Ace Your JavaScript Interview: Frequently Asked Questions and Answers

Prepare for your next JavaScript interview with this comprehensive guide to common questions and answers. This resource covers fundamental JavaScript concepts, modern features (ES6+), and best practices, helping you confidently demonstrate your JavaScript expertise.

Frequently Asked JavaScript Interview Questions

This section provides a collection of common JavaScript interview questions and answers, covering fundamental concepts, features, and best practices. It's designed to help both beginners and experienced JavaScript developers prepare for technical interviews.

JavaScript Interview Questions and Answers

1. What is JavaScript? JavaScript is a lightweight, interpreted programming language widely used for adding interactivity to web pages. It's object-based, runs in web browsers, and doesn't require compilation.
2. Key features of JavaScript? Lightweight, interpreted, network-centric, open-source, cross-platform, and complements both Java and HTML.
3. Who created JavaScript, and what was its original name? Brendan Eich (Netscape programmer) created it in 1995. Its original names were Mocha and LiveScript.
4. Advantages of JavaScript? Reduced server interaction, immediate feedback to users, high interactivity, richer user interfaces.
5. Disadvantages of JavaScript? No built-in support for multithreading or multiprocessing, limited file system access, and security concerns (client-side code can be viewed and manipulated by users).
6. Named functions in JavaScript? Functions with names declared using the `function` keyword (e.g., `function myFunction() { ... }`).
7. Types of JavaScript functions? Named and anonymous functions.
8. Anonymous functions? Functions without names, declared using function expressions (e.g., `let myFunc = function() { ... }`).
9. Assigning anonymous functions to variables? Yes, this is a common practice in JavaScript.
10. What is the `arguments` object? Provides access to the arguments passed to a function.
11. What is a closure? When an inner function has access to the variables of its surrounding scope(s), even after the outer function has finished executing.
12. Getting a character at a specific index? Use the `charAt()` method (e.g., `"hello".charAt(0)` returns "h").
13. JavaScript vs. JScript? JScript is Microsoft's version of JavaScript. They are essentially the same language but with some minor variations.
14. "Hello, world!" in JavaScript? console.log("Hello, world!"); or use `document.write()` or `document.getElementById().innerHTML`.
15. Key differences between Java and JavaScript? Java is a compiled, strongly-typed, object-oriented language primarily used for backend development and standalone applications. JavaScript is an interpreted, dynamically-typed, prototype-based scripting language mainly used for frontend web development and increasingly for backend (Node.js) and mobile applications. Key differences include typing, object models, execution environments, and concurrency models.
16. Using an external JavaScript file? Include a `<script>` tag in your HTML pointing to the .js file (e.g., `<script src="myScript.js"></script>`).
17. Is JavaScript case-sensitive? Yes.
18. What is BOM (Browser Object Model)? Provides ways to interact with browser windows (e.g., opening new windows, accessing browser history).
19. What is DOM (Document Object Model)? Represents the HTML document as a tree-like structure, allowing JavaScript to access and modify its content and structure dynamically.
20. What is the window object? The global object representing the browser window; it provides methods for interacting with the browser (e.g., `alert()`, `prompt()`, `confirm()`).
21. What is the history object? Provides methods to manipulate the browser's history (e.g., `back()`, `forward()`, `go()`).
22. What is JavaScript? JavaScript is a lightweight, interpreted programming language for creating interactive web pages. It's object-based and runs within web browsers.
23. Features of JavaScript? Lightweight, interpreted, well-suited for network-centric applications, complements Java and HTML, open-source, cross-platform.
24. Who created JavaScript, and what was its original name? Brendan Eich (Netscape) created it; its original names were Mocha and LiveScript.
25. Advantages of JavaScript? Less server interaction, immediate user feedback, high interactivity, richer user interfaces.
26. Disadvantages of JavaScript? No inherent multithreading or multiprocessing, limited file system access, and potential security vulnerabilities (client-side code can be accessed and manipulated).
27. Named functions? Functions with names (declared with `function functionName() { ... }`).
28. Types of functions? Named and anonymous functions.
29. Anonymous functions? Functions without names (declared using function expressions, like `let myFunc = function() { ... }`).
30. Assigning anonymous functions to variables? Yes, this is standard JavaScript practice.
31. What is the `arguments` object? Allows access to all arguments passed to a function.
32. What is a closure? When an inner function retains access to variables from its surrounding scope(s) even after the outer function has finished executing.
33. Getting a character at a specific index? Use the `charAt()` method.
34. JavaScript vs. JScript? JScript is Microsoft's version of JavaScript; they are very similar.
35. "Hello, world!" in JavaScript? console.log("Hello, world!");
36. Java vs. JavaScript? Java is a compiled, strongly-typed, object-oriented language for backend development and standalone applications. JavaScript is an interpreted, dynamically-typed, prototype-based language primarily for frontend web development, though increasingly used for backend (Node.js) and mobile development. They differ significantly in syntax, typing, and execution environments.
37. Using an external JavaScript file? Include a `<script>` tag in your HTML pointing to the `.js` file (e.g., `<script src="script.js"></script>`).
38. Is JavaScript case-sensitive? Yes.
39. What is BOM (Browser Object Model)? The BOM allows JavaScript to interact with the browser window (e.g., opening new windows, using the history object).
40. What is DOM (Document Object Model)? The DOM represents the HTML document as a tree-like structure, allowing JavaScript to access and manipulate page content and structure.
41. What is the `window` object? The global object representing the browser window; it has methods for creating popups, closing windows, etc.
42. What is the `history` object? Provides methods to navigate the browser's history (`back()`, `forward()`, `go()`).
43. How to write JavaScript comments? Single-line comments use `//`; multi-line comments use `/* ... */`.
44. How to create a function in JavaScript? Use the `function` keyword (e.g., `function myFunction() { ... }`).
45. Data types in JavaScript? Primitive types: String, Number, Boolean, BigInt, Undefined, Symbol, Null. Non-primitive types: Objects and Arrays.
46. `==` vs. `===`? `==` checks for loose equality (value only); `===` checks for strict equality (value and type).
47. Dynamically adding HTML with JavaScript? Use the `innerHTML` property.
48. Dynamically adding text with JavaScript? Use the `innerText` property.
49. Creating objects in JavaScript? Object literals ({key: value}), `new Object()`, or object constructors.
50. Creating arrays in JavaScript? Array literals ([value1, value2]), `new Array()`, or the `Array` constructor.
51. What does `isNaN()` do? Checks if a value is not a number.
52. Output of `10 + 20 + "30"`? `"3030"` (string concatenation).
53. Output of `"10" + 20 + 30`? `"102030"` (string concatenation).
54. Client-side vs. server-side JavaScript? Client-side runs in the browser; server-side runs on a web server (e.g., Node.js).
55. Cookie storage location? Varies depending on the operating system and browser; often in a specific folder within the user's profile.
56. `event.preventDefault()` vs. `event.stopPropagation()`? `preventDefault()` stops the default action of an event; `stopPropagation()` prevents the event from bubbling up the DOM tree.
57. What is JavaScript? JavaScript is a lightweight, interpreted programming language primarily used to make web pages interactive.
58. Features of JavaScript? Lightweight, interpreted, well-suited for network applications, complements Java and HTML, open source, cross-platform.
59. Creator of JavaScript and original name? Brendan Eich (Netscape) created it; original names were Mocha and LiveScript.
60. Advantages of JavaScript? Less server load, immediate user feedback, high interactivity, richer interfaces.
61. Disadvantages of JavaScript? No built-in multithreading or multiprocessing, limited file system access, potential security vulnerabilities.
62. Named function definition? function myFunction() { ... }
63. Types of functions? Named and anonymous functions.
64. Anonymous function definition? let myFunc = function() { ... } (function expression).
65. Assigning anonymous functions to variables? Yes, this is standard JavaScript practice.
66. The `arguments` object? Provides access to the arguments passed to a function.
67. What is a closure? An inner function has access to variables in its outer function's scope even after the outer function has completed.
68. Getting a character at an index? Use the `charAt()` string method.
69. JavaScript vs. JScript? Essentially the same language; JScript is Microsoft's version of Javascript.
70. "Hello, world!" in JavaScript? console.log("Hello, world!");
71. Key differences between Java and JavaScript? Java is compiled, strongly-typed, object-oriented; JavaScript is interpreted, dynamically-typed, prototype-based. They differ significantly in their purpose, syntax, and execution environment.
72. Using an external JavaScript file? Include a `<script>` tag in your HTML (e.g., `<script src="myScript.js"></script>`).
73. Is JavaScript case-sensitive? Yes.
74. What is the BOM (Browser Object Model)? Allows JavaScript to interact with the browser and its features (e.g., opening windows, accessing history).
75. What is the DOM (Document Object Model)? A programming interface for HTML and XML documents; it represents the page structure as a tree of objects.
76. Uses of the `window` object? Creating popups, managing windows, setting timers, etc. It's the global object in the browser environment.
77. Uses of the `history` object? Allows navigating the browser's history using methods like `back()`, `forward()`, and `go()`.
78. Writing comments in JavaScript? Single-line comments use `//`; multi-line comments use `/* ... */`.
79. Creating functions in JavaScript? Use the `function` keyword (e.g., `function myFunction() { ... }`).
80. JavaScript data types? Primitive types: String, Number, Boolean, BigInt, Undefined, Symbol, Null. Non-primitive types: Objects and Arrays.
81. `==` vs. `===`? `==` performs loose equality comparison (type coercion); `===` performs strict equality comparison (no type coercion).
82. Dynamically adding HTML? Use the `innerHTML` property.
83. Dynamically adding text? Use the `innerText` property.
84. Creating objects in JavaScript? Object literals, `new Object()`, or constructor functions.
85. Creating arrays in JavaScript? Array literals, `new Array()`, or the `Array` constructor.
86. The `isNaN()` function? Checks whether a value is NaN (Not a Number).
87. Output of `10 + 20 + "30"`? `"3030"` (string concatenation).
88. Output of `"10" + 20 + 30`? `"102030"` (string concatenation).
89. Client-side vs. server-side JavaScript? Client-side runs in the browser; server-side runs on a server (e.g., Node.js).
90. Cookie storage location? Depends on the operating system and browser.
91. `event.preventDefault()` vs. `event.stopPropagation()`? `preventDefault()` cancels the default behavior of an event; `stopPropagation()` prevents event bubbling.