JavaScript remains one of the most versatile and in-demand programming languages in 2025. Whether you are a fresher or a professional developer, mastering JavaScript is crucial for career advancement.
Interviews for JavaScript roles often cover a wide range of questions, from basic concepts to advanced problem-solving. This comprehensive guide compiles the Top 100 Most Asked JavaScript Interview Questions and Answers, structured to help you prepare efficiently and confidently.
Basic JavaScript Interview Questions and Answers
1. What is JavaScript?
JavaScript is a lightweight, interpreted scripting language used to create dynamic and interactive web content. It allows developers to enhance user interfaces, validate inputs, and build complex web applications. JavaScript operates on the client-side but can also be used server-side through environments like Node.js.
2. Is JavaScript a compiled or interpreted language?
JavaScript is traditionally an interpreted language, meaning code is executed line-by-line. However, modern JavaScript engines like Google’s V8 utilize Just-In-Time (JIT) compilation to improve performance by compiling code on the fly.
3. What are the key features of JavaScript?
- Lightweight and interpreted
- Prototype-based inheritance
- First-class functions
- Asynchronous event handling
- Cross-platform support
4. How is JavaScript different from Java?
Java is a statically typed, class-based programming language primarily used for backend systems and Android development. JavaScript is dynamically typed and mainly used for enhancing web page interactivity. Despite their names, they are not directly related.
5. What are variables in JavaScript?
Variables in JavaScript are containers for storing data values. They can be declared using var, let, or const. var is function-scoped, whereas let and const are block-scoped, with const creating a read-only reference to a value.
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6. What are data types supported by JavaScript?
JavaScript supports primitive types like String, Number, Boolean, Null, Undefined, Symbol, and BigInt. It also supports non-primitive types such as Objects, Arrays, and Functions.
7. What is the difference between == and === operators?
== compares two values after converting them to a common type (type coercion), while === compares both value and type, enforcing strict equality without type conversion.
8. What are JavaScript functions?
A JavaScript function is a block of reusable code designed to perform a specific task. Functions can be declared, expressed, or defined as arrow functions in JavaScript. They can accept inputs (parameters) and return outputs (values).
9. What is a JavaScript object?
JavaScript objects are collections of key-value pairs where values can be any data type. They are fundamental to building complex data structures and are widely used for representing real-world entities.
10. What are arrays in JavaScript?
Arrays are special objects used to store multiple values in a single variable. They are ordered, and elements are accessed via numerical indices starting from zero.
11. What is hoisting in JavaScript?
Hoisting is JavaScript’s behavior of moving declarations (not initializations) to the top of their containing scope before code execution. This applies to variables and functions declared with var and regular function declarations.
12. What is the difference between null and undefined?
undefined indicates that a variable has been declared but not assigned a value. null is an assignment value that represents no value or no object.
13. What is the use of the typeof operator?
The typeof operator is used to determine the type of a JavaScript variable or expression. It returns a string such as “number”, “string”, “boolean”, “object”, “function”, or “undefined”.
14. Explain scope in JavaScript.
Scope refers to the accessibility of variables and functions in different parts of the code. JavaScript has global scope, function scope, and block scope (with let and const).
15. What is an Immediately Invoked Function Expression (IIFE)?
An IIFE is a function that is executed immediately after its creation. It is often used to create a private scope and avoid polluting the global namespace.
(function() )();16. What is closure in JavaScript?
A closure is a function that retains access to its lexical scope even when the function is executed outside that scope. Closures are commonly used for data privacy and creating factory functions.
17. What is event bubbling and capturing?
Event bubbling means events propagate from the innermost element to outer elements. Event capturing (less common) means events are captured from the outermost element first before reaching the target element.
18. What is the event loop in JavaScript?
The event loop is a mechanism that manages the execution of asynchronous callbacks. It ensures that non-blocking operations like timers and network requests can run smoothly without freezing the main thread.
19. What are promises in JavaScript?
Promises are objects representing the eventual completion (or failure) of an asynchronous operation. They simplify asynchronous code management by replacing deeply nested callbacks with cleaner syntax.
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20. What is async/await in JavaScript?
async/await is syntactic sugar over promises, allowing asynchronous code to be written in a synchronous-looking manner. Functions marked as async return promises, and await pauses execution until a promise resolves.
21. What is the difference between let, var, and const?
var is function-scoped and can be redeclared and updated. let is block-scoped and can be updated but not redeclared within the same scope. const is also block-scoped but cannot be updated or redeclared.
22. What are template literals in JavaScript?
Template literals are string literals allowing embedded expressions. They are enclosed by backticks (`) and support multi-line strings and interpolation using $ .
23. What are arrow functions in JavaScript?
Arrow functions provide a concise syntax for writing functions. They do not bind their own this value, making them useful for certain callbacks and object methods.
const sum = (a, b) => a + b;24. What is destructuring assignment?
Destructuring allows unpacking values from arrays or properties from objects into distinct variables.
const [a, b] = [1, 2];
const = ;25. What is the spread operator?
The spread operator (...) expands iterable elements (like arrays or objects) into individual elements. It is used for array cloning, merging, or function arguments.
26. What is the difference between slice and splice in JavaScript?
slice() returns a shallow copy of a portion of an array into a new array without modifying the original array. splice() changes the original array by removing, replacing, or adding elements.
27. What are higher-order functions in JavaScript?
Higher-order functions are functions that accept other functions as arguments or return functions as their result. Examples include map(), filter(), and reduce().
28. What is currying in JavaScript?
Currying is a technique of transforming a function with multiple arguments into a series of functions, each taking one argument at a time.
function add(a) ;
}
const addFive = add(5);
console.log(addFive(10)); // 15
29. What is memoization in JavaScript?
Memoization is an optimization technique where the results of function calls are cached based on their inputs to avoid redundant computations.
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30. Explain the concept of callbacks in JavaScript.
Callbacks are functions passed into another function as an argument to be executed later, typically after an asynchronous operation is complete.
31. What are generator functions in JavaScript?
Generator functions are functions that can be paused and resumed using the yield keyword. They are defined with the function* syntax.
function* genFunc()
32. What is the ‘this’ keyword in JavaScript?
this refers to the object that is executing the current function. Its value can change depending on how a function is called (regular function call, method call, constructor, or bound function).
33. What is prototype chaining?
Prototype chaining is a mechanism by which JavaScript objects inherit properties from other objects via their prototype. It allows inheritance and reuse of code.
34. What are promises chaining in JavaScript?
Promise chaining is the process of executing asynchronous operations sequentially, where each .then() returns another promise, allowing cleaner code than deeply nested callbacks.
35. What is a pure function?
A pure function is a function that does not cause side effects and returns the same output for the same input. It does not modify external variables or state.
36. Explain debouncing in JavaScript.
Debouncing limits the rate at which a function is executed. It ensures the function is called after a specified delay only if no new triggering events have occurred during that delay.
37. Explain throttling in JavaScript.
Throttling ensures a function is called at most once in a specified period, regardless of how many triggering events occur. It is used to control execution frequency.
38. What is event delegation?
Event delegation is a technique where a single event listener is attached to a parent element, managing events triggered by its child elements using event propagation.
39. What is a JavaScript Set?
A Set is a built-in object that stores unique values of any type, whether primitive or object references. Sets automatically remove duplicate values.
const mySet = new Set([1, 2, 3, 3]);
console.log(mySet); // Set(3)
40. What is a WeakSet?
A WeakSet is a collection of objects where each object can only appear once and is held weakly, meaning the object can be garbage collected if there are no other references to it.
Intermediate JavaScript Interview Questions and Answers
41. What is the Temporal Dead Zone (TDZ) in JavaScript?
The Temporal Dead Zone is the time between the entering of a scope and the actual variable declaration, where variables declared with let and const cannot be accessed before initialization.
42. What are modules in JavaScript?
Modules are reusable pieces of code that can be exported from one file and imported into another. They allow better organization, separation of concerns, and encapsulation.
43. What is the difference between default export and named export?
Default export allows exporting one value per module, while named exports allow exporting multiple values. Named exports must be imported using their exact names.
44. What is the Reflect API in JavaScript?
The Reflect API provides a set of static methods for interceptable JavaScript operations, improving the way JavaScript programs interact with objects.
45. What is the Proxy object in JavaScript?
The Proxy object enables you to create a proxy for another object, allowing you to intercept and redefine fundamental operations like property lookup, assignment, and function invocation.
46. Explain shallow copy vs deep copy.
A shallow copy copies an object’s top-level properties but not nested objects. A deep copy recursively copies all levels of objects, creating entirely new references for all nested structures.
47. What are tagged template literals?
Tagged template literals allow customizing the behavior of template literals by passing them to a tag function that can process and manipulate the input.
48. What is optional chaining?
Optional chaining (?.) allows accessing deeply nested object properties safely, without causing an error if a reference is nullish (null or undefined).
const user = ;
console.log(user?.profile?.email); // undefined
49. What is nullish coalescing?
The nullish coalescing operator (??) returns the right-hand operand when the left-hand operand is null or undefined. It is useful for setting default values.
const name = null ?? "Guest";
console.log(name); // Guest
50. What are private fields in JavaScript classes?
Private fields are properties that cannot be accessed or modified outside the class definition. They are prefixed with # in the class body.
class Person
}
const nums = [1, 2, 3];
const newNums = [...nums, 4, 5];51. What is object destructuring?
Object destructuring is a syntax that allows extracting properties from an object and assigning them to variables with the same name.
const user = ;
const = user;
52. What is array destructuring?
Array destructuring allows unpacking values from arrays into distinct variables based on their positions.
const numbers = [1, 2, 3];
const [first, second] = numbers;
53. What is the purpose of the rest operator?
The rest operator (...) allows us to represent an indefinite number of arguments as an array. It can be used in function parameters or destructuring assignments.
54. Explain the difference between call, apply, and bind.
- call: Invokes a function with a given
thisvalue and arguments provided individually. - apply: Invokes a function with a given
thisvalue and arguments provided as an array. - bind: Returns a new function, binding
thispermanently without invoking immediately.
55. What is a WeakMap in JavaScript?
A WeakMap is a collection of key-value pairs where keys are objects and values can be arbitrary values. Keys are weakly referenced, meaning they can be garbage collected if there are no other references to them.
56. What is a WeakSet in JavaScript?
A WeakSet is similar to a Set, but it only stores objects and holds “weak” references, allowing objects to be garbage collected.
57. What is function composition?
Function composition is the process of combining two or more functions to produce a new function. The result of each function is passed as the argument to the next.
58. What is a service worker?
A service worker is a script that runs in the background of a browser, enabling features like offline capabilities, push notifications, and background sync without user interaction.
59. What is asynchronous programming?
Asynchronous programming is a technique that allows a program to perform long network requests, file operations, or timers without blocking the main execution thread. JavaScript uses callbacks, promises, and async/await to handle asynchronous operations.
60. What is a microtask queue in JavaScript?
The microtask queue is a queue for tasks that should be executed immediately after the currently executing script and before any rendering or other macro tasks. Promises use the microtask queue.
61. What are macro tasks?
Macro tasks are larger tasks scheduled to be executed later, such as setTimeout, setInterval, or rendering events. They run after the microtask queue is emptied.
62. What is JSON?
JSON (JavaScript Object Notation) is a lightweight data interchange format based on a subset of JavaScript syntax. It is commonly used to transmit data between a server and a client.
63. How do you parse JSON in JavaScript?
Use JSON.parse() to convert a JSON string into a JavaScript object, and JSON.stringify() to convert an object into a JSON string.
const obj = JSON.parse(' ');
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64. What are async iterators?
Async iterators are a special type of iterator that handle asynchronous operations. They are often used with for await...of loops to process data streams sequentially as they arrive.
65. What is for…of loop?
The for...of loop iterates over iterable objects (like arrays, strings, maps, and sets) and returns each value directly without needing to access its index.
66. What is the difference between map and forEach?
map()creates and returns a new array by applying a function to each element.forEach()executes a function for each array element but does not return anything.
67. What is Object.freeze()?
Object.freeze() is used to make an object immutable. It prevents new properties from being added, existing properties from being removed or modified.
68. What is Object.seal()?
Object.seal() prevents new properties from being added to an object but allows modification of existing properties.
69. What is the global object in JavaScript?
The global object is the highest-level object in the environment that provides global functions and variables. In browsers, it is window; in Node.js, it is global.
70. What are the different ways to create an object in JavaScript?
- Object literal syntax:
const obj = - Using
new Object() - Using Object.create()
- Using ES6 classes
71. What is a factory function?
A factory function is a function that returns a new object each time it is called, providing a way to create multiple instances without using classes or constructors.
72. What is a constructor function?
A constructor function is a function intended to be used with the new keyword to create object instances with shared properties and methods.
function Person(name)
const p = new Person('Alice');
73. What are method chaining and fluent interfaces?
Method chaining is the practice of calling multiple methods on the same object consecutively. A fluent interface is an object-oriented API designed to allow method chaining.
74. What is an Immediately Invoked Arrow Function?
It is an arrow function that is executed immediately upon creation, useful for creating a private scope.
(() => )();
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75. What are static methods in JavaScript classes?
Static methods are defined on the class itself and not on instances. They are called using the class name rather than an instance object.
class MathUtils
}
console.log(MathUtils.add(5, 10)); // 15
Advanced JavaScript Interview Questions and Answers
76. What are generator functions?
Generator functions are functions that can be paused and resumed. They are declared with an asterisk (*) and use the yield keyword to return values one by one as requested.
function* generatorFunc()
77. What is the difference between async/await and Promises?
async/await is syntactic sugar over Promises, making asynchronous code look synchronous. While Promises use .then() chaining, await pauses execution until the Promise resolves or rejects.
78. How does memory management work in JavaScript?
JavaScript automatically allocates memory when variables are declared and frees it using garbage collection when they are no longer accessible.
79. What is a memory leak?
A memory leak occurs when memory that is no longer needed is not released. Causes include lingering references in closures, forgotten timers, or global variables.
80. What is the difference between deep copy and shallow copy?
A shallow copy copies the immediate properties of an object but nested objects remain referenced. A deep copy duplicates everything recursively, creating independent copies.
81. What is function currying?
Currying is the process of transforming a function with multiple arguments into a sequence of functions, each taking one argument.
82. What is function throttling?
Throttling limits how often a function can be called in a given timeframe, useful for rate-limiting expensive operations like scrolling and resizing events.
83. What is the difference between prototypal inheritance and classical inheritance?
Prototypal inheritance involves cloning existing objects to create new ones, while classical inheritance involves classes and instances (as seen in languages like Java).
84. What are Web Workers?
Web Workers allow scripts to run in background threads separate from the main execution thread, preventing the UI from freezing during intensive computations.
85. What are Service Workers?
Service Workers are specialized Web Workers that act as a proxy between the web application and the network, enabling features like offline support and push notifications.
86. What is event delegation?
Event delegation attaches a single event listener to a parent element instead of multiple listeners on child elements. It leverages event bubbling to manage all events efficiently.
87. How can you make an object immutable?
Use Object.freeze() to make an object immutable by preventing modification of existing properties and addition of new properties.
88. What is the difference between synchronous and asynchronous code?
Synchronous code blocks further execution until the current task completes, while asynchronous code allows the program to continue running while waiting for the task to complete.
89. What are JavaScript modules?
Modules help organize code into reusable components. Using export and import keywords, modules enable splitting functionality across multiple files.
90. What is dynamic import?
Dynamic imports allow you to load modules asynchronously using the import() function, optimizing load times and bundle sizes in modern applications.
91. What is a Symbol in JavaScript?
A Symbol is a unique and immutable primitive value often used as the key for object properties to avoid property name collisions.
92. What is garbage collection?
Garbage collection automatically reclaims memory occupied by objects that are no longer reachable from the code, helping prevent memory leaks.
93. What are callback hell and how to avoid it?
Callback hell is the situation where callbacks are nested within other callbacks, making code difficult to read and maintain. It can be avoided using Promises or async/await.
94. What is event bubbling?
Event bubbling means that events propagate from the innermost target element up through its ancestors in the DOM hierarchy.
95. What are design patterns in JavaScript?
Design patterns are proven solutions to common software design problems. Examples include Singleton, Factory, Observer, and Module patterns.
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96. What is the singleton pattern?
The Singleton Pattern ensures that a class has only one instance and provides a global point of access to it.
97. What is a closure?
A closure is a function that captures variables from its surrounding lexical scope, even after that outer function has returned.
98. What is debouncing?
Debouncing limits the rate at which a function is executed by ensuring it only runs after a specified time has elapsed since the last event.
99. What are ternary operators?
A ternary operator is a shorthand for an if-else statement. Syntax: condition ? exprIfTrue : exprIfFalse.
const result = (age >= 18) ? 'Adult' : 'Minor';
100. What is an event target?
The event target is the DOM element on which the event was originally dispatched. It can be accessed using event.target inside event handlers.
Frequently Asked Questions (FAQ) on Javascript Interview Questions
1. What are the most common JavaScript interview questions asked to freshers?
Freshers are often asked about basic concepts such as variables, data types, control flow, loops, functions, arrays, objects, DOM manipulation, and basic asynchronous programming with callbacks.
2. What advanced JavaScript topics are important for interviews in 2025?
Closures, Promises, Async/Await, Event Loop, Prototypal Inheritance, Functional Programming Patterns, ES6+ Features, Web APIs, and Performance Optimization are crucial for advanced roles.
3. How can I best prepare for JavaScript coding interviews?
Practice writing clean JavaScript code, solve algorithm challenges, work with modern JavaScript libraries and frameworks, understand asynchronous programming thoroughly, and build small projects to demonstrate understanding.
4. What is the difference between == and === in JavaScript?
== compares values after type coercion, while === compares both value and type strictly, making it a safer and preferred choice in most cases.
5. Is JavaScript still in demand in 2025?
Yes, JavaScript remains one of the top programming languages in 2025, crucial for frontend development (React, Vue, Angular), backend (Node.js), mobile app development (React Native), and emerging technologies like Web3 and PWAs.
