JavaScript Interview Questions & Answers (100+)¶

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This guide contains 100+ JavaScript interview questions, ranging from basic to advanced. Each question includes a model answer, code snippets, and potential follow-up paths based on the candidate's response.

Part 1: Core JavaScript Basics (1-20)¶

1. What are the different data types in JavaScript?¶

JavaScript has Primitive and Non-Primitive (Reference) types.

Primitives:
1. String
2. Number
3. BigInt (ES2020)
4. Boolean
5. Undefined
6. Null
7. Symbol (ES6)
Non-Primitive:
1. Object (includes Arrays, Functions, etc.)

Candidate Response Paths:

Path A: Candidate mentions null vs undefined.
- Follow-up: "What is the result of typeof null?"
- Answer: "object". This is a known bug in JavaScript that hasn't been fixed for legacy reasons.

2. What is the difference between `var`, `let`, and `const`?¶

Feature	`var`	`let`	`const`
Scope	Function Scope	Block Scope	Block Scope
Hoisting	Yes (initialized `undefined`)	Yes (TDZ)	Yes (TDZ)
Reassignable	Yes	Yes	No
Redeclarable	Yes	No	No

Candidate Response Paths:

Path A: Candidate mentions TDZ (Temporal Dead Zone).
- Follow-up: "Can you explain what the Temporal Dead Zone is?"
- Answer: It's the period between the start of the block and the actual declaration of the variable. Accessing the variable during this time throws a ReferenceError.

3. What is Hoisting?¶

Hoisting is JavaScript's default behavior of moving declarations to the top of the current scope (script or function) during the compilation phase.

var: Hoisted and initialized with undefined.
Function Declarations: Fully hoisted (can call before definition).
let/const: Hoisted but not initialized (enter Temporal Dead Zone).

4. Explain `==` vs `===`.¶

== (Loose Equality): Performs Type Coercion before comparison.
=== (Strict Equality): Checks both Value and Type.

0 == false   // true
0 === false  // false
'1' == 1     // true
'1' === 1    // false

5. What is Type Coercion?¶

The automatic or implicit conversion of values from one data type to another (e.g., string to number).

Implicit: 1 + '2' -> '12'
Explicit: Number('2')

Candidate Response Paths:

Path A: Candidate discusses truthy/falsy.
- Follow-up: "Is [] truthy or falsy?"
- Answer: Truthy. But [] == false is true (coercion fun).

6. What is `NaN`?¶

It stands for "Not-a-Number". It is a value of the Number type. It results from invalid math operations (e.g., Math.sqrt(-1), 'a' * 1).

Properties: * NaN === NaN is false. * Use Number.isNaN(value) to check.

7. What are the Falsy values in JavaScript?¶

There are 6 falsy values (plus false itself): 1. false 2. 0 (and -0, 0n) 3. "" (Empty string) 4. null 5. undefined 6. NaN

Everything else is Truthy (e.g., [], {}, "0", "false").

8. What is the difference between `null` and `undefined`?¶

undefined: A variable has been declared but not assigned a value. It is the default value of uninitialized variables/parameters.
null: Represents the intentional absence of any object value. It is an assignment value.

9. Explain strict mode (`'use strict'`).¶

Strict mode is a way to opt in to a restricted variant of JavaScript. Benefits: 1. Eliminates some silent errors by throwing them. 2. Fixes mistakes that make it difficult for engines to perform optimizations. 3. Prohibits some syntax likely to be defined in future versions. Example: Prevents accidental global variables (x = 3.14 throws error if x not declared).

10. How do you check the type of a variable?¶

Using the typeof operator.

typeof "John" // "string"
typeof 3.14   // "number"
typeof {}     // "object"
typeof []     // "object"
typeof (() => {}) // "function"

Candidate Response Paths:

Path A: Candidate notes typeof array is object.
- Follow-up: "How do you specifically check if a variable is an Array?"
- Answer: Array.isArray(variable) or variable instanceof Array.

11. What is the difference between Function Declaration and Function Expression?¶

Declaration: function foo() {}. Hoisted (can be called before defined).
Expression: const foo = function() {}. Not hoisted (behaves like variable).

12. What is an IIFE (Immediately Invoked Function Expression)?¶

A function that runs as soon as it is defined. Used to create a local scope and avoid polluting the global namespace.

(function() {
    var secret = "I am safe here";
})();

13. What is the difference between `call`, `apply`, and `bind`?¶

All three allow you to control the value of this inside a function.

Method	Syntax	Execution
`call`	`func.call(thisObj, arg1, arg2)`	Immediately
`apply`	`func.apply(thisObj, [argArray])`	Immediately
`bind`	`const newFunc = func.bind(thisObj)`	Returns new function

14. What is `this` keyword?¶

this refers to the object that is executing the current function. * Global Context: window (browsers) or global (Node). * Object Method: The object itself. * Event Handler: The element that received the event. * Arrow Function: Lexical this (inherits from outer scope).

15. What are Arrow Functions?¶

A concise syntax for writing function expressions. Key Differences: 1. No own this (lexical scoping). 2. No arguments object. 3. Cannot be used as constructors (new fails).

16. What is a Callback Function?¶

A function passed into another function as an argument, which is then invoked inside the outer function to complete some kind of routine or action.

17. What is Scope?¶

Scope determines the accessibility (visibility) of variables. 1. Global Scope: Accessible everywhere. 2. Function Scope: Accessible only within the function. 3. Block Scope: (let, const) Accessible only within the block {}.

18. What is a Closure?¶

A closure is the combination of a function bundled together (enclosed) with references to its surrounding state (the lexical environment). In other words, a closure gives you access to an outer function’s scope from an inner function.

function makeAdder(x) {
  return function(y) {
    return x + y;
  };
}
const add5 = makeAdder(5);
console.log(add5(2)); // 7

19. Explain the module pattern.¶

A design pattern used to wrap a set of variables and functions together in a single scope. It is used to define private and public access levels (encapsulation) using closures (often IIFEs).

20. What is "Currying"?¶

Currying is the process of converting a function that takes multiple arguments into a sequence of functions that each take a single argument.

const add = a => b => a + b;
add(1)(2); // 3

Part 2: Objects, Arrays & Prototypes (21-40)¶

21. What is the Prototype Chain?¶

In JavaScript, objects have a special hidden property [[Prototype]] (exposed as __proto__). If you access a property that doesn't exist on the object, JS looks for it on the prototype, then the prototype's prototype, until it reaches null. This chain is used for inheritance.

22. What is the difference between `proto` and `prototype`?¶

__proto__: The actual object that is used in the lookup chain to resolve methods, etc. (exists on instances).
prototype: The object that will be used as the __proto__ of instances created when using new Constructor(). (exists on functions).

23. How do you create an object in JavaScript?¶

Object Literal: const o = {}
Constructor: const o = new Object()
Object.create(): const o = Object.create(proto)
Class (ES6): new MyClass()

24. What is `Object.freeze()` vs `Object.seal()`?¶

Object.freeze(): Totally immutable. Cannot add, remove, or modify properties.
Object.seal(): Cannot add or remove properties, but can modify existing property values.

25. How do you check if a key exists in an object?¶

"key" in obj (Check prototype chain too)
obj.hasOwnProperty("key") (Check own properties only)
Object.hasOwn(obj, "key") (Modern, safer version of hasOwnProperty)

26. What are the primitive wrapper objects?¶

String, Number, Boolean. They allow you to access properties/methods on primitives (e.g., "hello".toUpperCase()). JS temporarily wraps the primitive in an object, calls the method, and discards the object.

27. What is Destructuring Assignment?¶

A syntax that makes it possible to unpack values from arrays, or properties from objects, into distinct variables.

const user = { id: 42, isVerified: true };
const { id, isVerified } = user;

28. What is the Spread Operator (`...`)?¶

It expands an iterable (like an array or string) into more elements.

const arr1 = [1, 2];
const arr2 = [...arr1, 3, 4]; // [1, 2, 3, 4]

Also used for Object spread (shallow copy/merge).

29. What is the Rest Parameter (`...`)?¶

It allows a function to accept an indefinite number of arguments as an array.

function sum(...args) {
    return args.reduce((a, b) => a + b);
}

30. Explain Shallow Copy vs Deep Copy.¶

Shallow Copy: Copies the object, but nested objects are still references to the original. ({...obj}, Object.assign()).
Deep Copy: Recursively copies everything. (JSON.parse(JSON.stringify(obj)) or structuredClone()).

31. What is the difference between `map` and `forEach`?¶

map(): Returns a new array. Used to transform data.
forEach(): Returns undefined. Used for side effects.

32. How does `reduce()` work?¶

Executes a reducer function on each element of the array, resulting in a single output value.

const sum = [1, 2, 3].reduce((acc, curr) => acc + curr, 0);

33. What is `filter()`?¶

Creates a new array with all elements that pass the test implemented by the provided function.

34. What are Higher-Order Functions?¶

Functions that take other functions as arguments or return functions as their result. (e.g., map, filter, setTimeout).

35. What is the difference between `slice()` and `splice()`?¶

slice(start, end): Returns a new array (shallow copy). Does not modify original.
splice(start, count, ...items): Removes/Adds items. Modifies the original array.

36. What is a `Set`?¶

A collection of values where each value must be unique.

const unique = [...new Set([1, 2, 2, 3])]; // [1, 2, 3]

37. What is a `Map` vs `Object`?¶

Feature	Map	Object
Keys	Any type (incl. objects)	Strings & Symbols
Order	Insertion order	Not guaranteed (historically)
Size	`.size` property	Manually count
Iteration	Directly iterable	`Object.keys()`, etc.

38. What is a `WeakMap`?¶

A Map where keys must be objects and are held "weakly". If the key object is not referenced elsewhere, it can be garbage collected. Not iterable.

39. How do you sort an array of numbers?¶

[1, 10, 2].sort() sorts alphabetically by default -> [1, 10, 2]. Correct way:

[1, 10, 2].sort((a, b) => a - b); // [1, 2, 10]

40. What is `Array.from()`?¶

Creates a new Array instance from an array-like or iterable object (e.g., NodeList, arguments, String).

Array.from('foo'); // ["f", "o", "o"]

Part 3: Async JavaScript (41-60)¶

41. What is the Event Loop?¶

JavaScript is single-threaded. The Event Loop monitors the Call Stack and the Callback Queue. If the Call Stack is empty, it takes the first event from the queue and pushes it to the stack.

Diagram:

graph TD
    CallStack[Call Stack] -->|Execute| API[Web APIs]
    API -->|Callback| Queue[Callback Queue]
    Queue -->|Event Loop| CallStack

42. Explain Synchronous vs Asynchronous.¶

Synchronous: Code executes line-by-line, blocking execution until completion.
Asynchronous: Code executes without blocking (e.g., timers, network requests), providing a callback or promise to handle the result later.

43. What is a Promise?¶

An object representing the eventual completion (or failure) of an asynchronous operation and its resulting value. States: 1. Pending 2. Fulfilled (Resolved) 3. Rejected

44. What are the instance methods of a Promise?¶

.then(onSuccess, onFailure)
.catch(onFailure)
.finally(onAlways)

45. Explain `Promise.all()` vs `Promise.allSettled()`.¶

Promise.all(iterable): Waits for all to resolve. Rejects immediately if any promise rejects (fail-fast).
Promise.allSettled(iterable): Waits for all to finish (resolve or reject). Returns an array of status objects.

46. What is `Promise.race()`?¶

Returns a promise that resolves or rejects as soon as one of the promises in the iterable resolves or rejects.

47. Explain `async` and `await`.¶

Syntactic sugar built on top of Promises to write asynchronous code that looks synchronous. * async function returns a Promise. * await pauses execution until the Promise resolves.

async function fetchData() {
    try {
        const response = await fetch('/api');
        const data = await response.json();
    } catch (error) {
        console.error(error);
    }
}

48. What happens if you `await` a non-promise?¶

It is converted to a resolved Promise automatically. const x = await 5; // x is 5

49. Can you use `await` inside a loop?¶

Yes. * forEach: async callbacks run in parallel (doesn't wait). * for...of: await inside pauses the loop (sequential execution).

50. What is a Microtask vs Macrotask?¶

Microtasks: Promises, queueMicrotask, MutationObserver. (High priority, run immediately after current script).
Macrotasks: setTimeout, setInterval, setImmediate, I/O. (Run in next Event Loop tick).

Execution Order: Script -> All Microtasks -> Render -> One Macrotask.

51. What is the output?¶

console.log(1);
setTimeout(() => console.log(2), 0);
Promise.resolve().then(() => console.log(3));
console.log(4);

Answer: 1, 4, 3, 2 (Sync -> Sync -> Microtask -> Macrotask).

52. What is `setTimeout`?¶

Schedules code to be run after a specified delay (in ms). It is not part of JS engine, but a Web API provided by the browser/Node environment.

53. Why is `setTimeout(fn, 0)` useful?¶

It pushes the function to the end of the Call Stack (to the task queue), allowing the UI to update or the current execution to finish first.

54. What is Callback Hell?¶

The situation where callbacks are nested within other callbacks several levels deep, making code difficult to read and maintain. Solution: Use Promises or Async/Await.

55. What is `fetch` API?¶

A modern interface for making HTTP requests (replacement for XMLHttpRequest). It returns a Promise.

56. How do you cancel a fetch request?¶

Using an AbortController.

const controller = new AbortController();
fetch(url, { signal: controller.signal });
controller.abort(); // Cancels request

57. What is `JSON.parse` and `JSON.stringify`?¶

JSON.parse(): Parses a JSON string to a JS object.
JSON.stringify(): Converts a JS object to a JSON string.

58. What is debouncing?¶

Ensures that a function is not called again until a certain amount of time has passed since it was last called. (e.g., search input).

function debounce(func, delay) {
    let timeout;
    return (...args) => {
        clearTimeout(timeout);
        timeout = setTimeout(() => func(...args), delay);
    };
}

59. What is throttling?¶

Ensures that a function is called at most once in a specified time period. (e.g., window resize, scroll).

60. What is a Generator Function in async context?¶

Generators (function*) can yield Promises. Before async/await, libraries like co used generators to mimic sync-style async code.

Part 4: DOM & Web API (61-80)¶

61. What is the DOM?¶

Document Object Model. A tree structure representation of the HTML document. It allows JS to manipulate content, structure, and style.

62. How to select elements in DOM?¶

document.getElementById('id')
document.getElementsByClassName('class')
document.getElementsByTagName('div')
document.querySelector('.class') (Returns first match)
document.querySelectorAll('.class') (Returns NodeList)

63. `NodeList` vs `HTMLCollection`?¶

HTMLCollection: Live (updates automatically if DOM changes). Returned by getElementsByClassName.
NodeList: Static (mostly). Returned by querySelectorAll.

64. What is Event Bubbling?¶

When an event triggers on an element, it first runs the handlers on it, then on its parent, then all the way up on other ancestors. (Bottom -> Up).

65. What is Event Capturing (Trickling)?¶

The opposite of bubbling. The event goes down to the element. (Top -> Down). Enabled by passing { capture: true } to addEventListener.

66. How to stop propagation?¶

event.stopPropagation() stops the event from moving to the next part of the propagation flow (bubbling or capturing).

67. What is `event.preventDefault()`?¶

Prevents the browser's default behavior for the event (e.g., prevents form submission, or link navigation).

68. What is Event Delegation?¶

Instead of attaching event listeners to individual elements, you attach a single listener to a common parent. It utilizes event bubbling to handle events for child elements (even dynamically added ones).

document.getElementById('list').addEventListener('click', (e) => {
    if (e.target.tagName === 'LI') {
        console.log('List item clicked');
    }
});

69. What is the `dataset` property?¶

Allows access to custom data attributes (data-*) on elements.

<div id="user" data-id="123" data-role="admin"></div>
<script>
    const el = document.getElementById('user');
    console.log(el.dataset.id); // "123"
</script>

70. `innerHTML` vs `innerText` vs `textContent`?¶

innerHTML: Parses HTML string (Security risk: XSS).
innerText: Visible text (aware of CSS styling like display: none).
textContent: All text (including hidden elements). Faster.

71. What is the BOM (Browser Object Model)?¶

Objects provided by the browser to interact with everything outside the page content. Examples: window, navigator, location, history, screen.

72. How do you detect the browser name?¶

navigator.userAgent. (Though feature detection is preferred over browser detection).

73. What is `localStorage` vs `sessionStorage`?¶

localStorage: Data persists even after the browser is closed. (No expiration).
sessionStorage: Data survives page refresh but is cleared when the tab/window is closed.

74. What are Cookies?¶

Small data stored in the browser. Sent with every HTTP request to the server. Properties: Expires, HttpOnly (secure against XSS), Secure (HTTPS only).

75. How to create a new DOM element?¶

document.createElement('tagName').

76. What is a DocumentFragment?¶

A lightweight, minimal document object. It has no parent. Appending to it doesn't cause reflow. Appending the fragment to DOM moves all its children. Used for performance optimization when adding multiple elements.

77. What is Reflow and Repaint?¶

Reflow (Layout): Calculating positions and geometries of elements. (Expensive).
Repaint: Changing visibility (color, background) without changing layout.

78. What is Shadow DOM?¶

Encapsulation mechanism. Allows a hidden DOM tree to be attached to an element. Styles inside Shadow DOM don't leak out, and global styles don't leak in. Basis of Web Components.

79. What is `window.onload` vs `DOMContentLoaded`?¶

DOMContentLoaded: HTML is parsed and DOM is built. (External scripts/styles might not be loaded).
window.onload: Everything is loaded (images, styles, scripts).

80. How does `defer` and `async` work in script tags?¶

<script>: Blocks parsing, fetches, executes.
<script async>: Fetches in parallel, executes as soon as fetched (pauses parsing). Order not guaranteed.
<script defer>: Fetches in parallel, executes after HTML parsing is complete. Order guaranteed.

Part 5: ES6+ & Advanced Concepts (81-100)¶

81. What is a Class in ES6?¶

Syntactic sugar over prototype-based inheritance.

class Person {
    constructor(name) { this.name = name; }
    greet() { return `Hi, ${this.name}`; }
}

82. What are Modules (ESM)?¶

Official standard for modules. * export (Named or Default). * import. * File-based scope. Always strict mode.

83. What are Template Literals?¶

String literals allowing embedded expressions. Enclosed by backticks (`). Supports multi-line strings.

84. What is the Optional Chaining Operator (`?.`)?¶

Safely access nested properties. Returns undefined if reference is nullish.

const zip = user?.address?.zipCode;

85. What is the Nullish Coalescing Operator (`??`)?¶

Returns the right-hand operand when the left is null or undefined (not just falsy).

const x = 0 ?? 5; // 0
const y = 0 || 5; // 5

86. What is a Symbol?¶

A unique and immutable primitive. Often used as object keys to create private-like properties or avoid name collisions.

87. What is a Proxy?¶

An object that wraps another object and intercepts operations (like read, write, function call).

const handler = {
    get: (obj, prop) => prop in obj ? obj[prop] : "Not Found"
};
const p = new Proxy({}, handler);

88. What is `Reflect`?¶

A built-in object that provides methods for interceptable JavaScript operations. Complements Proxy.

89. What is Memory Leak in JS?¶

When objects are no longer needed but are still referenced, preventing Garbage Collection. Causes: Global variables, forgotton timers, closures, detached DOM nodes.

90. How does Garbage Collection work?¶

Mark-and-Sweep: 1. Roots (Global, Stack) are marked. 2. References are traversed and marked. 3. Unmarked objects (unreachable) are swept (deleted).

91. What is `BigInt`?¶

A primitive for integers larger than 2^53 - 1. Created by appending n. const huge = 1234567890123456789012345678901234567890n;

92. What is `globalThis`?¶

A standard way to access the global object across environments (window, self, global).

93. What is Currying vs Partial Application?¶

Currying: f(a, b, c) -> f(a)(b)(c).
Partial App: f(a, b, c) -> g(b, c) (where a is fixed).

94. What is Memoization?¶

Optimization technique. Caching the result of an expensive function call based on arguments and returning the cached result when the same inputs occur again.

95. What is the difference between `for...in` and `for...of`?¶

for...in: Iterates over keys (enumerable properties). (Works on Objects).
for...of: Iterates over values. (Works on Iterables: Array, Map, Set, String).

96. What is TCO (Tail Call Optimization)?¶

An optimization where a recursive function call is the last action in the function. The engine reuses the stack frame. (Supported in Safari, theoretically in ES6 spec).

97. What is a Polyfill?¶

Code that implements a feature on web browsers that do not support the feature.

98. What is Transpilation (Babel)?¶

Converting modern JS (ES6+) code into older JS (ES5) to ensure compatibility with older browsers.

99. What are Worker Threads (Web Workers)?¶

Allow running scripts in background threads, separate from the main execution thread. Prevents UI blocking during heavy computations. They don't have access to DOM.

100. What is Server-Side Rendering (SSR) vs Client-Side Rendering (CSR)?¶

CSR: Browser downloads minimal HTML and a JS bundle. JS fetches data and builds the UI. (Slower initial load, faster interaction).
SSR: Server renders HTML with data and sends it. (Faster initial load, SEO friendly).

End of Interview Questions

Study Guide¶

To deepen your understanding of these concepts, check out the following resources:

Official Documentation: Javascript Docs
Internal Style Guide: Javascript Style Guide

JavaScript Interview Questions & Answers (100+)¶

Part 1: Core JavaScript Basics (1-20)¶

1. What are the different data types in JavaScript?¶

2. What is the difference between var, let, and const?¶

3. What is Hoisting?¶

4. Explain == vs ===.¶

5. What is Type Coercion?¶

6. What is NaN?¶

7. What are the Falsy values in JavaScript?¶

8. What is the difference between null and undefined?¶

9. Explain strict mode ('use strict').¶

10. How do you check the type of a variable?¶

11. What is the difference between Function Declaration and Function Expression?¶

12. What is an IIFE (Immediately Invoked Function Expression)?¶

13. What is the difference between call, apply, and bind?¶

14. What is this keyword?¶

15. What are Arrow Functions?¶

16. What is a Callback Function?¶

17. What is Scope?¶

18. What is a Closure?¶

19. Explain the module pattern.¶

20. What is "Currying"?¶

Part 2: Objects, Arrays & Prototypes (21-40)¶

21. What is the Prototype Chain?¶

22. What is the difference between __proto__ and prototype?¶

23. How do you create an object in JavaScript?¶

24. What is Object.freeze() vs Object.seal()?¶

25. How do you check if a key exists in an object?¶

26. What are the primitive wrapper objects?¶

27. What is Destructuring Assignment?¶

28. What is the Spread Operator (...)?¶

29. What is the Rest Parameter (...)?¶

30. Explain Shallow Copy vs Deep Copy.¶

31. What is the difference between map and forEach?¶

32. How does reduce() work?¶

33. What is filter()?¶

34. What are Higher-Order Functions?¶

35. What is the difference between slice() and splice()?¶

36. What is a Set?¶

37. What is a Map vs Object?¶

38. What is a WeakMap?¶

39. How do you sort an array of numbers?¶

40. What is Array.from()?¶

Part 3: Async JavaScript (41-60)¶

41. What is the Event Loop?¶

42. Explain Synchronous vs Asynchronous.¶

43. What is a Promise?¶

44. What are the instance methods of a Promise?¶

45. Explain Promise.all() vs Promise.allSettled().¶

46. What is Promise.race()?¶

47. Explain async and await.¶

48. What happens if you await a non-promise?¶

49. Can you use await inside a loop?¶

50. What is a Microtask vs Macrotask?¶

51. What is the output?¶

52. What is setTimeout?¶

53. Why is setTimeout(fn, 0) useful?¶

54. What is Callback Hell?¶

55. What is fetch API?¶

56. How do you cancel a fetch request?¶

57. What is JSON.parse and JSON.stringify?¶

58. What is debouncing?¶

59. What is throttling?¶

60. What is a Generator Function in async context?¶

Part 4: DOM & Web API (61-80)¶

61. What is the DOM?¶

62. How to select elements in DOM?¶

63. NodeList vs HTMLCollection?¶

64. What is Event Bubbling?¶

65. What is Event Capturing (Trickling)?¶

66. How to stop propagation?¶

67. What is event.preventDefault()?¶

68. What is Event Delegation?¶

69. What is the dataset property?¶

70. innerHTML vs innerText vs textContent?¶

71. What is the BOM (Browser Object Model)?¶

72. How do you detect the browser name?¶

73. What is localStorage vs sessionStorage?¶

74. What are Cookies?¶

75. How to create a new DOM element?¶

2. What is the difference between `var`, `let`, and `const`?¶

4. Explain `==` vs `===`.¶

6. What is `NaN`?¶

8. What is the difference between `null` and `undefined`?¶

9. Explain strict mode (`'use strict'`).¶

13. What is the difference between `call`, `apply`, and `bind`?¶

14. What is `this` keyword?¶

22. What is the difference between `proto` and `prototype`?¶

24. What is `Object.freeze()` vs `Object.seal()`?¶

28. What is the Spread Operator (`...`)?¶

29. What is the Rest Parameter (`...`)?¶

31. What is the difference between `map` and `forEach`?¶

32. How does `reduce()` work?¶

33. What is `filter()`?¶

35. What is the difference between `slice()` and `splice()`?¶

36. What is a `Set`?¶

37. What is a `Map` vs `Object`?¶

38. What is a `WeakMap`?¶

40. What is `Array.from()`?¶

45. Explain `Promise.all()` vs `Promise.allSettled()`.¶

46. What is `Promise.race()`?¶

47. Explain `async` and `await`.¶

48. What happens if you `await` a non-promise?¶

49. Can you use `await` inside a loop?¶

52. What is `setTimeout`?¶

53. Why is `setTimeout(fn, 0)` useful?¶

55. What is `fetch` API?¶

57. What is `JSON.parse` and `JSON.stringify`?¶

63. `NodeList` vs `HTMLCollection`?¶

67. What is `event.preventDefault()`?¶

69. What is the `dataset` property?¶

70. `innerHTML` vs `innerText` vs `textContent`?¶

73. What is `localStorage` vs `sessionStorage`?¶

79. What is `window.onload` vs `DOMContentLoaded`?¶

80. How does `defer` and `async` work in script tags?¶

84. What is the Optional Chaining Operator (`?.`)?¶

85. What is the Nullish Coalescing Operator (`??`)?¶

88. What is `Reflect`?¶

91. What is `BigInt`?¶

92. What is `globalThis`?¶

95. What is the difference between `for...in` and `for...of`?¶