Preparing to become a Developer? This Top 70 JavaScript MCQs collection is your ultimate practice resource. It includes the most frequently asked and recently introduced JavaScript topics - ideal for interviews, coding assessments, and skill evaluation.
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1. What is the output of typeof null in JavaScript?
2. Which of the following is NOT a JavaScript data type?
3. Which of the following keywords is used to declare a constant variable in ES6?
4. What will be the output of console.log(2 + '2' - 2)?
5. Which method converts a JSON string into a JavaScript object?
6. What does a closure in JavaScript do?
7. What is the result of calling a function before its declaration when using a function declaration?
8. Which statement about arrow functions is TRUE?
9. Which variable scope is created when using let?
10. What will be the output of the following?let a = [1,2,3]; let b = a; b.push(4); console.log(a);
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11. What does Function.prototype.bind() do?
12. Which is the correct way to create a private variable using closures?
13. What is the effect of the delete operator on object properties?
14. Which statement is TRUE about function expressions vs declarations?
15. What happens when you return from a function without a value?
16. Which feature allows multi-line strings and interpolation with ${}?
17. What does the spread operator ... do when used with arrays?
18. Which of these is true about const with objects?
19. Which ES6 feature provides a concise way to define methods on object literals?
20. Which of the following was introduced for creating classes in ES6?
21. What does async/await help with?
22. Which function returns a Promise that resolves when the first promise resolves or rejects?
23. What part of the JavaScript runtime handles callbacks and timers outside the call stack?
24. Which queue processes microtasks like Promise.then callbacks?
25. What is the output order of this code?console.log('A'); setTimeout(()=>console.log('B'),0); Promise.resolve().then(()=>console.log('C')); console.log('D');
26. Which method selects the first matching element using a CSS selector?
27. Which property provides access to the element’s parent in the DOM?
28. Which event method prevents the default browser action?
29. Which API is best for making HTTP requests in modern browsers?
30. Which Web API allows you to store key-value pairs in the browser persistently?
31. Which statement imports the default export from a module?
32. Which bundler is commonly used for modern JavaScript apps?
33. What is tree-shaking in bundlers?
34. Which file typically defines project scripts and dependencies for Node.js projects?
35. Which command initializes a new npm project interactively?
36. Which global object in Node.js provides access to the file system module?
37. Which method reads a file asynchronously in Node.js using callbacks?
38. Which command installs a package and saves it to package.json dependencies?
39. Which statement describes Node.js?
40. Which pattern is commonly used in Node.js to handle asynchronous flows with improved readability?
41. Which practice helps prevent Cross-Site Scripting (XSS) attacks?
42. Which function is considered dangerous and can lead to security risks if used with untrusted input?
43. Which header helps mitigate clickjacking attacks when sent from server?
44. What is CORS (Cross-Origin Resource Sharing) primarily used for?
45. Which is a secure way to store secrets for client-side code?
46. Which technique helps reduce initial page load by loading JavaScript only when needed?
47. What is debouncing useful for in front-end development?
48. Which browser API helps do heavy work off the main thread?
49. Which property indicates layout shift for measuring visual stability (CLS)?
50. Which technique reduces network requests by combining files?
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51. Which browser tool can help you inspect network requests and responses?
52. Which source maps setting helps map minified code back to original source for debugging?
53. Which tool can help find security vulnerabilities in npm packages?
54. Which linting tool is widely used for JavaScript code quality checks?
55. Which testing framework is commonly used for unit tests in JavaScript?
56. What does optional chaining ?. do?
57. What is the Nullish Coalescing operator (??) used for?
58. Which feature helps with true-private fields in classes?
59. Which new-ish API enables background tasks and can be used for offline capabilities?
60. Which proposal aims to improve server-first rendering and streaming in modern web apps (commonly referenced in 2023-2025 ecosystem)?
61. What is prototypal inheritance in JavaScript?
62. Which is a common reason for memory leaks in JavaScript apps?
63. What is event delegation?
64. Which statement about JSON.stringify is TRUE?
65. Which of these is a correct way to shallow clone an object in modern JS?
66. Which operator checks both type and value equality?
67. Which built-in method returns the index of the first occurrence of a specified value in an array?
68. Which array method creates a new array with the results of calling a provided function on every element?
69. Which of the following is true about use strict?
70. What is the purpose of data attributes (attributes prefixed with data-) in HTML when used with JavaScript?
71. What will be logged?console.log([] == false);
72. Which method is used to freeze an object?
73. What is the purpose of WeakMap?
74. What does the optional catch binding allow?
75. Which hook is used in React for side effects?
76. What is hydration in SSR?
77. Which HTTP status indicates successful creation?
78. What is memoization used for?
79. What does BigInt allow?
80. What does Array.prototype.reduce() return?
81. Which method converts a string to an integer in JavaScript?
82. What will typeof NaN return?
83. Which array method modifies the original array?
84. What is the default value of an uninitialized variable declared with let?
85. Which keyword is used to handle errors?
86. Which symbol is used for rest parameters?
87. Which method checks if an array includes a value?
88. What does Object.keys() return?
89. Which statement creates a Promise?
90. Which lifecycle method runs after DOM content loads in vanilla JS?
91. Which operator spreads object properties?
92. Which method converts an object into JSON string?
93. Which method removes the last element from an array?
94. Which operator is used for exponentiation?
95. Which object represents the browser window?
96. Which tool transpiles modern JavaScript for older browsers?
97. Which function delays execution for specified milliseconds?
98. Which keyword stops loop execution?
99. Which operator is used to check if property exists in object?
100. Which API is used to observe DOM changes?
101. What is the primary purpose of WebSockets?
