1. What is JavaScript?

JavaScript is a versatile, high-level, interpreted programming language that is a core technology of the World Wide Web, along with HTML and CSS. It enables interactive web pages and is an essential part of web applications.

2. What are the different data types in JavaScript?

There are 8 main data types:

Primitives: string, number, boolean, undefined, null, bigint, symbol.
Objects: object (including arrays, functions, dates, etc.).

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

var: Function-scoped, can be re-declared and updated, hoisted (initialized as undefined).
let: Block-scoped, can be updated but not re-declared, hoisted (uninitialized, "Temporal Dead Zone").
const: Block-scoped, cannot be updated or re-declared, must be initialized at declaration.

4. What is "Hoisting"?

Hoisting is JavaScript's default behavior of moving declarations to the top of the current scope. Variable declarations (var) are hoisted as undefined, while function declarations are fully hoisted. let and const are hoisted but inaccessible until their declaration.

5. What is the difference between `==` and `===`?

== (Loose equality): Performs type coercion before comparison.
=== (Strict equality): Compares both value and type without coercion.

6. What is a "Closure"?

A closure is a function that remembers and accesses variables from its outer scope even after the outer function has finished executing.

7. What are "Template Literals"?

Introduced in ES6, template literals use backticks (`) and allow for multi-line strings and string interpolation using ${expression}.

8. What is the "Temporal Dead Zone"?

The period between the start of a block's execution and the point where a variable (declared with let or const) is initialized. Accessing the variable during this time results in a ReferenceError.

9. What is the difference between `null` and `undefined`?

null: An intentional absence of value; an object type.
undefined: A variable that has been declared but not yet assigned a value; a type in itself.

10. What is an "Arrow Function"?

A compact alternative to conventional function expressions using the => syntax. Arrow functions do not have their own this, arguments, or super context.

11. What is the purpose of `this` in JavaScript?

The this keyword refers to the object that is currently executing the function. Its value depends on how the function is called (global context, method, constructor, or event handler).

12. What is a "Promise"?

An object representing the eventual completion (or failure) of an asynchronous operation and its resulting value. A promise can be in one of three states: pending, fulfilled, or rejected.

13. What is "Async/Await"?

Syntactic sugar for working with Promises, built on top of generators. It allows you to write asynchronous code that looks and behaves like synchronous code.

14. What are "Callbacks"?

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.

15. What is "Callback Hell"?

A situation where multiple nested callbacks make the code difficult to read and maintain. This is often solved using Promises or Async/Await.

16. What is the difference between Synchronous and Asynchronous code?

Synchronous: Each operation waits for the previous one to complete (blocking).
Asynchronous: Operations can start and finish independently of the main execution flow (non-blocking).

17. What is the "Event Loop"?

A mechanism that allows JavaScript to perform non-blocking I/O operations by offloading tasks to the system kernel and then placing their results back onto the task queue when they're finished.

18. What are the different ways to create objects in JavaScript?

Object Literals: const obj = {};
new Object() constructor.
Object.create().
Constructor functions.
ES6 Classes.

19. What is "Prototype Inheritance"?

In JavaScript, objects have a private property which holds a link to another object called its prototype. That prototype object has a prototype of its own, and so on until an object is reached with null as its prototype.

20. What is a "Higher-Order Function"?

A function that either takes one or more functions as arguments or returns a function as its result (e.g., map, filter, reduce).

21. What is the difference between `map` and `forEach`?

map: Returns a new array with the results of calling a provided function on every element.
forEach: Executes a provided function once for each array element but doesn't return anything.

22. What is "Destructuring"?

A special syntax that allows you to "unpack" values from arrays or properties from objects into distinct variables.

23. What is the "Spread Operator" (`...`)?

Used to expand an iterable (like an array or string) into places where zero or more arguments or elements are expected.

24. What are "Rest Parameters" (`...`)?

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

25. What is "Strict Mode" (`"use strict"`)?

A way to opt into a restricted variant of JavaScript that eliminates some silent errors and makes it easier to write "secure" JavaScript.

26. What is the difference between an Array and an Object?

Array: An ordered collection of items (elements) accessed by index.
Object: An unordered collection of key-value pairs (properties) accessed by key.

27. What is an "Immediately Invoked Function Expression" (IIFE)?

A function that runs as soon as it is defined. It is used to create a private scope for variables.

28. What is the purpose of `call()`, `apply()`, and `bind()`?

call(): Invokes a function with a given this value and arguments provided individually.
apply(): Invokes a function with a given this value and arguments provided as an array.
bind(): Returns a new function with a fixed this value, without invoking it immediately.

29. What is the "DOM" (Document Object Model)?

A programming interface for HTML and XML documents. It represents the structure of a document as a tree of objects, which can be manipulated via JavaScript.

30. What is "Event Bubbling" and "Event Capturing"?

Bubbling: The event starts from the target element and moves up to its ancestors.
Capturing: The event starts from the root element and moves down to the target element.

31. What is `event.preventDefault()` and `event.stopPropagation()`?

preventDefault(): Cancels the default behavior of an event (e.g., preventing a form from submitting).
stopPropagation(): Prevents the event from bubbling up the DOM tree.

32. What is "Debouncing" and "Throttling"?

Debouncing: Ensures a function is called only after a certain period of inactivity (e.g., search input).
Throttling: Ensures a function is called only once in a specified time interval (e.g., scroll event).

33. What are ES6 "Classes"?

Syntactic sugar over JavaScript's existing prototype-based inheritance. Classes provide a clearer and more concise way to create objects and deal with inheritance.

34. What is the difference between `null` and `undefined`?

undefined: A variable has been declared but not assigned a value.
null: An intentional assignment of "no value" to a variable.

35. What is the purpose of the `Symbol` data type?

Used to create unique, anonymous, and immutable identifiers for object properties, ensuring they are not accidentally overwritten.

36. What is "Type Coercion"?

The automatic or implicit conversion of values from one data type to another (e.g., 5 + "5" results in "55").

37. What is the difference between a "Shallow Copy" and a "Deep Copy"?

Shallow Copy: Copies the top-level structure of an object; nested objects still refer to the same memory addresses.
Deep Copy: Copies the entire structure of an object, including all nested objects, creating completely independent copies.

38. What is a "Generator Function"?

A function that can be exited and later re-entered. Its context (variable bindings) is saved across re-entrances. Use the function* syntax and yield keyword.

39. What are "Modules" in JavaScript?

A way to divide JavaScript programs into several files, each exporting specific functionalities to be imported and used in other files (ESM: import/export).

40. What is the difference between `forEach` and `for...of`?

forEach: A method specific to arrays.
for...of: A modern loop that works with any iterable (arrays, strings, maps, sets, etc.).

41. What is the "Spread" vs "Rest" operator?

Spread: Expands an array into individual elements (used when calling a function or creating an array).
Rest: Collects individual elements into an array (used in function parameters or destructuring).

42. What is the "Global Execution Context" and "Function Execution Context"?

Global: Created when the script starts; contains global variables and the window/global object.
Function: Created whenever a function is invoked; contains the function's local variables and arguments.

43. What is the "Scope Chain"?

The mechanism by which JavaScript looks up variable values when they are used in nested functions. It starts from the current scope and goes up to the global scope.

44. What is a "Pure Function"?

A function that always returns the same output for the same input and has no side effects (doesn't modify global variables or perform I/O).

45. What is "Memoization"?

An optimization technique that involves caching the results of expensive function calls and returning the cached result when the same inputs occur again.

46. What is the "Reflect API"?

A built-in object that provides intermediate methods for interceptable JavaScript operations (like getting/setting properties), similar to the Proxy API.

47. What is the "Proxy API"?

Allows you to create an object that wraps another object and intercepts operations like property lookup, assignment, and function invocation.

48. What is the difference between `Map` and `Object`?

Map: Maintains the order of keys; keys can be of any type; has built-in size property.
Object: Keys must be strings or symbols; keys are not necessarily ordered; size must be calculated manually.

49. What is a "Set" and "WeakSet"?

Set: A collection of unique values of any type.
WeakSet: A collection of unique objects only; objects are weakly held (can be garbage collected).

50. What is "Currying"?

A transformation of a function with multiple arguments into a sequence of functions, each taking a single argument.

JavaScript Interview Questions

Table of Contents

What is JavaScript?

What are the different data types in JavaScript?

What is the difference between <code>var</code>, <code>let</code>, and <code>const</code>?

What is "Hoisting"?

What is the difference between <code>==</code> and <code>===</code>?

What is a "Closure"?

What are "Template Literals"?

What is the "Temporal Dead Zone"?

What is the difference between <code>null</code> and <code>undefined</code>?

What is an "Arrow Function"?

What is the purpose of <code>this</code> in JavaScript?

What is a "Promise"?

What is "Async/Await"?

What are "Callbacks"?

What is "Callback Hell"?

What is the difference between Synchronous and Asynchronous code?

What is the "Event Loop"?

What are the different ways to create objects in JavaScript?

What is "Prototype Inheritance"?

What is a "Higher-Order Function"?

What is the difference between <code>map</code> and <code>forEach</code>?

What is "Destructuring"?

What is the "Spread Operator" (<code>...</code>)?

What are "Rest Parameters" (<code>...</code>)?

What is "Strict Mode" (<code>"use strict"</code>)?

What is the difference between an Array and an Object?

What is an "Immediately Invoked Function Expression" (IIFE)?

What is the purpose of <code>call()</code>, <code>apply()</code>, and <code>bind()</code>?

What is the "DOM" (Document Object Model)?

What is "Event Bubbling" and "Event Capturing"?

What is <code>event.preventDefault()</code> and <code>event.stopPropagation()</code>?

What is "Debouncing" and "Throttling"?

What are ES6 "Classes"?

What is the difference between <code>null</code> and <code>undefined</code>?

What is the purpose of the <code>Symbol</code> data type?

What is "Type Coercion"?

What is the difference between a "Shallow Copy" and a "Deep Copy"?

What is a "Generator Function"?

What are "Modules" in JavaScript?

What is the difference between <code>forEach</code> and <code>for...of</code>?

What is the "Spread" vs "Rest" operator?

What is the "Global Execution Context" and "Function Execution Context"?

What is the "Scope Chain"?

What is a "Pure Function"?

What is "Memoization"?

What is the "Reflect API"?

What is the "Proxy API"?

What is the difference between <code>Map</code> and <code>Object</code>?

What is a "Set" and "WeakSet"?

What is "Currying"?