Top 20 JavaScript Interview Questions and Answers

Discover the Top 20 JavaScript Interview Questions and Answers to help you ace your next interview. Master key concepts and boost your confidence with Java.

 

1. What is JavaScript?

Answer: JavaScript is a high-level, interpreted programming language that is primarily used to create interactive and dynamic content on web pages. It is an essential part of web development, alongside HTML and CSS.

 

2. What are the different data types in JavaScript?

Answer: JavaScript supports several data types, including:

Primitive types: string, number, boolean, null, undefined, symbol, and bigint

Objects: Object, Array, Function, Date, RegExp, and more.

 

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

Answer: var: Function-scoped or globally-scoped if declared outside a function, can be re-declared and updated.

let: Block-scoped, can be updated but not re-declared within the same scope.

const: Block-scoped, cannot be updated or re-declared. However, the properties of objects declared with const can be modified.

 

4. What is a closure in JavaScript?

Answer: A closure is a function that retains access to its lexical scope, even when the function is executed outside that scope. This allows a function to “remember” the environment in which it was created.

 

5. Explain event delegation in JavaScript.

Answer: Event delegation is a technique where a single event listener is added to a parent element to manage events for its child elements. This is efficient as it reduces the number of event listeners and utilizes event bubbling.

 

6.What is the difference between == and === in JavaScript?

Answer: == (loose equality) checks for equality after performing type conversion if necessary, while === (strict equality) checks for equality without type conversion, meaning both value and type must be the same.

 

7. What is the purpose of this keyword in JavaScript?

Answer: The this keyword refers to the object it belongs to. Its value depends on the context in which it is used, such as within a method, a function, or in global scope.

 

8. What are promises in JavaScript?

Answer: Promises are objects that represent 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.

 

9. What is an arrow function and how is it different from a regular function?

Answer: Arrow functions provide a shorter syntax for writing functions and do not have their own binding. Instead, this is lexically inherited from the surrounding scope.

// Regular function

function add(a, b) {

  return a + b;

}

 

// Arrow function

const add = (a, b) => a + b;

 

10. What are the different ways to create an object in JavaScript?

Answer: Objects can be created in several ways:

Using object literals:

const obj = { key: ‘value’ };

Using the new keyword with a constructor function:

function Person(name) {

  this.name = name;

}

const person1 = new Person(‘John’);

Using Object.create() method:

const proto = { key: ‘value’ };

const obj = Object.create(proto);

 

11. What is asynchronous programming and how does JavaScript handle it?

Answer: Asynchronous programming allows the program to perform tasks without blocking the main thread. JavaScript handles asynchronous operations using callbacks, promises, and async/await syntax.

 

12. What is the event loop in JavaScript?

Answer: The event loop is a mechanism that allows JavaScript to perform non-blocking I/O operations, despite being single-threaded. It continuously checks the call stack and the task queue to execute code, collect events, and execute queued tasks.

 

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

Answer: call: Invokes a function with a specified value and arguments provided individually.

apply: Invokes a function with a specified value and arguments provided as an array.

bind: Creates a new function that, when called, has its this value set to a specified value, with a given sequence of arguments preceding any provided when the new function is called.

function greet(greeting, punctuation) {

  console.log(greeting + ‘, ‘ + this.name + punctuation);

}

const person = { name: ‘Alice’ };

 

greet.call(person, ‘Hello’, ‘!’); // Hello, Alice!

greet.apply(person, [‘Hi’, ‘.’]); // Hi, Alice.

const greetPerson = greet.bind(person, ‘Hey’);

greetPerson(‘?’); // Hey, Alice?

 

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14. Explain hoisting in JavaScript.

Answer: Hoisting is a JavaScript mechanism where variables and function declarations are moved to the top of their containing scope during the compilation phase. This means variables declared with var and functions can be used before their actual declaration in the code.

 

15. What is the difference between synchronous and asynchronous functions?

Answer: Synchronous functions are executed sequentially, blocking the execution of subsequent code until they finish. Asynchronous functions allow the program to continue executing subsequent code while they complete in the background, typically using callbacks, promises, or async/await for handling the result.

 

16. What is an event loop? What is the precedence of the event loop?

Answer: The event loop is a fundamental concept in JavaScript that allows for asynchronous programming by managing

the execution of code, collecting and processing events, and executing queued sub-tasks. It's a part of the

JavaScript runtime environment and is crucial for handling operations that might take some time to complete,

such as I/O operations, without blocking the main thread.

 

17. How the Event Loop Works

Answer: Call Stack: This is where the JavaScript engine keeps track of function calls. When a function is invoked, it's added

to the call stack. When the function completes, it's removed from the stack.

 

Web APIs: These are browser-provided APIs that handle operations like setTimeout, DOM events, HTTP requests

(via XMLHttpRequest or Fetch), and more. When these operations are completed, they send their callbacks to

the task queue.

Task Queue (or Callback Queue): This queue contains callbacks for events like user interactions, timers, or

network responses. These tasks are processed after the current execution context (call stack) is empty.

Microtask Queue: This is a special queue for microtasks, which include promises and MutationObserver

callbacks. Microtasks are given higher priority over tasks in the task queue.

Event Loop: The event loop continuously checks the call stack to see if it's empty. If the call stack is empty, it

looks at the microtask queue first and processes all microtasks before moving to the task queue. This ensures

that promises and other microtasks are handled as soon as possible.

 

18. Among setTimeout() and promise() which one will take precedence?

Answer: In JavaScript, when comparing setTimeout() and Promise (or more specifically, the .then() or .catch() methods of

a promise), promises take precedence over setTimeout() due to the way the event loop handles microtasks and macrotasks.

Explanation

Promises: When a promise resolves, its .then() and .catch() handlers are placed in the microtask queue.

Microtasks are given higher priority and are executed before any tasks in the macrotask queue.

setTimeout(): The callback function passed to setTimeout() is placed in the macrotask queue (also known as the

task queue). Tasks in this queue are processed only after all the microtasks have been completed.

Event Loop Order of Operations

Execute all code in the current call stack.

Process all the tasks in the microtask queue.

Process the next task from the macrotask queue.

Example

Consider the following code snippet to demonstrate the precedence:

console.log('Start');

setTimeout(() => {

console.log('setTimeout');

}, 0);

Promise.resolve().then(() => {

console.log('Promise');

});

console.log('End');

Execution Flow:

Call Stack: Execute console.log('Start') which logs "Start".

 

Call Stack: Execute setTimeout which schedules the callback in the macrotask queue.

Call Stack: Execute Promise.resolve().then(…) which schedules the .then() callback in the microtask queue.

Call Stack: Execute console.log('End') which logs "End".

After the call stack is empty:

Microtasks: Process the promise's .then() callback which logs "Promise".

Macrotasks: Process the setTimeout callback which logs "setTimeout".

So, the output will be:

Start

End

Promise

setTimeout

Summary

Promises and their .then() handlers are placed in the microtask queue and are executed before the tasks in the

macrotask queue.

setTimeout() callbacks are placed in the macrotask queue and are executed after all microtasks have been

processed.

Therefore, promises take precedence over setTimeout() in the event loop.

 

19. Where do you mostly use the rest operator?

Answer: The rest operator (…) in JavaScript is used to handle function arguments, array elements, and object properties

in a flexible way. Here are the primary scenarios where the rest operator is commonly used:

1. Function Parameters

The rest operator allows you to represent an indefinite number of arguments as an array. This is especially useful

when creating functions that accept a variable number of arguments.

Example:

function sum(…numbers) {

return numbers.reduce((total, num) => total + num, 0);

}

console.log(sum(1, 2, 3)); // 6

console.log(sum(4, 5, 6, 7)); // 22

2. Destructuring Arrays

The rest operator can be used in array destructuring to capture the remaining elements into a new array.

Example:

const [first, second, …rest] = [1, 2, 3, 4, 5];

console.log(first); // 1

console.log(second); // 2

console.log(rest); // [3, 4, 5]

3. Destructuring Objects

Similarly, the rest operator can be used in object destructuring to collect the remaining properties into a new

object.

 

Example:

const { a, b, …rest } = { a: 1, b: 2, c: 3, d: 4 };

console.log(a); // 1

console.log(b); // 2

console.log(rest); // { c: 3, d: 4 }

4. Function Arguments

When dealing with function arguments, especially in functions where you don't know how many arguments will

be passed, the rest operator can help you manage them efficiently.

Example:

function logArguments(…args) {

args.forEach(arg => console.log(arg));

}

logArguments('one', 'two', 'three');

// one

// two

// three

Summary

The rest operator is versatile and simplifies handling variable numbers of parameters and properties in functions,

arrays, and objects. Its main uses are:

Function Parameters: Capturing all arguments passed to a function.

Array Destructuring: Collecting remaining elements in an array.

Object Destructuring: Collecting remaining properties in an object.

Managing Function Arguments: Simplifying the handling of a variable number of arguments.

Using the rest operator makes code more concise and readable, especially when dealing with collections of data or dynamic parameters.

 

20. What are shallow copy and deep copy?

Answer: In JavaScript, the concepts of shallow copy and deep copy refer to the ways in which objects or arrays are

duplicated. The key difference lies in how these copies handle nested objects or arrays.

Shallow Copy

A shallow copy of an object or array is a new object or array that has copies of the references to the original

object or array's elements. If the original object contains other objects or arrays as its properties, the shallow

copy will have references to those same objects or arrays, not duplicates.

Deep Copy

A deep copy of an object or array is a new object or array that is an entirely independent copy of the original.

This means that all levels of nested objects or arrays are also copied, resulting in a completely separate object or

array that shares no references with the original.

Methods for Creating Copies

Shallow Copy Methods:

Object.assign():

let shallowCopy = Object.assign({}, originalObject);

Spread Operator (…):

 

let shallowCopy = { …originalObject };

let shallowCopyArray = […originalArray];

Array.prototype.slice():

 

let shallowCopyArray = originalArray.slice();

Deep Copy Methods:

Manual Recursion:

Write a custom function to recursively copy nested objects or arrays. This method is the most flexible and can

handle special cases, but it is more complex to implement.

 

function deepCopy(obj) {

if (obj === null || typeof obj !== 'object') return obj;

if (Array.isArray(obj)) {

return obj.map(item => deepCopy(item));

}

const copy = {};

for (let key in obj) {

if (obj.hasOwnProperty(key)) {

copy[key] = deepCopy(obj[key]);

}

}

return copy;

}

JSON.parse(JSON.stringify()):

This method is simpler and works well for plain objects and arrays but has limitations. It cannot handle functions,

undefined, Infinity, NaN, and other non-serializable data.

 

let deepCopy = JSON.parse(JSON.stringify(originalObject));

Summary

Shallow Copy: Creates a new object or array with copies of the original's references. Changes to nested objects

or arrays will affect both the original and the copy.

Deep Copy: Creates a new object or array that is entirely independent of the original, including all nested objects

or arrays. Changes to the deep copy will not affect the original.

Understanding the difference between shallow and deep copies is crucial for managing data in JavaScript,

especially when dealing with complex objects and nested structures.

 

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