cipher B Hill Cipher - substitution cipher based on linear algebra Machine Learning B NanoNeuron - 7 simple JS functions that illustrate how machines can actually learn (forward/backward propagation) B k-NN - k-nearest neighbors classification algorithm B k-Means - k-Means clustering algorithm Image Processing B Seam Carving - content-aware image resizing algorithm Uncategorized B Tower of Hanoi B Square Matrix Rotation - in-place algorithm B Jump Game - backtracking, dynamic programming (top-down + bottom-up) and greedy examples B Unique Paths - backtracking, dynamic programming and Pascal's Triangle based examples B Rain Terraces - trapping rain water problem (dynamic programming and brute force versions) B Recursive Staircase - count the number of ways to reach to the top (4 solutions) B Best Time To Buy Sell Stocks - divide and conquer and one-pass examples A N-Queens Problem A Knight's Tour Algorithms by Paradigm An algorithmic paradigm is a generic method or approach which underlies the design of a classof algorithms. It is an abstraction higher than the notion of an algorithm, just as analgorithm is an abstraction higher than a computer program. Brute Force - look at all the possibilities and selects the best solution B Linear Search B Rain Terraces - trapping rain water problem B Recursive Staircase - count the number of ways to reach to the top A Maximum Subarray A Travelling Salesman Problem - shortest possible route that visits each city and returns to the origin city A Discrete Fourier Transform - decompose a function of time (a signal) into the frequencies that make it up Greedy - choose the best option at the current time, without any consideration for the future B Jump Game A Unbound Knapsack Problem A Dijkstra Algorithm - finding the shortest path to all graph vertices A Prim’s Algorithm - finding Minimum Spanning Tree (MST) for weighted undirected graph A Kruskal’s Algorithm - finding Minimum Spanning Tree (MST) for weighted undirected graph Divide and Conquer - divide the problem into smaller parts and then solve those parts B Binary Search B Tower of Hanoi B Pascal's Triangle B Euclidean Algorithm - calculate the Greatest Common Divisor (GCD) B Merge Sort B Quicksort B Tree Depth-First Search (DFS) B Graph Depth-First Search (DFS) B Matrices - generating and traversing the matrices of different shapes B Jump Game B Fast Powering B Best Time To Buy Sell Stocks - divide and conquer and one-pass examples A Permutations (with and without repetitions) A Combinations (with and without repetitions) Dynamic Programming - build up a solution using previously found sub-solutions B Fibonacci Number B Jump Game B Unique Paths B Rain Terraces - trapping rain water problem B Recursive Staircase - count the number of ways to reach to the top B Seam Carving - content-aware image resizing algorithm A Levenshtein Distance - minimum edit distance between two sequences A Longest Common Subsequence (LCS) A Longest Common Substring A Longest Increasing Subsequence A Shortest Common Supersequence A 0/1 Knapsack Problem A Integer Partition A Maximum Subarray A Bellman-Ford Algorithm - finding the shortest path to all graph vertices A Floyd-Warshall Algorithm - find the shortest paths between all pairs of vertices A Regular Expression Matching Backtracking - similarly to brute force, try to generate all possible solutions, but each time you generate next solution you testif it satisfies all conditions, and only then continue generating subsequent solutions. Otherwise, backtrack, and go on adifferent path of finding a solution. Normally the DFS traversal of state-space is being used. B Jump Game B Unique Paths B Power Set - all subsets of a set A Hamiltonian Cycle - Visit every vertex exactly once A N-Queens Problem A Knight's Tour A Combination Sum - find all combinations that form specific sum Branch & Bound - remember the lowest-cost solution found at each stage of the backtrackingsearch, and use the cost of the lowest-cost solution found so far as a lower bound on the cost ofa least-cost solution to the problem, in order to discard partial solutions with costs larger than thelowest-cost solution found so far. Normally BFS traversal in combination with DFS traversal of state-spacetree is being used. How to use this repository Install all dependencies npm install Run ESLint You may want to run it to check code quality. npm run lint Run all tests npm test Run tests by name npm test -- 'LinkedList' Troubleshooting In case if linting or testing is failing try to delete the node_modules folder and re-install npm packages: rm -rf ./node_modulesnpm i Playground You may play with data-structures and algorithms in ./src/playground/playground.js file and writetests for it in ./src/playground/__test__/playground.test.js . Then just simply run the following command to test if your playground code works as expected: npm test -- 'playground' Useful Information References ▶ Data Structures and Algorithms on YouTube Big O Notation Big O notation is used to classify algorithms according to how their running time or space requirements grow as the input size grows.On the chart below you may find most common orders of growth of algorithms specified in Big O notation. Source: Big O Cheat Sheet . Below is the list of some of the most used Big O notations and their performance comparisons against different sizes of the input data. | Big O Notation | Computations for 10 elements | Computations for 100 elements | Computations for 1000 elements || -------------- | ---------------------------- | ----------------------------- | ------------------------------- || O(1) | 1 | 1 | 1 || O(log N) | 3 | 6 | 9 || O(N) | 10 | 100 | 1000 || O(N log N) | 30 | 600 | 9000 || O(N^2) | 100 | 10000 | 1000000 || O(2^N) | 1024 | 1.26e+29 | 1.07e+301 || O(N!) | 3628800 | 9.3e+157 | 4.02e+2567 | Data Structure Operations Complexity | Data Structure | Access | Search | Insertion | Deletion | Comments || ----------------------- | :-------: | :-------: | :-------: | :-------: | :-------- || Array | 1 | n | n | n | || Stack | n | n | 1 | 1 | || Queue | n | n | 1 | 1 | || Linked List | n | n | 1 | n | || Hash Table | - | n | n | n | In case of perfect hash function costs would be O(1) || Binary Search Tree | n | n | n | n | In case of balanced tree costs would be O(log(n)) || B-Tree | log(n) | log(n) | log(n) | log(n) | || Red-Black Tree | log(n) | log(n) | log(n) | log(n) | || AVL Tree | log(n) | log(n) | log(n) | log(n) | || Bloom Filter | - | 1 | 1 | - | False positives are possible while searching | Array Sorting Algorithms Complexity | Name | Best | Average | Worst | Memory | Stable | Comments || --------------------- | :-------------: | :-----------------: | :-----------------: | :-------: | :-------: | :-------- || Bubble sort | n | n 2 | n 2 | 1 | Yes | || Insertion sort | n | n 2 | n 2 | 1 | Yes | || Selection sort | n 2 | n 2 | n 2 | 1 | No | || Heap sort | n log(n) | n log(n) | n log(n) | 1 | No | || Merge sort | n log(n) | n log(n) | n log(n) | n | Yes | || Quick sort | n log(n) | n log(n) | n 2 | log(n) | No | Quicksort is usually done in-place with O(log(n)) stack space || Shell sort | n log(n) | depends on gap sequence | n (log(n)) 2 | 1 | No | || Counting sort | n + r | n + r | n + r | n + r | Yes | r - biggest number in array || Radix sort | n * k | n * k | n * k | n + k | Yes | k - length of longest key | Project Backers You may support this project via ❤️️ GitHub or ❤️️ Patreon . Folks who are backing this project ∑ = 0 To restore the repository download the bundle wget https://archive.org/download/github.com-trekhleb-javascript-algorithms_-_2021-04-28_09-20-24/trekhleb-javascript-algorithms_-_2021-04-28_09-20-24.bundle and run: git clone trekhleb-javascript-algorithms_-_2021-04-28_09-20-24.bundle Source: https://github.com/trekhleb/javascript-algorithms Uploader: trekhleb Upload date: 2021-04-28

Curso completo de Desenvolvimento Web Crie 6 projetosHTML JAVASCRIPT CSS PHP 8. Bootstrap - Criando Sites Responsivos 4. Configurando o Bootrstrap

Curso completo de Desenvolvimento Web Crie 6 projetosHTML JAVASCRIPT CSS PHP 9. JavaScript 4. Variáveis

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Curso completo de Desenvolvimento Web Crie 6 projetosHTML JAVASCRIPT CSS PHP 7. CSS3 4. Normalize css

Javascript Cuevana 01

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📝 Algorithms and data structures implemented in JavaScript with explanations and links to further readings JavaScript Algorithms and Data Structures This repository contains JavaScript based examples of manypopular algorithms and data structures. Each algorithm and data structure has its own separate READMEwith related explanations and links for further reading (including onesto YouTube videos). Read this in other languages: 简体中文 , 繁體中文 , 한국어 , 日本語 , Polski , Français , Español , Português , Русский , Türk , Italiana , Bahasa Indonesia , Українська , Arabic , Deutsch ☝ Note that this project is meant to be used for learning and researching purposesonly, and it is not meant to be used for production. Data Structures A data structure is a particular way of organizing and storing data in a computer so that it canbe accessed and modified efficiently. More precisely, a data structure is a collection of datavalues, the relationships among them, and the functions or operations that can be applied tothe data. B - Beginner, A - Advanced B Linked List B Doubly Linked List B Queue B Stack B Hash Table B Heap - max and min heap versions B Priority Queue A Trie A Tree A Binary Search Tree A AVL Tree A Red-Black Tree A Segment Tree - with min/max/sum range queries examples A Fenwick Tree (Binary Indexed Tree) A Graph (both directed and undirected) A Disjoint Set A Bloom Filter Algorithms An algorithm is an unambiguous specification of how to solve a class of problems. It isa set of rules that precisely define a sequence of operations. B - Beginner, A - Advanced Algorithms by Topic Math B Bit Manipulation - set/get/update/clear bits, multiplication/division by two, make negative etc. B Binary Floating Point - binary representation of the floating-point numbers. B Factorial B Fibonacci Number - classic and closed-form versions B Prime Factors - finding prime factors and counting them using Hardy-Ramanujan's theorem B Primality Test (trial division method) B Euclidean Algorithm - calculate the Greatest Common Divisor (GCD) B Least Common Multiple (LCM) B Sieve of Eratosthenes - finding all prime numbers up to any given limit B Is Power of Two - check if the number is power of two (naive and bitwise algorithms) B Pascal's Triangle B Complex Number - complex numbers and basic operations with them B Radian & Degree - radians to degree and backwards conversion B Fast Powering B Horner's method - polynomial evaluation B Matrices - matrices and basic matrix operations (multiplication, transposition, etc.) B Euclidean Distance - distance between two points/vectors/matrices A Integer Partition A Square Root - Newton's method A Liu Hui π Algorithm - approximate π calculations based on N-gons A Discrete Fourier Transform - decompose a function of time (a signal) into the frequencies that make it up Sets B Cartesian Product - product of multiple sets B Fisher–Yates Shuffle - random permutation of a finite sequence A Power Set - all subsets of a set (bitwise and backtracking solutions) A Permutations (with and without repetitions) A Combinations (with and without repetitions) A Longest Common Subsequence (LCS) A Longest Increasing Subsequence A Shortest Common Supersequence (SCS) A Knapsack Problem - "0/1" and "Unbound" ones A Maximum Subarray - "Brute Force" and "Dynamic Programming" (Kadane's) versions A Combination Sum - find all combinations that form specific sum Strings B Hamming Distance - number of positions at which the symbols are different A Levenshtein Distance - minimum edit distance between two sequences A Knuth–Morris–Pratt Algorithm (KMP Algorithm) - substring search (pattern matching) A Z Algorithm - substring search (pattern matching) A Rabin Karp Algorithm - substring search A Longest Common Substring A Regular Expression Matching Searches B Linear Search B Jump Search (or Block Search) - search in sorted array B Binary Search - search in sorted array B Interpolation Search - search in uniformly distributed sorted array Sorting B Bubble Sort B Selection Sort B Insertion Sort B Heap Sort B Merge Sort B Quicksort - in-place and non-in-place implementations B Shellsort B Counting Sort B Radix Sort Linked Lists B Straight Traversal B Reverse Traversal Trees B Depth-First Search (DFS) B Breadth-First Search (BFS) Graphs B Depth-First Search (DFS) B Breadth-First Search (BFS) B Kruskal’s Algorithm - finding Minimum Spanning Tree (MST) for weighted undirected graph A Dijkstra Algorithm - finding the shortest paths to all graph vertices from single vertex A Bellman-Ford Algorithm - finding the shortest paths to all graph vertices from single vertex A Floyd-Warshall Algorithm - find the shortest paths between all pairs of vertices A Detect Cycle - for both directed and undirected graphs (DFS and Disjoint Set based versions) A Prim’s Algorithm - finding Minimum Spanning Tree (MST) for weighted undirected graph A Topological Sorting - DFS method A Articulation Points - Tarjan's algorithm (DFS based) A Bridges - DFS based algorithm A Eulerian Path and Eulerian Circuit - Fleury's algorithm - Visit every edge exactly once A Hamiltonian Cycle - Visit every vertex exactly once A Strongly Connected Components - Kosaraju's algorithm A Travelling Salesman Problem - shortest possible route that visits each city and returns to the origin city Cryptography B Polynomial Hash - rolling hash function based on polynomial B Rail Fence Cipher - a transposition cipher algorithm for encoding messages B Caesar Cipher - simple substitution cipher B Hill Cipher - substitution cipher based on linear algebra Machine Learning B NanoNeuron - 7 simple JS functions that illustrate how machines can actually learn (forward/backward propagation) B k-NN - k-nearest neighbors classification algorithm B k-Means - k-Means clustering algorithm Image Processing B Seam Carving - content-aware image resizing algorithm Statistics B Weighted Random - select the random item from the list based on items' weights Evolutionary algorithms A Genetic algorithm - example of how the genetic algorithm may be applied for training the self-parking cars Uncategorized B Tower of Hanoi B Square Matrix Rotation - in-place algorithm B Jump Game - backtracking, dynamic programming (top-down + bottom-up) and greedy examples B Unique Paths - backtracking, dynamic programming and Pascal's Triangle based examples B Rain Terraces - trapping rain water problem (dynamic programming and brute force versions) B Recursive Staircase - count the number of ways to reach to the top (4 solutions) B Best Time To Buy Sell Stocks - divide and conquer and one-pass examples A N-Queens Problem A Knight's Tour Algorithms by Paradigm An algorithmic paradigm is a generic method or approach which underlies the design of a classof algorithms. It is an abstraction higher than the notion of an algorithm, just as analgorithm is an abstraction higher than a computer program. Brute Force - look at all the possibilities and selects the best solution B Linear Search B Rain Terraces - trapping rain water problem B Recursive Staircase - count the number of ways to reach to the top A Maximum Subarray A Travelling Salesman Problem - shortest possible route that visits each city and returns to the origin city A Discrete Fourier Transform - decompose a function of time (a signal) into the frequencies that make it up Greedy - choose the best option at the current time, without any consideration for the future B Jump Game A Unbound Knapsack Problem A Dijkstra Algorithm - finding the shortest path to all graph vertices A Prim’s Algorithm - finding Minimum Spanning Tree (MST) for weighted undirected graph A Kruskal’s Algorithm - finding Minimum Spanning Tree (MST) for weighted undirected graph Divide and Conquer - divide the problem into smaller parts and then solve those parts B Binary Search B Tower of Hanoi B Pascal's Triangle B Euclidean Algorithm - calculate the Greatest Common Divisor (GCD) B Merge Sort B Quicksort B Tree Depth-First Search (DFS) B Graph Depth-First Search (DFS) B Matrices - generating and traversing the matrices of different shapes B Jump Game B Fast Powering B Best Time To Buy Sell Stocks - divide and conquer and one-pass examples A Permutations (with and without repetitions) A Combinations (with and without repetitions) Dynamic Programming - build up a solution using previously found sub-solutions B Fibonacci Number B Jump Game B Unique Paths B Rain Terraces - trapping rain water problem B Recursive Staircase - count the number of ways to reach to the top B Seam Carving - content-aware image resizing algorithm A Levenshtein Distance - minimum edit distance between two sequences A Longest Common Subsequence (LCS) A Longest Common Substring A Longest Increasing Subsequence A Shortest Common Supersequence A 0/1 Knapsack Problem A Integer Partition A Maximum Subarray A Bellman-Ford Algorithm - finding the shortest path to all graph vertices A Floyd-Warshall Algorithm - find the shortest paths between all pairs of vertices A Regular Expression Matching Backtracking - similarly to brute force, try to generate all possible solutions, but each time you generate next solution you testif it satisfies all conditions, and only then continue generating subsequent solutions. Otherwise, backtrack, and go on adifferent path of finding a solution. Normally the DFS traversal of state-space is being used. B Jump Game B Unique Paths B Power Set - all subsets of a set A Hamiltonian Cycle - Visit every vertex exactly once A N-Queens Problem A Knight's Tour A Combination Sum - find all combinations that form specific sum Branch & Bound - remember the lowest-cost solution found at each stage of the backtrackingsearch, and use the cost of the lowest-cost solution found so far as a lower bound on the cost ofa least-cost solution to the problem, in order to discard partial solutions with costs larger than thelowest-cost solution found so far. Normally BFS traversal in combination with DFS traversal of state-spacetree is being used. How to use this repository Install all dependencies npm install Run ESLint You may want to run it to check code quality. npm run lint Run all tests npm test Run tests by name npm test -- 'LinkedList' Troubleshooting In case if linting or testing is failing try to delete the node_modules folder and re-install npm packages: rm -rf ./node_modulesnpm i Playground You may play with data-structures and algorithms in ./src/playground/playground.js file and writetests for it in ./src/playground/__test__/playground.test.js . Then just simply run the following command to test if your playground code works as expected: npm test -- 'playground' Useful Information References ▶ Data Structures and Algorithms on YouTube Big O Notation Big O notation is used to classify algorithms according to how their running time or space requirements grow as the input size grows.On the chart below you may find most common orders of growth of algorithms specified in Big O notation. Source: Big O Cheat Sheet . Below is the list of some of the most used Big O notations and their performance comparisons against different sizes of the input data. | Big O Notation | Computations for 10 elements | Computations for 100 elements | Computations for 1000 elements || -------------- | ---------------------------- | ----------------------------- | ------------------------------- || O(1) | 1 | 1 | 1 || O(log N) | 3 | 6 | 9 || O(N) | 10 | 100 | 1000 || O(N log N) | 30 | 600 | 9000 || O(N^2) | 100 | 10000 | 1000000 || O(2^N) | 1024 | 1.26e+29 | 1.07e+301 || O(N!) | 3628800 | 9.3e+157 | 4.02e+2567 | Data Structure Operations Complexity | Data Structure | Access | Search | Insertion | Deletion | Comments || ----------------------- | :-------: | :-------: | :-------: | :-------: | :-------- || Array | 1 | n | n | n | || Stack | n | n | 1 | 1 | || Queue | n | n | 1 | 1 | || Linked List | n | n | 1 | n | || Hash Table | - | n | n | n | In case of perfect hash function costs would be O(1) || Binary Search Tree | n | n | n | n | In case of balanced tree costs would be O(log(n)) || B-Tree | log(n) | log(n) | log(n) | log(n) | || Red-Black Tree | log(n) | log(n) | log(n) | log(n) | || AVL Tree | log(n) | log(n) | log(n) | log(n) | || Bloom Filter | - | 1 | 1 | - | False positives are possible while searching | Array Sorting Algorithms Complexity | Name | Best | Average | Worst | Memory | Stable | Comments || --------------------- | :-------------: | :-----------------: | :-----------------: | :-------: | :-------: | :-------- || Bubble sort | n | n 2 | n 2 | 1 | Yes | || Insertion sort | n | n 2 | n 2 | 1 | Yes | || Selection sort | n 2 | n 2 | n 2 | 1 | No | || Heap sort | n log(n) | n log(n) | n log(n) | 1 | No | || Merge sort | n log(n) | n log(n) | n log(n) | n | Yes | || Quick sort | n log(n) | n log(n) | n 2 | log(n) | No | Quicksort is usually done in-place with O(log(n)) stack space || Shell sort | n log(n) | depends on gap sequence | n (log(n)) 2 | 1 | No | || Counting sort | n + r | n + r | n + r | n + r | Yes | r - biggest number in array || Radix sort | n * k | n * k | n * k | n + k | Yes | k - length of longest key | Project Backers You may support this project via ❤️️ GitHub or ❤️️ Patreon . Folks who are backing this project ∑ = 0 ℹ️ A few more projects and articles about JavaScript and algorithms on trekhleb.dev To restore the repository download the bundle wget https://archive.org/download/github.com-trekhleb-javascript-algorithms_-_2021-12-07_13-33-17/trekhleb-javascript-algorithms_-_2021-12-07_13-33-17.bundle and run: git clone trekhleb-javascript-algorithms_-_2021-12-07_13-33-17.bundle Source: https://github.com/trekhleb/javascript-algorithms Uploader: trekhleb Upload date: 2021-12-07

A long list of (advanced) JavaScript questions, and their explanations :sparkles: JavaScript Questions --- I post multiple choice JavaScript questions on my [Instagram](https://www.instagram.com/theavocoder) **stories**, which I'll also post here! Last updated: June 12th From basic to advanced: test how well you know JavaScript, refresh your knowledge a bit, or prepare for your coding interview! :muscle: :rocket: I update this repo regularly with new questions. I added the answers in the **collapsed sections** below the questions, simply click on them to expand it. It's just for fun, good luck! :heart: Feel free to reach out to me! 😊 Instagram || Twitter || LinkedIn || Blog | Feel free to use them in a project! 😃 I would really appreciate a reference to this repo, I create the questions and explanations (yes I'm sad lol) and the community helps me so much to maintain and improve it! 💪🏼 Thank you and have fun! ||---| See 17 Available Translations 🇪🇸🇮🇹🇩🇪 🇫🇷🇷🇺🇨🇳🇵🇹 * [English](./en-EN/README.md)* [العربية](./ar-AR/README_AR.md)* [اللغة العامية - Egyptian Arabic](./ar-EG/README_ar-EG.md)* [Bosanski](./bs-BS/README-bs_BS.md) * [Deutsch](./de-DE/README.md) * [Español](./es-ES/README-ES.md)* [Français](./fr-FR/README_fr-FR.md)* [日本語](./ja-JA/README-ja_JA.md) * [한국어](./ko-KR/README-ko_KR.md)* [Nederlands](./nl-NL/README.md)* [Português Brasil](./pt-BR/README_pt_BR.md) * [Русский](./ru-RU/README.md)* [Українська мова](./ua-UA/README-ua_UA.md) * [Tiếng Việt](./vi-VI/README-vi.md)* [简体中文](./zh-CN/README-zh_CN.md)* [繁體中文](./zh-TW/README_zh-TW.md)* [Türkçe](./tr-TR/README-tr_TR.md)* [ไทย](./th-TH/README-th_TH.md)* [Indonesia](./id-ID/README.md) 1. What's the output? ```javascriptfunction sayHi() { console.log(name); console.log(age); var name = 'Lydia'; let age = 21;} sayHi();``` A: Lydia and undefined B: Lydia and ReferenceError C: ReferenceError and 21 D: undefined and ReferenceError Answer #### Answer: DWithin the function, we first declare the `name` variable with the `var` keyword. This means that the variable gets hoisted (memory space is set up during the creation phase) with the default value of `undefined`, until we actually get to the line where we define the variable. We haven't defined the variable yet on the line where we try to log the `name` variable, so it still holds the value of `undefined`.Variables with the `let` keyword (and `const`) are hoisted, but unlike `var`, don't get initialized . They are not accessible before the line we declare (initialize) them. This is called the "temporal dead zone". When we try to access the variables before they are declared, JavaScript throws a `ReferenceError`. 2. What's the output? ```javascriptfor (var i = 0; i < 3; i++) { setTimeout(() => console.log(i), 1);} for (let i = 0; i < 3; i++) { setTimeout(() => console.log(i), 1);}``` A: 0 1 2 and 0 1 2 B: 0 1 2 and 3 3 3 C: 3 3 3 and 0 1 2 Answer #### Answer: CBecause of the event queue in JavaScript, the `setTimeout` callback function is called _after_ the loop has been executed. Since the variable `i` in the first loop was declared using the `var` keyword, this value was global. During the loop, we incremented the value of `i` by `1` each time, using the unary operator `++`. By the time the `setTimeout` callback function was invoked, `i` was equal to `3` in the first example.In the second loop, the variable `i` was declared using the `let` keyword: variables declared with the `let` (and `const`) keyword are block-scoped (a block is anything between `{ }`). During each iteration, `i` will have a new value, and each value is scoped inside the loop. 3. What's the output? ```javascriptconst shape = { radius: 10, diameter() { return this.radius * 2; }, perimeter: () => 2 * Math.PI * this.radius,}; console.log(shape.diameter());console.log(shape.perimeter());``` A: 20 and 62.83185307179586 B: 20 and NaN C: 20 and 63 D: NaN and 63 Answer #### Answer: BNote that the value of `diameter` is a regular function, whereas the value of `perimeter` is an arrow function.With arrow functions, the `this` keyword refers to its current surrounding scope, unlike regular functions! This means that when we call `perimeter`, it doesn't refer to the shape object, but to its surrounding scope (window for example).There is no value `radius` on that object, which returns `NaN`. 4. What's the output? javascript+true;!'Lydia'; A: 1 and false B: false and NaN C: false and false Answer #### Answer: AThe unary plus tries to convert an operand to a number. `true` is `1`, and `false` is `0`.The string `'Lydia'` is a truthy value. What we're actually asking, is "is this truthy value falsy?". This returns `false`. 5. Which one is true? ```javascriptconst bird = { size: 'small',}; const mouse = { name: 'Mickey', small: true,};``` A: mouse.bird.size is not valid B: mouse[bird.size] is not valid C: mouse[bird["size"]] is not valid D: All of them are valid Answer #### Answer: AIn JavaScript, all object keys are strings (unless it's a Symbol). Even though we might not _type_ them as strings, they are always converted into strings under the hood.JavaScript interprets (or unboxes) statements. When we use bracket notation, it sees the first opening bracket `[` and keeps going until it finds the closing bracket `]`. Only then, it will evaluate the statement.`mouse[bird.size]`: First it evaluates `bird.size`, which is `"small"`. `mouse["small"]` returns `true`However, with dot notation, this doesn't happen. `mouse` does not have a key called `bird`, which means that `mouse.bird` is `undefined`. Then, we ask for the `size` using dot notation: `mouse.bird.size`. Since `mouse.bird` is `undefined`, we're actually asking `undefined.size`. This isn't valid, and will throw an error similar to `Cannot read property "size" of undefined`. 6. What's the output? ```javascriptlet c = { greeting: 'Hey!' };let d; d = c;c.greeting = 'Hello';console.log(d.greeting);``` A: Hello B: Hey! C: undefined D: ReferenceError E: TypeError Answer #### Answer: AIn JavaScript, all objects interact by _reference_ when setting them equal to each other.First, variable `c` holds a value to an object. Later, we assign `d` with the same reference that `c` has to the object. When you change one object, you change all of them. 7. What's the output? ```javascriptlet a = 3;let b = new Number(3);let c = 3; console.log(a == b);console.log(a === b);console.log(b === c);``` A: true false true B: false false true C: true false false D: false true true Answer #### Answer: C`new Number()` is a built-in function constructor. Although it looks like a number, it's not really a number: it has a bunch of extra features and is an object.When we use the `==` operator, it only checks whether it has the same _value_. They both have the value of `3`, so it returns `true`.However, when we use the `===` operator, both value _and_ type should be the same. It's not: `new Number()` is not a number, it's an **object**. Both return `false.` 8. What's the output? ```javascriptclass Chameleon { static colorChange(newColor) { this.newColor = newColor; return this.newColor; } constructor({ newColor = 'green' } = {}) { this.newColor = newColor; }} const freddie = new Chameleon({ newColor: 'purple' });console.log(freddie.colorChange('orange'));``` A: orange B: purple C: green D: TypeError Answer #### Answer: DThe `colorChange` function is static. Static methods are designed to live only on the constructor in which they are created, and cannot be passed down to any children. Since `freddie` is a child, the function is not passed down, and not available on the `freddie` instance: a `TypeError` is thrown. 9. What's the output? javascriptlet greeting;greetign = {}; // Typo!console.log(greetign); A: {} B: ReferenceError: greetign is not defined C: undefined Answer #### Answer: AIt logs the object, because we just created an empty object on the global object! When we mistyped `greeting` as `greetign`, the JS interpreter actually saw this as `global.greetign = {}` (or `window.greetign = {}` in a browser).In order to avoid this, we can use `"use strict"`. This makes sure that you have declared a variable before setting it equal to anything. 10. What happens when we do this? ```javascriptfunction bark() { console.log('Woof!');} bark.animal = 'dog';``` A: Nothing, this is totally fine! B: SyntaxError . You cannot add properties to a function this way. C: "Woof" gets logged. D: ReferenceError Answer #### Answer: AThis is possible in JavaScript, because functions are objects! (Everything besides primitive types are objects)A function is a special type of object. The code you write yourself isn't the actual function. The function is an object with properties. This property is invocable. 11. What's the output? ```javascriptfunction Person(firstName, lastName) { this.firstName = firstName; this.lastName = lastName;} const member = new Person('Lydia', 'Hallie');Person.getFullName = function() { return ${this.firstName} ${this.lastName} ;}; console.log(member.getFullName());``` A: TypeError B: SyntaxError C: Lydia Hallie D: undefined undefined Answer #### Answer: AIn JavaScript, functions are objects, and therefore, the method `getFullName` gets added to the constructor function object itself. For that reason, we can call `Person.getFullName()`, but `member.getFullName` throws a `TypeError`. If you want a method to be available to all object instances, you have to add it to the prototype property:```jsPerson.prototype.getFullName = function() { return `${this.firstName} ${this.lastName}`;};``` 12. What's the output? ```javascriptfunction Person(firstName, lastName) { this.firstName = firstName; this.lastName = lastName;} const lydia = new Person('Lydia', 'Hallie');const sarah = Person('Sarah', 'Smith'); console.log(lydia);console.log(sarah);``` A: Person {firstName: "Lydia", lastName: "Hallie"} and undefined B: Person {firstName: "Lydia", lastName: "Hallie"} and Person {firstName: "Sarah", lastName: "Smith"} C: Person {firstName: "Lydia", lastName: "Hallie"} and {} D: Person {firstName: "Lydia", lastName: "Hallie"} and ReferenceError Answer #### Answer: AFor `sarah`, we didn't use the `new` keyword. When using `new`, `this` refers to the new empty object we create. However, if you don't add `new`, `this` refers to the **global object**!We said that `this.firstName` equals `"Sarah"` and `this.lastName` equals `"Smith"`. What we actually did, is defining `global.firstName = 'Sarah'` and `global.lastName = 'Smith'`. `sarah` itself is left `undefined`, since we don't return a value from the `Person` function. 13. What are the three phases of event propagation? A: Target > Capturing > Bubbling B: Bubbling > Target > Capturing C: Target > Bubbling > Capturing D: Capturing > Target > Bubbling Answer #### Answer: DDuring the **capturing** phase, the event goes through the ancestor elements down to the target element. It then reaches the **target** element, and **bubbling** begins. 14. All object have prototypes. A: true B: false Answer #### Answer: BAll objects have prototypes, except for the **base object**. The base object is the object created by the user, or an object that is created using the `new` keyword. The base object has access to some methods and properties, such as `.toString`. This is the reason why you can use built-in JavaScript methods! All of such methods are available on the prototype. Although JavaScript can't find it directly on your object, it goes down the prototype chain and finds it there, which makes it accessible for you. 15. What's the output? ```javascriptfunction sum(a, b) { return a + b;} sum(1, '2');``` A: NaN B: TypeError C: "12" D: 3 Answer #### Answer: CJavaScript is a **dynamically typed language**: we don't specify what types certain variables are. Values can automatically be converted into another type without you knowing, which is called _implicit type coercion_. **Coercion** is converting from one type into another.In this example, JavaScript converts the number `1` into a string, in order for the function to make sense and return a value. During the addition of a numeric type (`1`) and a string type (`'2'`), the number is treated as a string. We can concatenate strings like `"Hello" + "World"`, so what's happening here is `"1" + "2"` which returns `"12"`. 16. What's the output? javascriptlet number = 0;console.log(number++);console.log(++number);console.log(number); A: 1 1 2 B: 1 2 2 C: 0 2 2 D: 0 1 2 Answer #### Answer: CThe **postfix** unary operator `++`:1. Returns the value (this returns `0`)2. Increments the value (number is now `1`)The **prefix** unary operator `++`:1. Increments the value (number is now `2`)2. Returns the value (this returns `2`)This returns `0 2 2`. 17. What's the output? ```javascriptfunction getPersonInfo(one, two, three) { console.log(one); console.log(two); console.log(three);} const person = 'Lydia';const age = 21; getPersonInfo ${person} is ${age} years old ;``` A: "Lydia" 21 ["", " is ", " years old"] B: ["", " is ", " years old"] "Lydia" 21 C: "Lydia" ["", " is ", " years old"] 21 Answer #### Answer: BIf you use tagged template literals, the value of the first argument is always an array of the string values. The remaining arguments get the values of the passed expressions! 18. What's the output? `` javascriptfunction checkAge(data) { if (data === { age: 18 }) { console.log('You are an adult!'); } else if (data == { age: 18 }) { console.log('You are still an adult.'); } else { console.log( Hmm.. You don't have an age I guess`); }} checkAge({ age: 18 });``` A: You are an adult! B: You are still an adult. C: Hmm.. You don't have an age I guess Answer #### Answer: CWhen testing equality, primitives are compared by their _value_, while objects are compared by their _reference_. JavaScript checks if the objects have a reference to the same location in memory.The two objects that we are comparing don't have that: the object we passed as a parameter refers to a different location in memory than the object we used in order to check equality.This is why both `{ age: 18 } === { age: 18 }` and `{ age: 18 } == { age: 18 }` return `false`. 19. What's the output? ```javascriptfunction getAge(...args) { console.log(typeof args);} getAge(21);``` A: "number" B: "array" C: "object" D: "NaN" Answer #### Answer: CThe rest parameter (`...args`) lets us "collect" all remaining arguments into an array. An array is an object, so `typeof args` returns `"object"` 20. What's the output? ```javascriptfunction getAge() { 'use strict'; age = 21; console.log(age);} getAge();``` A: 21 B: undefined C: ReferenceError D: TypeError Answer #### Answer: CWith `"use strict"`, you can make sure that you don't accidentally declare global variables. We never declared the variable `age`, and since we use `"use strict"`, it will throw a reference error. If we didn't use `"use strict"`, it would have worked, since the property `age` would have gotten added to the global object. 21. What's the value of sum ? javascriptconst sum = eval('10*10+5'); A: 105 B: "105" C: TypeError D: "10*10+5" Answer #### Answer: A`eval` evaluates codes that's passed as a string. If it's an expression, like in this case, it evaluates the expression. The expression is `10 * 10 + 5`. This returns the number `105`. 22. How long is cool_secret accessible? javascriptsessionStorage.setItem('cool_secret', 123); A: Forever, the data doesn't get lost. B: When the user closes the tab. C: When the user closes the entire browser, not only the tab. D: When the user shuts off their computer. Answer #### Answer: BThe data stored in `sessionStorage` is removed after closing the _tab_.If you used `localStorage`, the data would've been there forever, unless for example `localStorage.clear()` is invoked. 23. What's the output? ```javascriptvar num = 8;var num = 10; console.log(num);``` A: 8 B: 10 C: SyntaxError D: ReferenceError Answer #### Answer: BWith the `var` keyword, you can declare multiple variables with the same name. The variable will then hold the latest value.You cannot do this with `let` or `const` since they're block-scoped. 24. What's the output? ```javascriptconst obj = { 1: 'a', 2: 'b', 3: 'c' };const set = new Set([1, 2, 3, 4, 5]); obj.hasOwnProperty('1');obj.hasOwnProperty(1);set.has('1');set.has(1);``` A: false true false true B: false true true true C: true true false true D: true true true true Answer #### Answer: CAll object keys (excluding Symbols) are strings under the hood, even if you don't type it yourself as a string. This is why `obj.hasOwnProperty('1')` also returns true.It doesn't work that way for a set. There is no `'1'` in our set: `set.has('1')` returns `false`. It has the numeric type `1`, `set.has(1)` returns `true`. 25. What's the output? javascriptconst obj = { a: 'one', b: 'two', a: 'three' };console.log(obj); A: { a: "one", b: "two" } B: { b: "two", a: "three" } C: { a: "three", b: "two" } D: SyntaxError Answer #### Answer: CIf you have two keys with the same name, the key will be replaced. It will still be in its first position, but with the last specified value. 26. The JavaScript global execution context creates two things for you: the global object, and the "this" keyword. A: true B: false C: it depends Answer #### Answer: AThe base execution context is the global execution context: it's what's accessible everywhere in your code. 27. What's the output? javascriptfor (let i = 1; i < 5; i++) { if (i === 3) continue; console.log(i);} A: 1 2 B: 1 2 3 C: 1 2 4 D: 1 3 4 Answer #### Answer: CThe `continue` statement skips an iteration if a certain condition returns `true`. 28. What's the output? ```javascriptString.prototype.giveLydiaPizza = () => { return 'Just give Lydia pizza already!';}; const name = 'Lydia'; name.giveLydiaPizza();``` A: "Just give Lydia pizza already!" B: TypeError: not a function C: SyntaxError D: undefined Answer #### Answer: A`String` is a built-in constructor, which we can add properties to. I just added a method to its prototype. Primitive strings are automatically converted into a string object, generated by the string prototype function. So, all strings (string objects) have access to that method! 29. What's the output? ```javascriptconst a = {};const b = { key: 'b' };const c = { key: 'c' }; a[b] = 123;a[c] = 456; console.log(a[b]);``` A: 123 B: 456 C: undefined D: ReferenceError Answer #### Answer: BObject keys are automatically converted into strings. We are trying to set an object as a key to object `a`, with the value of `123`.However, when we stringify an object, it becomes `"[object Object]"`. So what we are saying here, is that `a["[object Object]"] = 123`. Then, we can try to do the same again. `c` is another object that we are implicitly stringifying. So then, `a["[object Object]"] = 456`.Then, we log `a[b]`, which is actually `a["[object Object]"]`. We just set that to `456`, so it returns `456`. 30. What's the output? ```javascriptconst foo = () => console.log('First');const bar = () => setTimeout(() => console.log('Second'));const baz = () => console.log('Third'); bar();foo();baz();``` A: First Second Third B: First Third Second C: Second First Third D: Second Third First Answer #### Answer: BWe have a `setTimeout` function and invoked it first. Yet, it was logged last.This is because in browsers, we don't just have the runtime engine, we also have something called a `WebAPI`. The `WebAPI` gives us the `setTimeout` function to start with, and for example the DOM.After the _callback_ is pushed to the WebAPI, the `setTimeout` function itself (but not the callback!) is popped off the stack. Now, `foo` gets invoked, and `"First"` is being logged. `foo` is popped off the stack, and `baz` gets invoked. `"Third"` gets logged. The WebAPI can't just add stuff to the stack whenever it's ready. Instead, it pushes the callback function to something called the _queue_. This is where an event loop starts to work. An **event loop** looks at the stack and task queue. If the stack is empty, it takes the first thing on the queue and pushes it onto the stack. `bar` gets invoked, `"Second"` gets logged, and it's popped off the stack. 31. What is the event.target when clicking the button? ```html Click! ``` A: Outer div B: Inner div C: button D: An array of all nested elements. Answer #### Answer: CThe deepest nested element that caused the event is the target of the event. You can stop bubbling by `event.stopPropagation` 32. When you click the paragraph, what's the logged output? ```html Click here! ``` A: p div B: div p C: p D: div Answer #### Answer: AIf we click `p`, we see two logs: `p` and `div`. During event propagation, there are 3 phases: capturing, target, and bubbling. By default, event handlers are executed in the bubbling phase (unless you set `useCapture` to `true`). It goes from the deepest nested element outwards. 33. What's the output? ```javascriptconst person = { name: 'Lydia' }; function sayHi(age) { return ${this.name} is ${age} ;} console.log(sayHi.call(person, 21));console.log(sayHi.bind(person, 21));``` A: undefined is 21 Lydia is 21 B: function function C: Lydia is 21 Lydia is 21 D: Lydia is 21 function Answer #### Answer: DWith both, we can pass the object to which we want the `this` keyword to refer to. However, `.call` is also _executed immediately_!`.bind.` returns a _copy_ of the function, but with a bound context! It is not executed immediately. 34. What's the output? ```javascriptfunction sayHi() { return (() => 0)();} console.log(typeof sayHi());``` A: "object" B: "number" C: "function" D: "undefined" Answer #### Answer: BThe `sayHi` function returns the returned value of the immediately invoked function expression (IIFE). This function returned `0`, which is type `"number"`.FYI: there are only 7 built-in types: `null`, `undefined`, `boolean`, `number`, `string`, `object`, and `symbol`. `"function"` is not a type, since functions are objects, it's of type `"object"`. 35. Which of these values are falsy? javascript0;new Number(0);('');(' ');new Boolean(false);undefined; A: 0 , '' , undefined B: 0 , new Number(0) , '' , new Boolean(false) , undefined C: 0 , '' , new Boolean(false) , undefined D: All of them are falsy Answer #### Answer: AThere are 8 falsy values:- `undefined`- `null`- `NaN`- `false`- `''` (empty string)- `0`- `-0`- `0n` (BigInt(0))Function constructors, like `new Number` and `new Boolean` are truthy. 36. What's the output? javascriptconsole.log(typeof typeof 1); A: "number" B: "string" C: "object" D: "undefined" Answer #### Answer: B`typeof 1` returns `"number"`.`typeof "number"` returns `"string"` 37. What's the output? javascriptconst numbers = [1, 2, 3];numbers[10] = 11;console.log(numbers); A: [1, 2, 3, 7 x null, 11] B: [1, 2, 3, 11] C: [1, 2, 3, 7 x empty, 11] D: SyntaxError Answer #### Answer: CWhen you set a value to an element in an array that exceeds the length of the array, JavaScript creates something called "empty slots". These actually have the value of `undefined`, but you will see something like:`[1, 2, 3, 7 x empty, 11]`depending on where you run it (it's different for every browser, node, etc.) 38. What's the output? javascript(() => { let x, y; try { throw new Error(); } catch (x) { (x = 1), (y = 2); console.log(x); } console.log(x); console.log(y);})(); A: 1 undefined 2 B: undefined undefined undefined C: 1 1 2 D: 1 undefined undefined Answer #### Answer: AThe `catch` block receives the argument `x`. This is not the same `x` as the variable when we pass arguments. This variable `x` is block-scoped.Later, we set this block-scoped variable equal to `1`, and set the value of the variable `y`. Now, we log the block-scoped variable `x`, which is equal to `1`.Outside of the `catch` block, `x` is still `undefined`, and `y` is `2`. When we want to `console.log(x)` outside of the `catch` block, it returns `undefined`, and `y` returns `2`. 39. Everything in JavaScript is either a... A: primitive or object B: function or object C: trick question! only objects D: number or object Answer #### Answer: AJavaScript only has primitive types and objects.Primitive types are `boolean`, `null`, `undefined`, `bigint`, `number`, `string`, and `symbol`.What differentiates a primitive from an object is that primitives do not have any properties or methods; however, you'll note that `'foo'.toUpperCase()` evaluates to `'FOO'` and does not result in a `TypeError`. This is because when you try to access a property or method on a primitive like a string, JavaScript will implicitly wrap the primitive type using one of the wrapper classes, i.e. `String`, and then immediately discard the wrapper after the expression evaluates. All primitives except for `null` and `undefined` exhibit this behaviour. 40. What's the output? javascript[[0, 1], [2, 3]].reduce( (acc, cur) => { return acc.concat(cur); }, [1, 2],); A: [0, 1, 2, 3, 1, 2] B: [6, 1, 2] C: [1, 2, 0, 1, 2, 3] D: [1, 2, 6] Answer #### Answer: C`[1, 2]` is our initial value. This is the value we start with, and the value of the very first `acc`. During the first round, `acc` is `[1,2]`, and `cur` is `[0, 1]`. We concatenate them, which results in `[1, 2, 0, 1]`.Then, `[1, 2, 0, 1]` is `acc` and `[2, 3]` is `cur`. We concatenate them, and get `[1, 2, 0, 1, 2, 3]` 41. What's the output? javascript!!null;!!'';!!1; A: false true false B: false false true C: false true true D: true true false Answer #### Answer: B`null` is falsy. `!null` returns `true`. `!true` returns `false`.`""` is falsy. `!""` returns `true`. `!true` returns `false`.`1` is truthy. `!1` returns `false`. `!false` returns `true`. 42. What does the setInterval method return in the browser? javascriptsetInterval(() => console.log('Hi'), 1000); A: a unique id B: the amount of milliseconds specified C: the passed function D: undefined Answer #### Answer: AIt returns a unique id. This id can be used to clear that interval with the `clearInterval()` function. 43. What does this return? javascript[...'Lydia']; A: ["L", "y", "d", "i", "a"] B: ["Lydia"] C: [[], "Lydia"] D: [["L", "y", "d", "i", "a"]] Answer #### Answer: AA string is an iterable. The spread operator maps every character of an iterable to one element. 44. What's the output? ```javascriptfunction* generator(i) { yield i; yield i * 2;} const gen = generator(10); console.log(gen.next().value);console.log(gen.next().value);``` A: [0, 10], [10, 20] B: 20, 20 C: 10, 20 D: 0, 10 and 10, 20 Answer #### Answer: CRegular functions cannot be stopped mid-way after invocation. However, a generator function can be "stopped" midway, and later continue from where it stopped. Every time a generator function encounters a `yield` keyword, the function yields the value specified after it. Note that the generator function in that case doesn’t _return_ the value, it _yields_ the value.First, we initialize the generator function with `i` equal to `10`. We invoke the generator function using the `next()` method. The first time we invoke the generator function, `i` is equal to `10`. It encounters the first `yield` keyword: it yields the value of `i`. The generator is now "paused", and `10` gets logged.Then, we invoke the function again with the `next()` method. It starts to continue where it stopped previously, still with `i` equal to `10`. Now, it encounters the next `yield` keyword, and yields `i * 2`. `i` is equal to `10`, so it returns `10 * 2`, which is `20`. This results in `10, 20`. 45. What does this return? ```javascriptconst firstPromise = new Promise((res, rej) => { setTimeout(res, 500, 'one');}); const secondPromise = new Promise((res, rej) => { setTimeout(res, 100, 'two');}); Promise.race([firstPromise, secondPromise]).then(res => console.log(res));``` A: "one" B: "two" C: "two" "one" D: "one" "two" Answer #### Answer: BWhen we pass multiple promises to the `Promise.race` method, it resolves/rejects the _first_ promise that resolves/rejects. To the `setTimeout` method, we pass a timer: 500ms for the first promise (`firstPromise`), and 100ms for the second promise (`secondPromise`). This means that the `secondPromise` resolves first with the value of `'two'`. `res` now holds the value of `'two'`, which gets logged. 46. What's the output? ```javascriptlet person = { name: 'Lydia' };const members = [person];person = null; console.log(members);``` A: null B: [null] C: [{}] D: [{ name: "Lydia" }] Answer #### Answer: DFirst, we declare a variable `person` with the value of an object that has a `name` property. Then, we declare a variable called `members`. We set the first element of that array equal to the value of the `person` variable. Objects interact by _reference_ when setting them equal to each other. When you assign a reference from one variable to another, you make a _copy_ of that reference. (note that they don't have the _same_ reference!) Then, we set the variable `person` equal to `null`. We are only modifying the value of the `person` variable, and not the first element in the array, since that element has a different (copied) reference to the object. The first element in `members` still holds its reference to the original object. When we log the `members` array, the first element still holds the value of the object, which gets logged. 47. What's the output? ```javascriptconst person = { name: 'Lydia', age: 21,}; for (const item in person) { console.log(item);}``` A: { name: "Lydia" }, { age: 21 } B: "name", "age" C: "Lydia", 21 D: ["name", "Lydia"], ["age", 21] Answer #### Answer: BWith a `for-in` loop, we can iterate through object keys, in this case `name` and `age`. Under the hood, object keys are strings (if they're not a Symbol). On every loop, we set the value of `item` equal to the current key it’s iterating over. First, `item` is equal to `name`, and gets logged. Then, `item` is equal to `age`, which gets logged. 48. What's the output? javascriptconsole.log(3 + 4 + '5'); A: "345" B: "75" C: 12 D: "12" Answer #### Answer: BOperator associativity is the order in which the compiler evaluates the expressions, either left-to-right or right-to-left. This only happens if all operators have the _same_ precedence. We only have one type of operator: `+`. For addition, the associativity is left-to-right.`3 + 4` gets evaluated first. This results in the number `7`.`7 + '5'` results in `"75"` because of coercion. JavaScript converts the number `7` into a string, see question 15. We can concatenate two strings using the `+`operator. `"7" + "5"` results in `"75"`. 49. What's the value of num ? javascriptconst num = parseInt('7*6', 10); A: 42 B: "42" C: 7 D: NaN Answer #### Answer: COnly the first numbers in the string is returned. Based on the _radix_ (the second argument in order to specify what type of number we want to parse it to: base 10, hexadecimal, octal, binary, etc.), the `parseInt` checks whether the characters in the string are valid. Once it encounters a character that isn't a valid number in the radix, it stops parsing and ignores the following characters.`*` is not a valid number. It only parses `"7"` into the decimal `7`. `num` now holds the value of `7`. 50. What's the output? javascript[1, 2, 3].map(num => { if (typeof num === 'number') return; return num * 2;}); A: [] B: [null, null, null] C: [undefined, undefined, undefined] D: [ 3 x empty ] Answer #### Answer: CWhen mapping over the array, the value of `num` is equal to the element it’s currently looping over. In this case, the elements are numbers, so the condition of the if statement `typeof num === "number"` returns `true`. The map function creates a new array and inserts the values returned from the function.However, we don’t return a value. When we don’t return a value from the function, the function returns `undefined`. For every element in the array, the function block gets called, so for each element we return `undefined`. 51. What's the output? ```javascriptfunction getInfo(member, year) { member.name = 'Lydia'; year = '1998';} const person = { name: 'Sarah' };const birthYear = '1997'; getInfo(person, birthYear); console.log(person, birthYear);``` A: { name: "Lydia" }, "1997" B: { name: "Sarah" }, "1998" C: { name: "Lydia" }, "1998" D: { name: "Sarah" }, "1997" Answer #### Answer: AArguments are passed by _value_, unless their value is an object, then they're passed by _reference_. `birthYear` is passed by value, since it's a string, not an object. When we pass arguments by value, a _copy_ of that value is created (see question 46).The variable `birthYear` has a reference to the value `"1997"`. The argument `year` also has a reference to the value `"1997"`, but it's not the same value as `birthYear` has a reference to. When we update the value of `year` by setting `year` equal to `"1998"`, we are only updating the value of `year`. `birthYear` is still equal to `"1997"`.The value of `person` is an object. The argument `member` has a (copied) reference to the _same_ object. When we modify a property of the object `member` has a reference to, the value of `person` will also be modified, since they both have a reference to the same object. `person`'s `name` property is now equal to the value `"Lydia"` 52. What's the output? ```javascriptfunction greeting() { throw 'Hello world!';} function sayHi() { try { const data = greeting(); console.log('It worked!', data); } catch (e) { console.log('Oh no an error:', e); }} sayHi();``` A: It worked! Hello world! B: Oh no an error: undefined C: SyntaxError: can only throw Error objects D: Oh no an error: Hello world! Answer #### Answer: DWith the `throw` statement, we can create custom errors. With this statement, you can throw exceptions. An exception can be a string , a number , a boolean or an object . In this case, our exception is the string `'Hello world!'`.With the `catch` statement, we can specify what to do if an exception is thrown in the `try` block. An exception is thrown: the string `'Hello world!'`. `e` is now equal to that string, which we log. This results in `'Oh an error: Hello world!'`. 53. What's the output? ```javascriptfunction Car() { this.make = 'Lamborghini'; return { make: 'Maserati' };} const myCar = new Car();console.log(myCar.make);``` A: "Lamborghini" B: "Maserati" C: ReferenceError D: TypeError Answer #### Answer: BWhen you return a property, the value of the property is equal to the _returned_ value, not the value set in the constructor function. We return the string `"Maserati"`, so `myCar.make` is equal to `"Maserati"`. 54. What's the output? ```javascript(() => { let x = (y = 10);})(); console.log(typeof x);console.log(typeof y);``` A: "undefined", "number" B: "number", "number" C: "object", "number" D: "number", "undefined" Answer #### Answer: A`let x = (y = 10);` is actually shorthand for:```javascripty = 10;let x = y;```When we set `y` equal to `10`, we actually add a property `y` to the global object (`window` in browser, `global` in Node). In a browser, `window.y` is now equal to `10`.Then, we declare a variable `x` with the value of `y`, which is `10`. Variables declared with the `let` keyword are _block scoped_, they are only defined within the block they're declared in; the immediately invoked function expression (IIFE) in this case. When we use the `typeof` operator, the operand `x` is not defined: we are trying to access `x` outside of the block it's declared in. This means that `x` is not defined. Values who haven't been assigned a value or declared are of type `"undefined"`. `console.log(typeof x)` returns `"undefined"`.However, we created a global variable `y` when setting `y` equal to `10`. This value is accessible anywhere in our code. `y` is defined, and holds a value of type `"number"`. `console.log(typeof y)` returns `"number"`. 55. What's the output? ```javascriptclass Dog { constructor(name) { this.name = name; }} Dog.prototype.bark = function() { console.log( Woof I am ${this.name} );}; const pet = new Dog('Mara'); pet.bark(); delete Dog.prototype.bark; pet.bark();``` A: "Woof I am Mara" , TypeError B: "Woof I am Mara" , "Woof I am Mara" C: "Woof I am Mara" , undefined D: TypeError , TypeError Answer #### Answer: AWe can delete properties from objects using the `delete` keyword, also on the prototype. By deleting a property on the prototype, it is not available anymore in the prototype chain. In this case, the `bark` function is not available anymore on the prototype after `delete Dog.prototype.bark`, yet we still try to access it.When we try to invoke something that is not a function, a `TypeError` is thrown. In this case `TypeError: pet.bark is not a function`, since `pet.bark` is `undefined`. 56. What's the output? ```javascriptconst set = new Set([1, 1, 2, 3, 4]); console.log(set);``` A: [1, 1, 2, 3, 4] B: [1, 2, 3, 4] C: {1, 1, 2, 3, 4} D: {1, 2, 3, 4} Answer #### Answer: DThe `Set` object is a collection of _unique_ values: a value can only occur once in a set.We passed the iterable `[1, 1, 2, 3, 4]` with a duplicate value `1`. Since we cannot have two of the same values in a set, one of them is removed. This results in `{1, 2, 3, 4}`. 57. What's the output? javascript// counter.jslet counter = 10;export default counter; ```javascript// index.jsimport myCounter from './counter'; myCounter += 1; console.log(myCounter);``` A: 10 B: 11 C: Error D: NaN Answer #### Answer: CAn imported module is _read-only_: you cannot modify the imported module. Only the module that exports them can change its value.When we try to increment the value of `myCounter`, it throws an error: `myCounter` is read-only and cannot be modified. 58. What's the output? ```javascriptconst name = 'Lydia';age = 21; console.log(delete name);console.log(delete age);``` A: false , true B: "Lydia" , 21 C: true , true D: undefined , undefined Answer #### Answer: AThe `delete` operator returns a boolean value: `true` on a successful deletion, else it'll return `false`. However, variables declared with the `var`, `const` or `let` keyword cannot be deleted using the `delete` operator.The `name` variable was declared with a `const` keyword, so its deletion is not successful: `false` is returned. When we set `age` equal to `21`, we actually added a property called `age` to the global object. You can successfully delete properties from objects this way, also the global object, so `delete age` returns `true`. 59. What's the output? ```javascriptconst numbers = [1, 2, 3, 4, 5];const [y] = numbers; console.log(y);``` A: [[1, 2, 3, 4, 5]] B: [1, 2, 3, 4, 5] C: 1 D: [1] Answer #### Answer: CWe can unpack values from arrays or properties from objects through destructuring. For example:```javascript[a, b] = [1, 2];``` The value of `a` is now `1`, and the value of `b` is now `2`. What we actually did in the question, is:```javascript[y] = [1, 2, 3, 4, 5];``` This means that the value of `y` is equal to the first value in the array, which is the number `1`. When we log `y`, `1` is returned. 60. What's the output? ```javascriptconst user = { name: 'Lydia', age: 21 };const admin = { admin: true, ...user }; console.log(admin);``` A: { admin: true, user: { name: "Lydia", age: 21 } } B: { admin: true, name: "Lydia", age: 21 } C: { admin: true, user: ["Lydia", 21] } D: { admin: true } Answer #### Answer: BIt's possible to combine objects using the spread operator `...`. It lets you create copies of the key/value pairs of one object, and add them to another object. In this case, we create copies of the `user` object, and add them to the `admin` object. The `admin` object now contains the copied key/value pairs, which results in `{ admin: true, name: "Lydia", age: 21 }`. 61. What's the output? ```javascriptconst person = { name: 'Lydia' }; Object.defineProperty(person, 'age', { value: 21 }); console.log(person);console.log(Object.keys(person));``` A: { name: "Lydia", age: 21 } , ["name", "age"] B: { name: "Lydia", age: 21 } , ["name"] C: { name: "Lydia"} , ["name", "age"] D: { name: "Lydia"} , ["age"] Answer #### Answer: BWith the `defineProperty` method, we can add new properties to an object, or modify existing ones. When we add a property to an object using the `defineProperty` method, they are by default _not enumerable_. The `Object.keys` method returns all _enumerable_ property names from an object, in this case only `"name"`.Properties added using the `defineProperty` method are immutable by default. You can override this behavior using the `writable`, `configurable` and `enumerable` properties. This way, the `defineProperty` method gives you a lot more control over the properties you're adding to an object. 62. What's the output? ```javascriptconst settings = { username: 'lydiahallie', level: 19, health: 90,}; const data = JSON.stringify(settings, ['level', 'health']);console.log(data);``` A: "{"level":19, "health":90}" B: "{"username": "lydiahallie"}" C: "["level", "health"]" D: "{"username": "lydiahallie", "level":19, "health":90}" Answer #### Answer: AThe second argument of `JSON.stringify` is the _replacer_. The replacer can either be a function or an array, and lets you control what and how the values should be stringified.If the replacer is an _array_, only the property names included in the array will be added to the JSON string. In this case, only the properties with the names `"level"` and `"health"` are included, `"username"` is excluded. `data` is now equal to `"{"level":19, "health":90}"`.If the replacer is a _function_, this function gets called on every property in the object you're stringifying. The value returned from this function will be the value of the property when it's added to the JSON string. If the value is `undefined`, this property is excluded from the JSON string. 63. What's the output? ```javascriptlet num = 10; const increaseNumber = () => num++;const increasePassedNumber = number => number++; const num1 = increaseNumber();const num2 = increasePassedNumber(num1); console.log(num1);console.log(num2);``` A: 10 , 10 B: 10 , 11 C: 11 , 11 D: 11 , 12 Answer #### Answer: AThe unary operator `++` _first returns_ the value of the operand, _then increments_ the value of the operand. The value of `num1` is `10`, since the `increaseNumber` function first returns the value of `num`, which is `10`, and only increments the value of `num` afterwards.`num2` is `10`, since we passed `num1` to the `increasePassedNumber`. `number` is equal to `10`(the value of `num1`. Again, the unary operator `++` _first returns_ the value of the operand, _then increments_ the value of the operand. The value of `number` is `10`, so `num2` is equal to `10`. 64. What's the output? ```javascriptconst value = { number: 10 }; const multiply = (x = { ...value }) => { console.log((x.number *= 2));}; multiply();multiply();multiply(value);multiply(value);``` A: 20 , 40 , 80 , 160 B: 20 , 40 , 20 , 40 C: 20 , 20 , 20 , 40 D: NaN , NaN , 20 , 40 Answer #### Answer: CIn ES6, we can initialize parameters with a default value. The value of the parameter will be the default value, if no other value has been passed to the function, or if the value of the parameter is `"undefined"`. In this case, we spread the properties of the `value` object into a new object, so `x` has the default value of `{ number: 10 }`.The default argument is evaluated at _call time_! Every time we call the function, a _new_ object is created. We invoke the `multiply` function the first two times without passing a value: `x` has the default value of `{ number: 10 }`. We then log the multiplied value of that number, which is `20`.The third time we invoke multiply, we do pass an argument: the object called `value`. The `*=` operator is actually shorthand for `x.number = x.number * 2`: we modify the value of `x.number`, and log the multiplied value `20`.The fourth time, we pass the `value` object again. `x.number` was previously modified to `20`, so `x.number *= 2` logs `40`. 65. What's the output? javascript[1, 2, 3, 4].reduce((x, y) => console.log(x, y)); A: 1 2 and 3 3 and 6 4 B: 1 2 and 2 3 and 3 4 C: 1 undefined and 2 undefined and 3 undefined and 4 undefined D: 1 2 and undefined 3 and undefined 4 Answer #### Answer: DThe first argument that the `reduce` method receives is the _accumulator_, `x` in this case. The second argument is the _current value_, `y`. With the reduce method, we execute a callback function on every element in the array, which could ultimately result in one single value.In this example, we are not returning any values, we are simply logging the values of the accumulator and the current value.The value of the accumulator is equal to the previously returned value of the callback function. If you don't pass the optional `initialValue` argument to the `reduce` method, the accumulator is equal to the first element on the first call.On the first call, the accumulator (`x`) is `1`, and the current value (`y`) is `2`. We don't return from the callback function, we log the accumulator and current value: `1` and `2` get logged.If you don't return a value from a function, it returns `undefined`. On the next call, the accumulator is `undefined`, and the current value is `3`. `undefined` and `3` get logged.On the fourth call, we again don't return from the callback function. The accumulator is again `undefined`, and the current value is `4`. `undefined` and `4` get logged. 66. With which constructor can we successfully extend the Dog class? ```javascriptclass Dog { constructor(name) { this.name = name; }}; class Labrador extends Dog { // 1 constructor(name, size) { this.size = size; } // 2 constructor(name, size) { super(name); this.size = size; } // 3 constructor(size) { super(name); this.size = size; } // 4 constructor(name, size) { this.name = name; this.size = size; } };``` A: 1 B: 2 C: 3 D: 4 Answer #### Answer: BIn a derived class, you cannot access the `this` keyword before calling `super`. If you try to do that, it will throw a ReferenceError: 1 and 4 would throw a reference error.With the `super` keyword, we call that parent class's constructor with the given arguments. The parent's constructor receives the `name` argument, so we need to pass `name` to `super`.The `Labrador` class receives two arguments, `name` since it extends `Dog`, and `size` as an extra property on the `Labrador` class. They both need to be passed to the constructor function on `Labrador`, which is done correctly using constructor 2. 67. What's the output? ```javascript// index.jsconsole.log('running index.js');import { sum } from './sum.js';console.log(sum(1, 2)); // sum.jsconsole.log('running sum.js');export const sum = (a, b) => a + b;``` A: running index.js , running sum.js , 3 B: running sum.js , running index.js , 3 C: running sum.js , 3 , running index.js D: running index.js , undefined , running sum.js Answer #### Answer: BWith the `import` keyword, all imported modules are _pre-parsed_. This means that the imported modules get run _first_, the code in the file which imports the module gets executed _after_.This is a difference between `require()` in CommonJS and `import`! With `require()`, you can load dependencies on demand while the code is being run. If we would have used `require` instead of `import`, `running index.js`, `running sum.js`, `3` would have been logged to the console. 68. What's the output? javascriptconsole.log(Number(2) === Number(2));console.log(Boolean(false) === Boolean(false));console.log(Symbol('foo') === Symbol('foo')); A: true , true , false B: false , true , false C: true , false , true D: true , true , true Answer #### Answer: AEvery Symbol is entirely unique. The purpose of the argument passed to the Symbol is to give the Symbol a description. The value of the Symbol is not dependent on the passed argument. As we test equality, we are creating two entirely new symbols: the first `Symbol('foo')`, and the second `Symbol('foo')`. These two values are unique and not equal to each other, `Symbol('foo') === Symbol('foo')` returns `false`. 69. What's the output? javascriptconst name = 'Lydia Hallie';console.log(name.padStart(13));console.log(name.padStart(2)); A: "Lydia Hallie" , "Lydia Hallie" B: " Lydia Hallie" , " Lydia Hallie" ( "[13x whitespace]Lydia Hallie" , "[2x whitespace]Lydia Hallie" ) C: " Lydia Hallie" , "Lydia Hallie" ( "[1x whitespace]Lydia Hallie" , "Lydia Hallie" ) D: "Lydia Hallie" , "Lyd" , Answer #### Answer: CWith the `padStart` method, we can add padding to the beginning of a string. The value passed to this method is the _total_ length of the string together with the padding. The string `"Lydia Hallie"` has a length of `12`. `name.padStart(13)` inserts 1 space at the start of the string, because 12 + 1 is 13.If the argument passed to the `padStart` method is smaller than the length of the array, no padding will be added. 70. What's the output? javascriptconsole.log('🥑' + '💻'); A: "🥑💻" B: 257548 C: A string containing their code points D: Error Answer #### Answer: AWith the `+` operator, you can concatenate strings. In this case, we are concatenating the string `"🥑"` with the string `"💻"`, resulting in `"🥑💻"`. 71. How can we log the values that are commented out after the console.log statement? ```javascriptfunction* startGame() { const answer = yield 'Do you love JavaScript?'; if (answer !== 'Yes') { return "Oh wow... Guess we're gone here"; } return 'JavaScript loves you back ❤️';} const game = startGame();console.log(/* 1 /); // Do you love JavaScript?console.log(/ 2 */); // JavaScript loves you back ❤️``` A: game.next("Yes").value and game.next().value B: game.next.value("Yes") and game.next.value() C: game.next().value and game.next("Yes").value D: game.next.value() and game.next.value("Yes") Answer #### Answer: CA generator function "pauses" its execution when it sees the `yield` keyword. First, we have to let the function yield the string "Do you love JavaScript?", which can be done by calling `game.next().value`.Every line is executed, until it finds the first `yield` keyword. There is a `yield` keyword on the first line within the function: the execution stops with the first yield! _This means that the variable `answer` is not defined yet!_When we call `game.next("Yes").value`, the previous `yield` is replaced with the value of the parameters passed to the `next()` function, `"Yes"` in this case. The value of the variable `answer` is now equal to `"Yes"`. The condition of the if-statement returns `false`, and `JavaScript loves you back ❤️` gets logged. 72. What's the output? javascriptconsole.log(String.raw`Hello\nworld`); A: Hello world! B: Hello       world C: Hello\nworld D: Hello\n       world Answer #### Answer: C`String.raw` returns a string where the escapes (`\n`, `\v`, `\t` etc.) are ignored! Backslashes can be an issue since you could end up with something like:`` const path = `C:\Documents\Projects\table.html` ``Which would result in:`"C:DocumentsProjects able.html"`With `String.raw`, it would simply ignore the escape and print:`C:\Documents\Projects\table.html`In this case, the string is `Hello\nworld`, which gets logged. 73. What's the output? ```javascriptasync function getData() { return await Promise.resolve('I made it!');} const data = getData();console.log(data);``` A: "I made it!" B: Promise {: "I made it!"} C: Promise {} D: undefined Answer #### Answer: CAn async function always returns a promise. The `await` still has to wait for the promise to resolve: a pending promise gets returned when we call `getData()` in order to set `data` equal to it.If we wanted to get access to the resolved value `"I made it"`, we could have used the `.then()` method on `data`:`data.then(res => console.log(res))`This would've logged `"I made it!"` 74. What's the output? ```javascriptfunction addToList(item, list) { return list.push(item);} const result = addToList('apple', ['banana']);console.log(result);``` A: ['apple', 'banana'] B: 2 C: true D: undefined Answer #### Answer: BThe `.push()` method returns the _length_ of the new array! Previously, the array contained one element (the string `"banana"`) and had a length of `1`. After adding the string `"apple"` to the array, the array contains two elements, and has a length of `2`. This gets returned from the `addToList` function.The `push` method modifies the original array. If you wanted to return the _array_ from the function rather than the _length of the array_, you should have returned `list` after pushing `item` to it. 75. What's the output? ```javascriptconst box = { x: 10, y: 20 }; Object.freeze(box); const shape = box;shape.x = 100; console.log(shape);``` A: { x: 100, y: 20 } B: { x: 10, y: 20 } C: { x: 100 } D: ReferenceError Answer #### Answer: B`Object.freeze` makes it impossible to add, remove, or modify properties of an object (unless the property's value is another object).When we create the variable `shape` and set it equal to the frozen object `box`, `shape` also refers to a frozen object. You can check whether an object is frozen by using `Object.isFrozen`. In this case, `Object.isFrozen(shape)` returns true, since the variable `shape` has a reference to a frozen object.Since `shape` is frozen, and since the value of `x` is not an object, we cannot modify the property `x`. `x` is still equal to `10`, and `{ x: 10, y: 20 }` gets logged. 76. What's the output? ```javascriptconst { name: myName } = { name: 'Lydia' }; console.log(name);``` A: "Lydia" B: "myName" C: undefined D: ReferenceError Answer #### Answer: DWhen we unpack the property `name` from the object on the right-hand side, we assign its value `"Lydia"` to a variable with the name `myName`.With `{ name: myName }`, we tell JavaScript that we want to create a new variable called `myName` with the value of the `name` property on the right-hand side.Since we try to log `name`, a variable that is not defined, a ReferenceError gets thrown. 77. Is this a pure function? javascriptfunction sum(a, b) { return a + b;} A: Yes B: No Answer #### Answer: AA pure function is a function that _always_ returns the same result, if the same arguments are passed.The `sum` function always returns the same result. If we pass `1` and `2`, it will _always_ return `3` without side effects. If we pass `5` and `10`, it will _always_ return `15`, and so on. This is the definition of a pure function. 78. What is the output? `` javascriptconst add = () => { const cache = {}; return num => { if (num in cache) { return From cache! ${cache[num]} ; } else { const result = num + 10; cache[num] = result; return Calculated! ${result}`; } };}; const addFunction = add();console.log(addFunction(10));console.log(addFunction(10));console.log(addFunction(5 * 2));``` A: Calculated! 20 Calculated! 20 Calculated! 20 B: Calculated! 20 From cache! 20 Calculated! 20 C: Calculated! 20 From cache! 20 From cache! 20 D: Calculated! 20 From cache! 20 Error Answer #### Answer: CThe `add` function is a _memoized_ function. With memoization, we can cache the results of a function in order to speed up its execution. In this case, we create a `cache` object that stores the previously returned values.If we call the `addFunction` function again with the same argument, it first checks whether it has already gotten that value in its cache. If that's the case, the caches value will be returned, which saves on execution time. Else, if it's not cached, it will calculate the value and store it afterwards.We call the `addFunction` function three times with the same value: on the first invocation, the value of the function when `num` is equal to `10` isn't cached yet. The condition of the if-statement `num in cache` returns `false`, and the else block gets executed: `Calculated! 20` gets logged, and the value of the result gets added to the cache object. `cache` now looks like `{ 10: 20 }`.The second time, the `cache` object contains the value that gets returned for `10`. The condition of the if-statement `num in cache` returns `true`, and `'From cache! 20'` gets logged.The third time, we pass `5 * 2` to the function which gets evaluated to `10`. The `cache` object contains the value that gets returned for `10`. The condition of the if-statement `num in cache` returns `true`, and `'From cache! 20'` gets logged. 79. What is the output? ```javascriptconst myLifeSummedUp = ['☕', '💻', '🍷', '🍫']; for (let item in myLifeSummedUp) { console.log(item);} for (let item of myLifeSummedUp) { console.log(item);}``` A: 0 1 2 3 and "☕" "💻" "🍷" "🍫" B: "☕" "💻" "🍷" "🍫" and "☕" "💻" "🍷" "🍫" C: "☕" "💻" "🍷" "🍫" and 0 1 2 3 D: 0 1 2 3 and {0: "☕", 1: "💻", 2: "🍷", 3: "🍫"} Answer #### Answer: AWith a _for-in_ loop, we can iterate over **enumerable** properties. In an array, the enumerable properties are the "keys" of array elements, which are actually their indexes. You could see an array as:`{0: "☕", 1: "💻", 2: "🍷", 3: "🍫"}`Where the keys are the enumerable properties. `0` `1` `2` `3` get logged.With a _for-of_ loop, we can iterate over **iterables**. An array is an iterable. When we iterate over the array, the variable "item" is equal to the element it's currently iterating over, `"☕"` `"💻"` `"🍷"` `"🍫"` get logged. 80. What is the output? javascriptconst list = [1 + 2, 1 * 2, 1 / 2];console.log(list); A: ["1 + 2", "1 * 2", "1 / 2"] B: ["12", 2, 0.5] C: [3, 2, 0.5] D: [1, 1, 1] Answer #### Answer: CArray elements can hold any value. Numbers, strings, objects, other arrays, null, boolean values, undefined, and other expressions such as dates, functions, and calculations.The element will be equal to the returned value. `1 + 2` returns `3`, `1 * 2` returns `2`, and `1 / 2` returns `0.5`. 81. What is the output? `` javascriptfunction sayHi(name) { return Hi there, ${name}`;} console.log(sayHi());``` A: Hi there, B: Hi there, undefined C: Hi there, null D: ReferenceError Answer #### Answer: BBy default, arguments have the value of `undefined`, unless a value has been passed to the function. In this case, we didn't pass a value for the `name` argument. `name` is equal to `undefined` which gets logged.In ES6, we can overwrite this default `undefined` value with default parameters. For example:`function sayHi(name = "Lydia") { ... }`In this case, if we didn't pass a value or if we passed `undefined`, `name` would always be equal to the string `Lydia` 82. What is the output? ```javascriptvar status = '😎'; setTimeout(() => { const status = '😍'; const data = { status: '🥑', getStatus() { return this.status; }, }; console.log(data.getStatus()); console.log(data.getStatus.call(this));}, 0);``` A: "🥑" and "😍" B: "🥑" and "😎" C: "😍" and "😎" D: "😎" and "😎" Answer #### Answer: BThe value of the `this` keyword is dependent on where you use it. In a **method**, like the `getStatus` method, the `this` keyword refers to _the object that the method belongs to_. The method belongs to the `data` object, so `this` refers to the `data` object. When we log `this.status`, the `status` property on the `data` object gets logged, which is `"🥑"`.With the `call` method, we can change the object to which the `this` keyword refers. In **functions**, the `this` keyword refers to the _the object that the function belongs to_. We declared the `setTimeout` function on the _global object_, so within the `setTimeout` function, the `this` keyword refers to the _global object_. On the global object, there is a variable called _status_ with the value of `"😎"`. When logging `this.status`, `"😎"` gets logged. 83. What is the output? ```javascriptconst person = { name: 'Lydia', age: 21,}; let city = person.city;city = 'Amsterdam'; console.log(person);``` A: { name: "Lydia", age: 21 } B: { name: "Lydia", age: 21, city: "Amsterdam" } C: { name: "Lydia", age: 21, city: undefined } D: "Amsterdam" Answer #### Answer: AWe set the variable `city` equal to the value of the property called `city` on the `person` object. There is no property on this object called `city`, so the variable `city` has the value of `undefined`.Note that we are _not_ referencing the `person` object itself! We simply set the variable `city` equal to the current value of the `city` property on the `person` object.Then, we set `city` equal to the string `"Amsterdam"`. This doesn't change the person object: there is no reference to that object.When logging the `person` object, the unmodified object gets returned. 84. What is the output? ```javascriptfunction checkAge(age) { if (age < 18) { const message = "Sorry, you're too young."; } else { const message = "Yay! You're old enough!"; } return message;} console.log(checkAge(21));``` A: "Sorry, you're too young." B: "Yay! You're old enough!" C: ReferenceError D: undefined Answer #### Answer: CVariables with the `const` and `let` keyword are _block-scoped_. A block is anything between curly brackets (`{ }`). In this case, the curly brackets of the if/else statements. You cannot reference a variable outside of the block it's declared in, a ReferenceError gets thrown. 85. What kind of information would get logged? javascriptfetch('https://www.website.com/api/user/1') .then(res => res.json()) .then(res => console.log(res)); A: The result of the fetch method. B: The result of the second invocation of the fetch method. C: The result of the callback in the previous .then() . D: It would always be undefined. Answer #### Answer: CThe value of `res` in the second `.then` is equal to the returned value of the previous `.then`. You can keep chaining `.then`s like this, where the value is passed to the next handler. 86. Which option is a way to set hasName equal to true , provided you cannot pass true as an argument? javascriptfunction getName(name) { const hasName = //} A: !!name B: name C: new Boolean(name) D: name.length Answer #### Answer: AWith `!!name`, we determine whether the value of `name` is truthy or falsy. If name is truthy, which we want to test for, `!name` returns `false`. `!false` (which is what `!!name` practically is) returns `true`.By setting `hasName` equal to `name`, you set `hasName` equal to whatever value you passed to the `getName` function, not the boolean value `true`.`new Boolean(true)` returns an object wrapper, not the boolean value itself.`name.length` returns the length of the passed argument, not whether it's `true`. 87. What's the output? javascriptconsole.log('I want pizza'[0]); A: """ B: "I" C: SyntaxError D: undefined Answer #### Answer: BIn order to get an character on a specific index in a string, you can use bracket notation. The first character in the string has index 0, and so on. In this case we want to get the element which index is 0, the character `"I'`, which gets logged.Note that this method is not supported in IE7 and below. In that case, use `.charAt()` 88. What's the output? ```javascriptfunction sum(num1, num2 = num1) { console.log(num1 + num2);} sum(10);``` A: NaN B: 20 C: ReferenceError D: undefined Answer #### Answer: BYou can set a default parameter's value equal to another parameter of the function, as long as they've been defined _before_ the default parameter. We pass the value `10` to the `sum` function. If the `sum` function only receives 1 argument, it means that the value for `num2` is not passed, and the value of `num1` is equal to the passed value `10` in this case. The default value of `num2` is the value of `num1`, which is `10`. `num1 + num2` returns `20`.If you're trying to set a default parameter's value equal to a parameter which is defined _after_ (to the right), the parameter's value hasn't been initialized yet, which will throw an error. 89. What's the output? ```javascript// module.jsexport default () => 'Hello world';export const name = 'Lydia'; // index.jsimport * as data from './module'; console.log(data);``` A: { default: function default(), name: "Lydia" } B: { default: function default() } C: { default: "Hello world", name: "Lydia" } D: Global object of module.js Answer #### Answer: AWith the `import * as name` syntax, we import _all exports_ from the `module.js` file into the `index.js` file as a new object called `data` is created. In the `module.js` file, there are two exports: the default export, and a named export. The default export is a function which returns the string `"Hello World"`, and the named export is a variable called `name` which has the value of the string `"Lydia"`.The `data` object has a `default` property for the default export, other properties have the names of the named exports and their corresponding values. 90. What's the output? ```javascriptclass Person { constructor(name) { this.name = name; }} const member = new Person('John');console.log(typeof member);``` A: "class" B: "function" C: "object" D: "string" Answer #### Answer: CClasses are syntactical sugar for function constructors. The equivalent of the `Person` class as a function constructor would be:```javascriptfunction Person() { this.name = name;}```Calling a function constructor with `new` results in the creation of an instance of `Person`, `typeof` keyword returns `"object"` for an instance. `typeof member` returns `"object"`. 91. What's the output? ```javascriptlet newList = [1, 2, 3].push(4); console.log(newList.push(5));``` A: [1, 2, 3, 4, 5] B: [1, 2, 3, 5] C: [1, 2, 3, 4] D: Error Answer #### Answer: DThe `.push` method returns the _new length_ of the array, not the array itself! By setting `newList` equal to `[1, 2, 3].push(4)`, we set `newList` equal to the new length of the array: `4`.Then, we try to use the `.push` method on `newList`. Since `newList` is the numerical value `4`, we cannot use the `.push` method: a TypeError is thrown. 92. What's the output? ```javascriptfunction giveLydiaPizza() { return 'Here is pizza!';} const giveLydiaChocolate = () => "Here's chocolate... now go hit the gym already."; console.log(giveLydiaPizza.prototype);console.log(giveLydiaChocolate.prototype);``` A: { constructor: ...} { constructor: ...} B: {} { constructor: ...} C: { constructor: ...} {} D: { constructor: ...} undefined Answer #### Answer: DRegular functions, such as the `giveLydiaPizza` function, have a `prototype` property, which is an object (prototype object) with a `constructor` property. Arrow functions however, such as the `giveLydiaChocolate` function, do not have this `prototype` property. `undefined` gets returned when trying to access the `prototype` property using `giveLydiaChocolate.prototype`. 93. What's the output? ```javascriptconst person = { name: 'Lydia', age: 21,}; for (const [x, y] of Object.entries(person)) { console.log(x, y);}``` A: name Lydia and age 21 B: ["name", "Lydia"] and ["age", 21] C: ["name", "age"] and undefined D: Error Answer #### Answer: A`Object.entries(person)` returns an array of nested arrays, containing the keys and objects:`[ [ 'name', 'Lydia' ], [ 'age', 21 ] ]`Using the `for-of` loop, we can iterate over each element in the array, the subarrays in this case. We can destructure the subarrays instantly in the for-of loop, using `const [x, y]`. `x` is equal to the first element in the subarray, `y` is equal to the second element in the subarray.The first subarray is `[ "name", "Lydia" ]`, with `x` equal to `"name"`, and `y` equal to `"Lydia"`, which get logged.The second subarray is `[ "age", 21 ]`, with `x` equal to `"age"`, and `y` equal to `21`, which get logged. 94. What's the output? ```javascriptfunction getItems(fruitList, ...args, favoriteFruit) { return [...fruitList, ...args, favoriteFruit]} getItems(["banana", "apple"], "pear", "orange")``` A: ["banana", "apple", "pear", "orange"] B: [["banana", "apple"], "pear", "orange"] C: ["banana", "apple", ["pear"], "orange"] D: SyntaxError Answer #### Answer: D`...args` is a rest parameter. The rest parameter's value is an array containing all remaining arguments, **and can only be the last parameter**! In this example, the rest parameter was the second parameter. This is not possible, and will throw a syntax error.```javascriptfunction getItems(fruitList, favoriteFruit, ...args) { return [...fruitList, ...args, favoriteFruit];}getItems(['banana', 'apple'], 'pear', 'orange');```The above example works. This returns the array `[ 'banana', 'apple', 'orange', 'pear' ]` 95. What's the output? ```javascriptfunction nums(a, b) { if (a > b) console.log('a is bigger'); else console.log('b is bigger'); return a + b;} console.log(nums(4, 2));console.log(nums(1, 2));``` A: a is bigger , 6 and b is bigger , 3 B: a is bigger , undefined and b is bigger , undefined C: undefined and undefined D: SyntaxError Answer #### Answer: BIn JavaScript, we don't _have_ to write the semicolon (`;`) explicitly, however the JavaScript engine still adds them after statements. This is called **Automatic Semicolon Insertion**. A statement can for example be variables, or keywords like `throw`, `return`, `break`, etc.Here, we wrote a `return` statement, and another value `a + b` on a _new line_. However, since it's a new line, the engine doesn't know that it's actually the value that we wanted to return. Instead, it automatically added a semicolon after `return`. You could see this as:```javascriptreturn;a + b;```This means that `a + b` is never reached, since a function stops running after the `return` keyword. If no value gets returned, like here, the function returns `undefined`. Note that there is no automatic insertion after `if/else` statements! 96. What's the output? ```javascriptclass Person { constructor() { this.name = 'Lydia'; }} Person = class AnotherPerson { constructor() { this.name = 'Sarah'; }}; const member = new Person();console.log(member.name);``` A: "Lydia" B: "Sarah" C: Error: cannot redeclare Person D: SyntaxError Answer #### Answer: BWe can set classes equal to other classes/function constructors. In this case, we set `Person` equal to `AnotherPerson`. The name on this constructor is `Sarah`, so the name property on the new `Person` instance `member` is `"Sarah"`. 97. What's the output? ```javascriptconst info = {}; console.log(info);console.log(Object.keys(info));``` A: {Symbol('a'): 'b'} and ["{Symbol('a')"] B: {} and [] C: { a: "b" } and ["a"] D: {Symbol('a'): 'b'} and [] Answer #### Answer: DA Symbol is not _enumerable_. The Object.keys method returns all _enumerable_ key properties on an object. The Symbol won't be visible, and an empty array is returned. When logging the entire object, all properties will be visible, even non-enumerable ones.This is one of the many qualities of a symbol: besides representing an entirely unique value (which prevents accidental name collision on objects, for example when working with 2 libraries that want to add properties to the same object), you can also "hide" properties on objects this way (although not entirely. You can still access symbols using the `Object.getOwnPropertySymbols()` method). 98. What's the output? ```javascriptconst getList = ([x, ...y]) => [x, y]const getUser = user => { name: user.name, age: user.age } const list = [1, 2, 3, 4]const user = { name: "Lydia", age: 21 } console.log(getList(list))console.log(getUser(user))``` A: [1, [2, 3, 4]] and undefined B: [1, [2, 3, 4]] and { name: "Lydia", age: 21 } C: [1, 2, 3, 4] and { name: "Lydia", age: 21 } D: Error and { name: "Lydia", age: 21 } Answer #### Answer: AThe `getList` function receives an array as its argument. Between the parentheses of the `getList` function, we destructure this array right away. You could see this as:`[x, ...y] = [1, 2, 3, 4]`With the rest parameter `...y`, we put all "remaining" arguments in an array. The remaining arguments are `2`, `3` and `4` in this case. The value of `y` is an array, containing all the rest parameters. The value of `x` is equal to `1` in this case, so when we log `[x, y]`, `[1, [2, 3, 4]]` gets logged.The `getUser` function receives an object. With arrow functions, we don't _have_ to write curly brackets if we just return one value. However, if you want to return an _object_ from an arrow function, you have to write it between parentheses, otherwise no value gets returned! The following function would have returned an object:`const getUser = user => ({ name: user.name, age: user.age })`Since no value gets returned in this case, the function returns `undefined`. 99. What's the output? ```javascriptconst name = 'Lydia'; console.log(name());``` A: SyntaxError B: ReferenceError C: TypeError D: undefined Answer #### Answer: CThe variable `name` holds the value of a string, which is not a function, thus cannot invoke.TypeErrors get thrown when a value is not of the expected type. JavaScript expected `name` to be a function since we're trying to invoke it. It was a string however, so a TypeError gets thrown: name is not a function!SyntaxErrors get thrown when you've written something that isn't valid JavaScript, for example when you've written the word `return` as `retrun`.ReferenceErrors get thrown when JavaScript isn't able to find a reference to a value that you're trying to access. 100. What's the value of output? ```javascript// 🎉✨ This is my 100th question! ✨🎉 const output = ${[] && 'Im'}possible!You should${'' && n't } see a therapist after so much JavaScript lol ;``` A: possible! You should see a therapist after so much JavaScript lol B: Impossible! You should see a therapist after so much JavaScript lol C: possible! You shouldn't see a therapist after so much JavaScript lol D: Impossible! You shouldn't see a therapist after so much JavaScript lol Answer #### Answer: B`[]` is a truthy value. With the `&&` operator, the right-hand value will be returned if the left-hand value is a truthy value. In this case, the left-hand value `[]` is a truthy value, so `"Im'` gets returned.`""` is a falsy value. If the left-hand value is falsy, nothing gets returned. `n't` doesn't get returned. 101. What's the value of output? ```javascriptconst one = false || {} || null;const two = null || false || '';const three = [] || 0 || true; console.log(one, two, three);``` A: false null [] B: null "" true C: {} "" [] D: null null true Answer #### Answer: CWith the `||` operator, we can return the first truthy operand. If all values are falsy, the last operand gets returned.`(false || {} || null)`: the empty object `{}` is a truthy value. This is the first (and only) truthy value, which gets returned. `one` is equal to `{}`.`(null || false || "")`: all operands are falsy values. This means that the last operand, `""` gets returned. `two` is equal to `""`.`([] || 0 || "")`: the empty array`[]` is a truthy value. This is the first truthy value, which gets returned. `three` is equal to `[]`. 102. What's the value of output? ```javascriptconst myPromise = () => Promise.resolve('I have resolved!'); function firstFunction() { myPromise().then(res => console.log(res)); console.log('second');} async function secondFunction() { console.log(await myPromise()); console.log('second');} firstFunction();secondFunction();``` A: I have resolved! , second and I have resolved! , second B: second , I have resolved! and second , I have resolved! C: I have resolved! , second and second , I have resolved! D: second , I have resolved! and I have resolved! , second Answer #### Answer: DWith a promise, we basically say _I want to execute this function, but I'll put it aside for now while it's running since this might take a while. Only when a certain value is resolved (or rejected), and when the call stack is empty, I want to use this value._We can get this value with both `.then` and the `await` keyword in an `async` function. Although we can get a promise's value with both `.then` and `await`, they work a bit differently.In the `firstFunction`, we (sort of) put the myPromise function aside while it was running, but continued running the other code, which is `console.log('second')` in this case. Then, the function resolved with the string `I have resolved`, which then got logged after it saw that the callstack was empty.With the await keyword in `secondFunction`, we literally pause the execution of an async function until the value has been resolved before moving to the next line.This means that it waited for the `myPromise` to resolve with the value `I have resolved`, and only once that happened, we moved to the next line: `second` got logged. 103. What's the value of output? ```javascriptconst set = new Set(); set.add(1);set.add('Lydia');set.add({ name: 'Lydia' }); for (let item of set) { console.log(item + 2);}``` A: 3 , NaN , NaN B: 3 , 7 , NaN C: 3 , Lydia2 , [object Object]2 D: "12" , Lydia2 , [object Object]2 Answer #### Answer: CThe `+` operator is not only used for adding numerical values, but we can also use it to concatenate strings. Whenever the JavaScript engine sees that one or more values are not a number, it coerces the number into a string.The first one is `1`, which is a numerical value. `1 + 2` returns the number 3.However, the second one is a string `"Lydia"`. `"Lydia"` is a string and `2` is a number: `2` gets coerced into a string. `"Lydia"` and `"2"` get concatenated, which results in the string `"Lydia2"`.`{ name: "Lydia" }` is an object. Neither a number nor an object is a string, so it stringifies both. Whenever we stringify a regular object, it becomes `"[object Object]"`. `"[object Object]"` concatenated with `"2"` becomes `"[object Object]2"`. 104. What's its value? javascriptPromise.resolve(5); A: 5 B: Promise {: 5} C: Promise {: 5} D: Error Answer #### Answer: CWe can pass any type of value we want to `Promise.resolve`, either a promise or a non-promise. The method itself returns a promise with the resolved value (``). If you pass a regular function, it'll be a resolved promise with a regular value. If you pass a promise, it'll be a resolved promise with the resolved value of that passed promise.In this case, we just passed the numerical value `5`. It returns a resolved promise with the value `5`. 105. What's its value? ```javascriptfunction compareMembers(person1, person2 = person) { if (person1 !== person2) { console.log('Not the same!'); } else { console.log('They are the same!'); }} const person = { name: 'Lydia' }; compareMembers(person);``` A: Not the same! B: They are the same! C: ReferenceError D: SyntaxError Answer #### Answer: BObjects are passed by reference. When we check objects for strict equality (`===`), we're comparing their references.We set the default value for `person2` equal to the `person` object, and passed the `person` object as the value for `person1`.This means that both values have a reference to the same spot in memory, thus they are equal.The code block in the `else` statement gets run, and `They are the same!` gets logged. 106. What's its value? ```javascriptconst colorConfig = { red: true, blue: false, green: true, black: true, yellow: false,}; const colors = ['pink', 'red', 'blue']; console.log(colorConfig.colors[1]);``` A: true B: false C: undefined D: TypeError Answer #### Answer: DIn JavaScript, we have two ways to access properties on an object: bracket notation, or dot notation. In this example, we use dot notation (`colorConfig.colors`) instead of bracket notation (`colorConfig["colors"]`).With dot notation, JavaScript tries to find the property on the object with that exact name. In this example, JavaScript tries to find a property called `colors` on the `colorConfig` object. There is no property called `colors`, so this returns `undefined`. Then, we try to access the value of the first element by using `[1]`. We cannot do this on a value that's `undefined`, so it throws a `TypeError`: `Cannot read property '1' of undefined`.JavaScript interprets (or unboxes) statements. When we use bracket notation, it sees the first opening bracket `[` and keeps going until it finds the closing bracket `]`. Only then, it will evaluate the statement. If we would've used `colorConfig[colors[1]]`, it would have returned the value of the `red` property on the `colorConfig` object. 107. What's its value? javascriptconsole.log('❤️' === '❤️'); A: true B: false Answer #### Answer: AUnder the hood, emojis are unicodes. The unicodes for the heart emoji is `"U+2764 U+FE0F"`. These are always the same for the same emojis, so we're comparing two equal strings to each other, which returns true. 108. Which of these methods modifies the original array? ```javascriptconst emojis = ['✨', '🥑', '😍']; emojis.map(x => x + '✨');emojis.filter(x => x !== '🥑');emojis.find(x => x !== '🥑');emojis.reduce((acc, cur) => acc + '✨');emojis.slice(1, 2, '✨');emojis.splice(1, 2, '✨');``` A: All of them B: map reduce slice splice C: map slice splice D: splice Answer #### Answer: DWith `splice` method, we modify the original array by deleting, replacing or adding elements. In this case, we removed 2 items from index 1 (we removed `'🥑'` and `'😍'`) and added the ✨ emoji instead.`map`, `filter` and `slice` return a new array, `find` returns an element, and `reduce` returns a reduced value. 109. What's the output? ```javascriptconst food = ['🍕', '🍫', '🥑', '🍔'];const info = { favoriteFood: food[0] }; info.favoriteFood = '🍝'; console.log(food);``` A: ['🍕', '🍫', '🥑', '🍔'] B: ['🍝', '🍫', '🥑', '🍔'] C: ['🍝', '🍕', '🍫', '🥑', '🍔'] D: ReferenceError Answer #### Answer: AWe set the value of the `favoriteFood` property on the `info` object equal to the string with the pizza emoji, `'🍕'`. A string is a primitive data type. In JavaScript, primitive data types don't interact by reference.In JavaScript, primitive data types (everything that's not an object) interact by _value_. In this case, we set the value of the `favoriteFood` property on the `info` object equal to the value of the first element in the `food` array, the string with the pizza emoji in this case (`'🍕'`). A string is a primitive data type, and interact by value (see my [blogpost](https://www.theavocoder.com/complete-javascript/2018/12/21/by-value-vs-by-reference) if you're interested in learning more)Then, we change the value of the `favoriteFood` property on the `info` object. The `food` array hasn't changed, since the value of `favoriteFood` was merely a _copy_ of the value of the first element in the array, and doesn't have a reference to the same spot in memory as the element on `food[0]`. When we log food, it's still the original array, `['🍕', '🍫', '🥑', '🍔']`. 110. What does this method do? javascriptJSON.parse(); A: Parses JSON to a JavaScript value B: Parses a JavaScript object to JSON C: Parses any JavaScript value to JSON D: Parses JSON to a JavaScript object only Answer #### Answer: AWith the `JSON.parse()` method, we can parse JSON string to a JavaScript value.```javascript// Stringifying a number into valid JSON, then parsing the JSON string to a JavaScript value:const jsonNumber = JSON.stringify(4); // '4'JSON.parse(jsonNumber); // 4// Stringifying an array value into valid JSON, then parsing the JSON string to a JavaScript value:const jsonArray = JSON.stringify([1, 2, 3]); // '[1, 2, 3]'JSON.parse(jsonArray); // [1, 2, 3]// Stringifying an object into valid JSON, then parsing the JSON string to a JavaScript value:const jsonArray = JSON.stringify({ name: 'Lydia' }); // '{"name":"Lydia"}'JSON.parse(jsonArray); // { name: 'Lydia' }``` 111. What's the output? ```javascriptlet name = 'Lydia'; function getName() { console.log(name); let name = 'Sarah';} getName();``` A: Lydia B: Sarah C: undefined D: ReferenceError Answer #### Answer: DEach function has its own _execution context_ (or _scope_). The `getName` function first looks within its own context (scope) to see if it contains the variable `name` we're trying to access. In this case, the `getName` function contains its own `name` variable: we declare the variable `name` with the `let` keyword, and with the value of `'Sarah'`.Variables with the `let` keyword (and `const`) are hoisted, but unlike `var`, don't get initialized . They are not accessible before the line we declare (initialize) them. This is called the "temporal dead zone". When we try to access the variables before they are declared, JavaScript throws a `ReferenceError`.If we wouldn't have declared the `name` variable within the `getName` function, the javascript engine would've looked down the _scope chain_. The outer scope has a variable called `name` with the value of `Lydia`. In that case, it would've logged `Lydia`.```javascriptlet name = 'Lydia';function getName() { console.log(name);}getName(); // Lydia``` 112. What's the output? ```javascriptfunction* generatorOne() { yield ['a', 'b', 'c'];} function* generatorTwo() { yield* ['a', 'b', 'c'];} const one = generatorOne();const two = generatorTwo(); console.log(one.next().value);console.log(two.next().value);``` A: a and a B: a and undefined C: ['a', 'b', 'c'] and a D: a and ['a', 'b', 'c'] Answer #### Answer: CWith the `yield` keyword, we `yield` values in a generator function. With the `yield*` keyword, we can yield values from another generator function, or iterable object (for example an array).In `generatorOne`, we yield the entire array `['a', 'b', 'c']` using the `yield` keyword. The value of `value` property on the object returned by the `next` method on `one` (`one.next().value`) is equal to the entire array `['a', 'b', 'c']`.```javascriptconsole.log(one.next().value); // ['a', 'b', 'c']console.log(one.next().value); // undefined```In `generatorTwo`, we use the `yield*` keyword. This means that the first yielded value of `two`, is equal to the first yielded value in the iterator. The iterator is the array `['a', 'b', 'c']`. The first yielded value is `a`, so the first time we call `two.next().value`, `a` is returned.```javascriptconsole.log(two.next().value); // 'a'console.log(two.next().value); // 'b'console.log(two.next().value); // 'c'console.log(two.next().value); // undefined``` 113. What's the output? javascriptconsole.log(`${(x => x)('I love')} to program`); A: I love to program B: undefined to program C: ${(x => x)('I love') to program D: TypeError Answer #### Answer: AExpressions within template literals are evaluated first. This means that the string will contain the returned value of the expression, the immediately invoked function `(x => x)('I love')` in this case. We pass the value `'I love'` as an argument to the `x => x` arrow function. `x` is equal to `'I love'`, which gets returned. This results in `I love to program`. 114. What will happen? ```javascriptlet config = { alert: setInterval(() => { console.log('Alert!'); }, 1000),}; config = null;``` A: The setInterval callback won't be invoked B: The setInterval callback gets invoked once C: The setInterval callback will still be called every second D: We never invoked config.alert() , config is null Answer #### Answer: CNormally when we set objects equal to `null`, those objects get _garbage collected_ as there is no reference anymore to that object. However, since the callback function within `setInterval` is an arrow function (thus bound to the `config` object), the callback function still holds a reference to the `config` object. As long as there is a reference, the object won't get garbage collected. Since it's not garbage collected, the `setInterval` callback function will still get invoked every 1000ms (1s). 115. Which method(s) will return the value 'Hello world!' ? ```javascriptconst myMap = new Map();const myFunc = () => 'greeting'; myMap.set(myFunc, 'Hello world!'); //1myMap.get('greeting');//2myMap.get(myFunc);//3myMap.get(() => 'greeting');``` A: 1 B: 2 C: 2 and 3 D: All of them Answer #### Answer: BWhen adding a key/value pair using the `set` method, the key will be the value of the first argument passed to the `set` function, and the value will be the second argument passed to the `set` function. The key is the _function_ `() => 'greeting'` in this case, and the value `'Hello world'`. `myMap` is now `{ () => 'greeting' => 'Hello world!' }`.1 is wrong, since the key is not `'greeting'` but `() => 'greeting'`.3 is wrong, since we're creating a new function by passing it as a parameter to the `get` method. Object interact by _reference_. Functions are objects, which is why two functions are never strictly equal, even if they are identical: they have a reference to a different spot in memory. 116. What's the output? ```javascriptconst person = { name: 'Lydia', age: 21,}; const changeAge = (x = { ...person }) => (x.age += 1);const changeAgeAndName = (x = { ...person }) => { x.age += 1; x.name = 'Sarah';}; changeAge(person);changeAgeAndName(); console.log(person);``` A: {name: "Sarah", age: 22} B: {name: "Sarah", age: 23} C: {name: "Lydia", age: 22} D: {name: "Lydia", age: 23} Answer #### Answer: CBoth the `changeAge` and `changeAgeAndName` functions have a default parameter, namely a _newly_ created object `{ ...person }`. This object has copies of all the key/values in the `person` object.First, we invoke the `changeAge` function and pass the `person` object as its argument. This function increases the value of the `age` property by 1. `person` is now `{ name: "Lydia", age: 22 }`.Then, we invoke the `changeAgeAndName` function, however we don't pass a parameter. Instead, the value of `x` is equal to a _new_ object: `{ ...person }`. Since it's a new object, it doesn't affect the values of the properties on the `person` object. `person` is still equal to `{ name: "Lydia", age: 22 }`. 117. Which of the following options will return 6 ? javascriptfunction sumValues(x, y, z) { return x + y + z;} A: sumValues([...1, 2, 3]) B: sumValues([...[1, 2, 3]]) C: sumValues(...[1, 2, 3]) D: sumValues([1, 2, 3]) Answer #### Answer: CWith the spread operator `...`, we can _spread_ iterables to individual elements. The `sumValues` function receives three arguments: `x`, `y` and `z`. `...[1, 2, 3]` will result in `1, 2, 3`, which we pass to the `sumValues` function. 118. What's the output? ```javascriptlet num = 1;const list = ['🥳', '🤠', '🥰', '🤪']; console.log(list[(num += 1)]);``` A: 🤠 B: 🥰 C: SyntaxError D: ReferenceError Answer #### Answer: BWith the `+=` operand, we're incrementing the value of `num` by `1`. `num` had the initial value `1`, so `1 + 1` is `2`. The item on the second index in the `list` array is 🥰, `console.log(list[2])` prints 🥰. 119. What's the output? `` javascriptconst person = { firstName: 'Lydia', lastName: 'Hallie', pet: { name: 'Mara', breed: 'Dutch Tulip Hound', }, getFullName() { return ${this.firstName} ${this.lastName}`; },}; console.log(person.pet?.name);console.log(person.pet?.family?.name);console.log(person.getFullName?.());console.log(member.getLastName?.());``` A: undefined undefined undefined undefined B: Mara undefined Lydia Hallie ReferenceError C: Mara null Lydia Hallie null D: null ReferenceError null ReferenceError Answer #### Answer: BWith the optional chaining operator `?.`, we no longer have to explicitly check whether the deeper nested values are valid or not. If we're trying to access a property on an `undefined` or `null` value (_nullish_), the expression short-circuits and returns `undefined`.`person.pet?.name`: `person` has a property named `pet`: `person.pet` is not nullish. It has a property called `name`, and returns `Mara`.`person.pet?.family?.name`: `person` has a property named `pet`: `person.pet` is not nullish. `pet` does _not_ have a property called `family`, `person.pet.family` is nullish. The expression returns `undefined`.`person.getFullName?.()`: `person` has a property named `getFullName`: `person.getFullName()` is not nullish and can get invoked, which returns `Lydia Hallie`.`member.getLastName?.()`: `member` is not defined: `member.getLastName()` is nullish. The expression returns `undefined`. 120. What's the output? ```javascriptconst groceries = ['banana', 'apple', 'peanuts']; if (groceries.indexOf('banana')) { console.log('We have to buy bananas!');} else { console.log( We don't have to buy bananas! );}``` A: We have to buy bananas! B: We don't have to buy bananas C: undefined D: 1 Answer #### Answer: BWe passed the condition `groceries.indexOf("banana")` to the if-statement. `groceries.indexOf("banana")` returns `0`, which is a falsy value. Since the condition in the if-statement is falsy, the code in the `else` block runs, and `We don't have to buy bananas!` gets logged. 121. What's the output? ```javascriptconst config = { languages: [], set language(lang) { return this.languages.push(lang); },}; console.log(config.language);``` A: function language(lang) { this.languages.push(lang } B: 0 C: [] D: undefined Answer #### Answer: DThe `language` method is a `setter`. Setters don't hold an actual value, their purpose is to _modify_ properties. When calling a `setter` method, `undefined` gets returned. 122. What's the output? ```javascriptconst name = 'Lydia Hallie'; console.log(!typeof name === 'object');console.log(!typeof name === 'string');``` A: false true B: true false C: false false D: true true Answer #### Answer: C`typeof name` returns `"string"`. The string `"string"` is a truthy value, so `!typeof name` returns the boolean value `false`. `false === "object"` and `false === "string"` both return`false`.(If we wanted to check whether the type was (un)equal to a certain type, we should've written `!==` instead of `!typeof`) 123. What's the output? ```javascriptconst add = x => y => z => { console.log(x, y, z); return x + y + z;}; add(4)(5)(6);``` A: 4 5 6 B: 6 5 4 C: 4 function function D: undefined undefined 6 Answer #### Answer: AThe `add` function returns an arrow function, which returns an arrow function, which returns an arrow function (still with me?). The first function receives an argument `x` with the value of `4`. We invoke the second function, which receives an argument `y` with the value `5`. Then we invoke the third function, which receives an argument `z` with the value `6`. When we're trying to access the value `x`, `y` and `z` within the last arrow function, the JS engine goes up the scope chain in order to find the values for `x` and `y` accordingly. This returns `4` `5` `6`. 124. What's the output? ```javascriptasync function* range(start, end) { for (let i = start; i { const gen = range(1, 3); for await (const item of gen) { console.log(item); }})();``` A: Promise {1} Promise {2} Promise {3} B: Promise {} Promise {} Promise {} C: 1 2 3 D: undefined undefined undefined Answer #### Answer: CThe generator function `range` returns an async object with promises for each item in the range we pass: `Promise{1}`, `Promise{2}`, `Promise{3}`. We set the variable `gen` equal to the async object, after which we loop over it using a `for await ... of` loop. We set the variable `item` equal to the returned Promise values: first `Promise{1}`, then `Promise{2}`, then `Promise{3}`. Since we're _awaiting_ the value of `item`, the resolved promsie, the resolved _values_ of the promises get returned: `1`, `2`, then `3`. 125. What's the output? ```javascriptconst myFunc = ({ x, y, z }) => { console.log(x, y, z);}; myFunc(1, 2, 3);``` A: 1 2 3 B: {1: 1} {2: 2} {3: 3} C: { 1: undefined } undefined undefined D: undefined undefined undefined Answer #### Answer: D`myFunc` expects an object with properties `x`, `y` and `z` as its argument. Since we're only passing three separate numeric values (1, 2, 3) instead of one object with properties `x`, `y` and `z` ({x: 1, y: 2, z: 3}), `x`, `y` and `z` have their default value of `undefined`. 126. What's the output? ```javascriptfunction getFine(speed, amount) { const formattedSpeed = new Intl.NumberFormat('en-US', { style: 'unit', unit: 'mile-per-hour' }).format(speed); const formattedAmount = new Intl.NumberFormat('en-US', { style: 'currency', currency: 'USD' }).format(amount); return The driver drove ${formattedSpeed} and has to pay ${formattedAmount} ;} console.log(getFine(130, 300))``` A: The driver drove 130 and has to pay 300 B: The driver drove 130 mph and has to pay \$300.00 C: The driver drove undefined and has to pay undefined D: The driver drove 130.00 and has to pay 300.00 Answer #### Answer: BWith the `Intl.NumberFormat` method, we can format numeric values to any locale. We format the numeric value `130` to the `en-US` locale as a `unit` in `mile-per-hour`, which results in `130 mph`. The numeric value `300` to the `en-US` locale as a `currentcy` in `USD` results in `$300.00`. 127. What's the output? ```javascriptconst spookyItems = ['👻', '🎃', '🕸'];({ item: spookyItems[3] } = { item: '💀' }); console.log(spookyItems);``` A: ["👻", "🎃", "🕸"] B: ["👻", "🎃", "🕸", "💀"] C: ["👻", "🎃", "🕸", { item: "💀" }] D: ["👻", "🎃", "🕸", "[object Object]"] Answer #### Answer: BBy destructuring objects, we can unpack values from the right-hand object, and assign the unpacked value to the value of the same property name on the left-hand object. In this case, we're assigning the value "💀" to `spookyItems[3]`. This means that we're modifying the `spookyItems` array, we're adding the "💀" to it. When logging `spookyItems`, `["👻", "🎃", "🕸", "💀"]` gets logged. 128. What's the output? ```javascriptconst name = 'Lydia Hallie';const age = 21; console.log(Number.isNaN(name));console.log(Number.isNaN(age)); console.log(isNaN(name));console.log(isNaN(age));``` A: true false true false B: true false false false C: false false true false D: false true false true Answer #### Answer: CWith the `Number.isNaN` method, you can check if the value you pass is a _numeric value_ and equal to `NaN`. `name` is not a numeric value, so `Number.isNaN(name)` returns `false`. `age` is a numeric value, but is not equal to `NaN`, so `Number.isNaN(age)` returns `false`.With the `isNaN` method, you can check if the value you pass is not a number. `name` is not a number, so `isNaN(name)` returns true. `age` is a number, so `isNaN(age)` returns `false`. 129. What's the output? ```javascriptconst randomValue = 21; function getInfo() { console.log(typeof randomValue); const randomValue = 'Lydia Hallie';} getInfo();``` A: "number" B: "string" C: undefined D: ReferenceError Answer #### Answer: DVariables declared with the `const` keyword are not referencable before their initialization: this is called the _temporal dead zone_. In the `getInfo` function, the variable `randomValue` is scoped in the functional scope of `getInfo`. On the line where we want to log the value of `typeof randomValue`, the variable `randomValue` isn't initialized yet: a `ReferenceError` gets thrown! The engine didn't go down the scope chain since we declared the variable `randomValue` in the `getInfo` function. 130. What's the output? ```javascriptconst myPromise = Promise.resolve('Woah some cool data'); (async () => { try { console.log(await myPromise); } catch { throw new Error( Oops didn't work ); } finally { console.log('Oh finally!'); }})();``` A: Woah some cool data B: Oh finally! C: Woah some cool data Oh finally! D: Oops didn't work Oh finally! Answer #### Answer: CIn the `try` block, we're logging the awaited value of the `myPromise` variable: `"Woah some cool data"`. Since no errors were thrown in the `try` block, the code in the `catch` block doesn't run. The code in the `finally` block _always_ runs, `"Oh finally!"` gets logged. 131. What's the output? ```javascriptconst emojis = ['🥑', ['✨', '✨', ['🍕', '🍕']]]; console.log(emojis.flat(1));``` A: ['🥑', ['✨', '✨', ['🍕', '🍕']]] B: ['🥑', '✨', '✨', ['🍕', '🍕']] C: ['🥑', ['✨', '✨', '🍕', '🍕']] D: ['🥑', '✨', '✨', '🍕', '🍕'] Answer #### Answer: BWith the `flat` method, we can create a new, flattened array. The depth of the flattened array depends on the value that we pass. In this case, we passed the value `1` (which we didn't have to, that's the default value), meaning that only the arrays on the first depth will be concatenated. `['🥑']` and `['✨', '✨', ['🍕', '🍕']]` in this case. Concatenating these two arrays results in `['🥑', '✨', '✨', ['🍕', '🍕']]`. 132. What's the output? ```javascriptclass Counter { constructor() { this.count = 0; } increment() { this.count++; }} const counterOne = new Counter();counterOne.increment();counterOne.increment(); const counterTwo = counterOne;counterTwo.increment(); console.log(counterOne.count);``` A: 0 B: 1 C: 2 D: 3 Answer #### Answer: D`counterOne` is an instance of the `Counter` class. The counter class contains a `count` property on its constructor, and an `increment` method. First, we invoked the `increment` method twice by calling `counterOne.increment()`. Currently, `counterOne.count` is `2`. Then, we create a new variable `counterTwo`, and set it equal to `counterOne`. Since objects interact by reference, we're just creating a new reference to the same spot in memory that `counterOne` points to. Since it has the same spot in memory, any changes made to the object that `counterTwo` has a reference to, also apply to `counterOne`. Currently, `counterTwo.count` is `2`.We invoke the `counterTwo.increment()`, which sets the `count` to `3`. Then, we log the count on `counterOne`, which logs `3`. 133. What's the output? ```javascriptconst myPromise = Promise.resolve(Promise.resolve('Promise!')); function funcOne() { myPromise.then(res => res).then(res => console.log(res)); setTimeout(() => console.log('Timeout!', 0)); console.log('Last line!');} async function funcTwo() { const res = await myPromise; console.log(await res); setTimeout(() => console.log('Timeout!', 0)); console.log('Last line!');} funcOne();funcTwo();``` A: Promise! Last line! Promise! Last line! Last line! Promise! B: Last line! Timeout! Promise! Last line! Timeout! Promise! C: Promise! Last line! Last line! Promise! Timeout! Timeout! D: Last line! Promise! Promise! Last line! Timeout! Timeout! Answer #### Answer: DFirst, we invoke `funcOne`. On the first line of `funcOne`, we call the `myPromise` promise, which is an _asynchronous_ operation. While the engine is busy completing the promise, it keeps on running the function `funcOne`. The next line is the _asynchronous_ `setTimeout` function, from which the callback is sent to the Web API. (see my article on the event loop here.)Both the promise and the timeout are asynchronous operations, the function keeps on running while it's busy completing the promise and handling the `setTimeout` callback. This means that `Last line!` gets logged first, since this is not an asynchonous operation. This is the last line of `funcOne`, the promise resolved, and `Promise!` gets logged. However, since we're invoking `funcTwo()`, the call stack isn't empty, and the callback of the `setTimeout` function cannot get added to the callstack yet.In `funcTwo` we're, first _awaiting_ the myPromise promise. With the `await` keyword, we pause the execution of the function until the promise has resolved (or rejected). Then, we log the awaited value of `res` (since the promise itself returns a promise). This logs `Promise!`.The next line is the _asynchronous_ `setTimeout` function, from which the callback is sent to the Web API.We get to the last line of `funcTwo`, which logs `Last line!` to the console. Now, since `funcTwo` popped off the call stack, the call stack is empty. The callbacks waiting in the queue (`() => console.log("Timeout!")` from `funcOne`, and `() => console.log("Timeout!")` from `funcTwo`) get added to the call stack one by one. The first callback logs `Timeout!`, and gets popped off the stack. Then, the second callback logs `Timeout!`, and gets popped off the stack. This logs `Last line! Promise! Promise! Last line! Timeout! Timeout!` 134. How can we invoke sum in index.js from sum.js? ```javascript// sum.jsexport default function sum(x) { return x + x;} // index.jsimport * as sum from './sum';``` A: sum(4) B: sum.sum(4) C: sum.default(4) D: Default aren't imported with * , only named exports Answer #### Answer: CWith the asterisk `*`, we import all exported values from that file, both default and named. If we had the following file:```javascript// info.jsexport const name = 'Lydia';export const age = 21;export default 'I love JavaScript';// index.jsimport * as info from './info';console.log(info);```The following would get logged:```javascript{ default: "I love JavaScript", name: "Lydia", age: 21}```For the `sum` example, it means that the imported value `sum` looks like this:```javascript{ default: function sum(x) { return x + x } }```We can invoke this function, by calling `sum.default` 135. What's the output? ```javascriptconst handler = { set: () => console.log('Added a new property!'), get: () => console.log('Accessed a property!'),}; const person = new Proxy({}, handler); person.name = 'Lydia';person.name;``` A: Added a new property! B: Accessed a property! C: Added a new property! Accessed a property! D: Nothing gets logged Answer #### Answer: CWith a Proxy object, we can add custom behavior to an object that we pass to it as the second argument. In this case, we pass the `handler` object which contained to properties: `set` and `get`. `set` gets invoked whenever we _set_ property values, `get` gets invoked whenever we _get_ (access) property values.The first argument is an empty object `{}`, which is the value of `person`. To this object, the custom behavior specified in the `handler` object gets added. If we add a property to the `person` object, `set` will get invoked. If we access a property on the `person` object, `get` gets invoked.First, we added a new property `name` to the proxy object (`person.name = "Lydia"`). `set` gets invoked, and logs `"Added a new property!"`.Then, we access a property value on the proxy object, the `get` property on the handler object got invoked. `"Accessed a property!"` gets logged. 136. Which of the following will modify the person object? ```javascriptconst person = { name: 'Lydia Hallie' }; Object.seal(person);``` A: person.name = "Evan Bacon" B: person.age = 21 C: delete person.name D: Object.assign(person, { age: 21 }) Answer #### Answer: AWith `Object.seal` we can prevent new properies from being _added_, or existing properties to be _removed_.However, you can still modify the value of existing properties. 137. Which of the following will modify the person object? ```javascriptconst person = { name: 'Lydia Hallie', address: { street: '100 Main St', },}; Object.freeze(person);``` A: person.name = "Evan Bacon" B: delete person.address C: person.address.street = "101 Main St" D: person.pet = { name: "Mara" } Answer #### Answer: CThe `Object.freeze` method _freezes_ an object. No properties can be added, modified, or removed.However, it only _shallowly_ freezes the object, meaning that only _direct_ properties on the object are frozen. If the property is another object, like `address` in this case, the properties on that object aren't frozen, and can be modified. 138. What's the output? ```javascriptconst add = x => x + x; function myFunc(num = 2, value = add(num)) { console.log(num, value);} myFunc();myFunc(3);``` A: 2 4 and 3 6 B: 2 NaN and 3 NaN C: 2 Error and 3 6 D: 2 4 and 3 Error Answer #### Answer: AFirst, we invoked `myFunc()` without passing any arguments. Since we didn't pass arguments, `num` and `value` got their default values: num is `2`, and `value` the returned value of the function `add`. To the `add` function, we pass `num` as an argument, which had the value of `2`. `add` returns `4`, which is the value of `value`.Then, we invoked `myFunc(3)` and passed the value `3` as the value for the argument `num`. We didn't pass an argument for `value`. Since we didn't pass a value for the `value` argument, it got the default value: the returned value of the `add` function. To `add`, we pass `num`, which has the value of `3`. `add` returns `6`, which is the value of `value`. 139. What's the output? ```javascriptclass Counter { #number = 10 increment() { this.#number++ } getNum() { return this.#number }} const counter = new Counter()counter.increment() console.log(counter.#number)``` A: 10 B: 11 C: undefined D: SyntaxError Answer #### Answer: DIn ES2020, we can add private variables in classes by using the `#`. We cannot access these variables outside of the class. When we try to log `counter.#number`, a SyntaxError gets thrown: we cannot acccess it outside the `Counter` class! 140. What's missing? ```javascriptconst teams = [ { name: 'Team 1', members: ['Paul', 'Lisa'] }, { name: 'Team 2', members: ['Laura', 'Tim'] },]; function* getMembers(members) { for (let i = 0; i < members.length; i++) { yield members[i]; }} function* getTeams(teams) { for (let i = 0; i < teams.length; i++) { // ✨ SOMETHING IS MISSING HERE ✨ }} const obj = getTeams(teams);obj.next(); // { value: "Paul", done: false }obj.next(); // { value: "Lisa", done: false }``` A: yield getMembers(teams[i].members) B: yield* getMembers(teams[i].members) C: return getMembers(teams[i].members) D: return yield getMembers(teams[i].members) Answer #### Answer: BIn order to iterate over the `members` in each element in the `teams` array, we need to pass `teams[i].members` to the `getMembers` generator function. The generator function returns a generator object. In order to iterate over each element in this generator object, we need to use `yield*`.If we would've written `yield`, `return yield`, or `return`, the entire generator function would've gotten returned the first time we called the `next` method. 141. What's the output? ```javascriptconst person = { name: 'Lydia Hallie', hobbies: ['coding'],}; function addHobby(hobby, hobbies = person.hobbies) { hobbies.push(hobby); return hobbies;} addHobby('running', []);addHobby('dancing');addHobby('baking', person.hobbies); console.log(person.hobbies);``` A: ["coding"] B: ["coding", "dancing"] C: ["coding", "dancing", "baking"] D: ["coding", "running", "dancing", "baking"] Answer #### Answer: CThe `addHobby` function receives two arguments, `hobby` and `hobbies` with the default value of the `hobbies` array on the `person` object.First, we invoke the `addHobby` function, and pass `"running"` as the value for `hobby` and an empty array as the value for `hobbies`. Since we pass an empty array as the value for `y`, `"running"` gets added to this empty array.Then, we invoke the `addHobby` function, and pass `"dancing"` as the value for `hobby`. We didn't pass a value for `hobbies`, so it gets the default value, the `hobbies` property on the `person` object. We push the hobby `dancing` to the `person.hobbies` array.Last, we invoke the `addHobby` function, and pass `"bdaking"` as the value for `hobby`, and the `person.hobbies` array as the value for `hobbies`. We push the hobby `baking` to the `person.hobbies` array.After pushing `dancing` and `baking`, the value of `person.hobbies` is `["coding", "dancing", "baking"]` 142. What's the output? ```javascriptclass Bird { constructor() { console.log("I'm a bird. 🦢"); }} class Flamingo extends Bird { constructor() { console.log("I'm pink. 🌸"); super(); }} const pet = new Flamingo();``` A: I'm pink. 🌸 B: I'm pink. 🌸 I'm a bird. 🦢 C: I'm a bird. 🦢 I'm pink. 🌸 D: Nothing, we didn't call any method Answer #### Answer: BWe create the variable `pet` which is an instance of the `Flamingo` class. When we instantiate this instance, the `constructor` on `Flamingo` gets called. First, `"I'm pink. 🌸"` gets logged, after which we call `super()`. `super()` calls the constructor of the parent class, `Bird`. The constructor in `Bird` gets called, and logs `"I'm a bird. 🦢"`. 143. Which of the options result(s) in an error? ```javascriptconst emojis = ['🎄', '🎅🏼', '🎁', '⭐']; /* 1 / emojis.push('🦌');/ 2 / emojis.splice(0, 2);/ 3 / emojis = [...emojis, '🥂'];/ 4 */ emojis.length = 0;``` A: 1 B: 1 and 2 C: 3 and 4 D: 3 Answer #### Answer: DThe `const` keyword simply means we cannot _redeclare_ the value of that variable, it's _read-only_. However, the value itself isn't immutable. The properties on the `emojis` array can be modified, for example by pushing new values, splicing them, or setting the length of the array to 0. 144. What do we need to add to the person object to get ["Lydia Hallie", 21] as the output of [...person] ? ```javascriptconst person = { name: "Lydia Hallie", age: 21} [...person] // ["Lydia Hallie", 21]``` A: Nothing, object are iterable by default B: *[Symbol.iterator]() { for (let x in this) yield* this[x] } C: *[Symbol.iterator]() { yield* Object.values(this) } D: *[Symbol.iterator]() { for (let x in this) yield this } Answer #### Answer: CObjects aren't iterable by default. An iterable is an iterable if the iterator protocol is present. We can add this manually by adding the iterator symbol `[Symbol.iterator]`, which has to return a generator object, for example by making it a generator function `*[Symbol.iterator]() {}`. This generator function has to yield the `Object.values` of the `person` object if we want it to return the array `["Lydia Hallie", 21]`: `yield* Object.values(this)`. 145. What's the output? ```javascriptlet count = 0;const nums = [0, 1, 2, 3]; nums.forEach(num => { if (num) count += 1}) console.log(count)``` A: 1 B: 2 C: 3 D: 4 Answer #### Answer: CThe `if` condition within the `forEach` loop checks whether the value of `num` is truthy or falsy. Since the first number in the `nums` array is `0`, a falsy value, the `if` statement's code block won't be executed. `count` only gets incremented for the other 3 numbers in the `nums` array, `1`, `2` and `3`. Since `count` gets incremented by `1` 3 times, the value of `count` is `3`. 146. What's the output? ```javascriptfunction getFruit(fruits) { console.log(fruits?.[1]?.[1])} getFruit([['🍊', '🍌'], ['🍍']])getFruit()getFruit([['🍍'], ['🍊', '🍌']])``` A: null , undefined , 🍌 B: [] , null , 🍌 C: [] , [] , 🍌 D: undefined , undefined , 🍌 Answer #### Answer: DThe `?` allows us to optionally access deeper nested properties within objects. We're trying to log the item on index `1` within the subarray that's on index `1` of the `fruits` array. If the subarray on index `1` in the `fruits` array doesn't exist, it'll simply return `undefined`. If the subarray on index `1` in the `fruits` array exists, but this subarray doesn't have an item on its `1` index, it'll also return `undefined`. First, we're trying to log the second item in the `['🍍']` subarray of `[['🍊', '🍌'], ['🍍']]`. This subarray only contains one item, which means there is no item on index `1`, and returns `undefined`.Then, we're invoking the `getFruits` function without passing a value as an argument, which means that `fruits` has a value of `undefined` by default. Since we're conditionally chaining the item on index `1` of`fruits`, it returns `undefined` since this item on index `1` does not exist. Lastly, we're trying to log the second item in the `['🍊', '🍌']` subarray of `['🍍'], ['🍊', '🍌']`. The item on index `1` within this subarray is `🍌`, which gets logged. 147. What's the output? ```javascriptclass Calc { constructor() { this.count = 0 } increase() { this.count ++} } const calc = new Calc()new Calc().increase() console.log(calc.count)``` A: 0 B: 1 C: undefined D: ReferenceError Answer #### Answer: AWe set the variable `calc` equal to a new instance of the `Calc` class. Then, we instantiate a new instance of `Calc`, and invoke the `increase` method on this instance. Since the count property is within the constructor of the `Calc` class, the count property is not shared on the prototype of `Calc`. This means that the value of count has not been updated for the instance calc points to, count is still `0`. 148. What's the output? ```javascriptconst user = { email: "[email protected]", password: "12345"} const updateUser = ({ email, password }) => { if (email) { Object.assign(user, { email }) } if (password) { user.password = password}return user } const updatedUser = updateUser({ email: "[email protected]" }) console.log(updatedUser === user)``` A: false B: true C: TypeError D: ReferenceError Answer #### Answer: BThe `updateUser` function updates the values of the `email` and `password` properties on user, if their values are passed to the function, after which the function returns the `user` object. The returned value of the `updateUser` function is the `user` object, which means that the value of updatedUser is a reference to the same `user` object that `user` points to. `updatedUser === user` equals `true`. 149. What's the output? ```javascriptconst fruit = ['🍌', '🍊', '🍎'] fruit.slice(0, 1)fruit.splice(0, 1)fruit.unshift('🍇') console.log(fruit)``` A: ['🍌', '🍊', '🍎'] B: ['🍊', '🍎'] C: ['🍇', '🍊', '🍎'] D: ['🍇', '🍌', '🍊', '🍎'] Answer #### Answer: CFirst, we invoke the `slice` method on the fruit array. The slice method does not modify the original array, but returns the value that it sliced off the array: the banana emoji.Then, we invoke the `splice` method on the fruit array. The splice method does modify the original array, which means that the fruit array now consists of `['🍊', '🍎']`.At last, we invoke the `unshift` method on the `fruit` array, which modifies the original array by adding the provided value, ‘🍇’ in this case, as the first element in the array. The fruit array now consists of `['🍇', '🍊', '🍎']`. 150. What's the output? ```javascriptconst animals = {};let dog = { emoji: '🐶' }let cat = { emoji: '🐈' } animals[dog] = { ...dog, name: "Mara" }animals[cat] = { ...cat, name: "Sara" } console.log(animals[dog])``` A: { emoji: "🐶", name: "Mara" } B: { emoji: "🐈", name: "Sara" } C: undefined D: ReferenceError Answer #### Answer: BObject keys are converted to strings. Since the value of `dog` is an object, `animals[dog]` actually means that we’re creating a new property called `"object Object"` equal to the new object. `animals["object Object"]` is now equal to `{ emoji: "🐶", name: "Mara"}`.`cat` is also an object, which means that `animals[cat]` actually means that we’re overwriting the value of `animals[``"``object Object``"``]` with the new cat properties. Logging `animals[dog]`, or actually `animals["object Object"]` since converting the `dog` object to a string results `"object Object"`, returns the `{ emoji: "🐈", name: "Sara" }`. 151. What's the output? ```javascriptconst user = { email: "[email protected]", updateEmail: email => { this.email = email }} user.updateEmail("[email protected]")console.log(user.email)``` A: [email protected] B: [email protected] C: undefined D: ReferenceError Answer #### Answer: AThe `updateEmail` function is an arrow function, and is not bound to the `user` object. This means that the `this` keyword is not referring to the `user` object, but refers to the global scope in this case. The value of `email` within the `user` object does not get updated. When logging the value of `user.email`, the original value of `[email protected]` gets returned. 152. What's the output? ```javascriptconst promise1 = Promise.resolve('First')const promise2 = Promise.resolve('Second')const promise3 = Promise.reject('Third')const promise4 = Promise.resolve('Fourth') const runPromises = async () => { const res1 = await Promise.all([promise1, promise2]) const res2 = await Promise.all([promise3, promise4]) return [res1, res2]} runPromises() .then(res => console.log(res)) .catch(err => console.log(err))``` A: [['First', 'Second'], ['Fourth']] B: [['First', 'Second'], ['Third', 'Fourth']] C: [['First', 'Second']] D: 'Third' Answer #### Answer: DThe `Promise.all` method runs the passed promises in parallel. If one promise fails, the `Promise.all` method _rejects_ with the value of the rejected promise. In this case, `promise3` rejected with the value `"Third"`. We’re catching the rejected value in the chained `catch` method on the `runPromises` invocation to catch any errors within the `runPromises` function. Only `"Third"` gets logged, since `promise3` rejected with this value. 153. What should the value of method be to log { name: "Lydia", age: 22 } ? ```javascriptconst keys = ["name", "age"]const values = ["Lydia", 22] const method = /* ?? */Object method // { name: "Lydia", age: 22 }``` A: entries B: values C: fromEntries D: forEach Answer #### Answer: CThe `fromEntries` method turns a 2d array into an object. The first element in each subarray will be the key, and the second element in each subarray will be the value. In this case, we’re mapping over the `keys` array, which returns an array which first element is the item on the key array on the current index, and the second element is the item of the values array on the current index. This creates an array of subarrays containing the correct keys and values, which results in `{ name: "Lydia", age: 22 }` 154. What's the output? ```javascriptconst createMember = ({ email, address = {}}) => { const validEmail = /.+\@.+..+/.test(email) if (!validEmail) throw new Error("Valid email pls") return { email, address: address ? address : null} } const member = createMember({ email: "[email protected]" })console.log(member)``` A: { email: "[email protected]", address: null } B: { email: "[email protected]" } C: { email: "[email protected]", address: {} } D: { email: "[email protected]", address: undefined } Answer #### Answer: CThe default value of `address` is an empty object `{}`. When we set the variable `member` equal to the object returned by the `createMember` function, we didn't pass a value for address, which means that the value of address is the default empty object `{}`. An empty object is a truthy value, which means that the condition of the `address ? address : null` conditional returns `true`. The value of address is the empty object `{}`. 155. What's the output? ```javascriptlet randomValue = { name: "Lydia" }randomValue = 23 if (!typeof randomValue === "string") { console.log("It's not a string!")} else { console.log("Yay it's a string!")}``` A: It's not a string! B: Yay it's a string! C: TypeError D: undefined Answer #### Answer: BThe condition within the `if` statement checks whether the value of `!typeof randomValue` is equal to `"string"`. The `!` operator converts the value to a boolean value. If the value is truthy, the returned value will be `false`, if the value is falsy, the returned value will be `true`. In this case, the returned value of `typeof randomValue` is the truthy value `"number"`, meaning that the value of `!typeof randomValue` is the boolean value `false`.`!typeof randomValue === "string"` always returns false, since we're actually checking `false === "string"`. Since the condition returned `false`, the code block of the `else` statement gets run, and `Yay it's a string!` gets logged. To restore the repository download the bundle wget https://archive.org/download/github.com-lydiahallie-javascript-questions_-_2020-10-02_18-59-54/lydiahallie-javascript-questions_-_2020-10-02_18-59-54.bundle and run: git clone lydiahallie-javascript-questions_-_2020-10-02_18-59-54.bundle Source: https://github.com/lydiahallie/javascript-questions Uploader: lydiahallie Upload date: 2020-10-02

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Tic Tac Toe (Noughts and Crosses) game in action.  Created using Scalar Vector Graphics (SVG) . Animation works for Google Chrome, Mozilla (Firefox and Seamonkey), and Opera browsers. The concept for using the path element is from http://stackoverflow.com/questions/3256943/define-a-circle-arc-animation-in-svg (Answer #2 by User:Erik Dahlström) and the links mentioned therein.  There are three (3) 7-zipped folders in this directory, one each for Tic-Tac-Toe games starting on Cells 1 (corner square), 2 (side square), and 5 (center square) (see Wikipedia Tic-tac-toe article for more info).  The file INFO-TicTacToe-svgXOp-c125.txt includes info regarding the items in this directory.  Please be aware that although each of the three 7-zipped files is less than 1MB in size, the unzipped files in the three folders total ~380.5MB  (380489916 Bytes) and can occupy a disk space > 600MB  (~604930048 Bytes) . Three sample svgs that are similar to those in the 7-zipped folders and that have been validated as SVG 1.1 at http://validator.w3.org/ are also included in this directory.  To re-validate, click on the corresponding link that follows: TicTacToe-152374698XOp.svg TicTacToe-51283794XOp.svg TicTacToe-6549127XOp.svg To download files, use the " All Files: HTTPS" link appearing in the " View the book " panel located on the left side of this page.