JavaScript Questions

Question 1

What is the difference between let and const?

Answer 1

let allows reassignment, while const does not allow reassignment.

const ensures immutability of the variable binding, not the value itself.

Question 2

What is a callback?

Answer 2

A callback is a function passed as an argument to another function, executed once a task is complete.

Question 3

When would you use a callback?

Answer 3

When handling asynchronous code, such as fetching data from an API or dealing with user interactions.

Question 4

What is the difference between == and ===?

Answer 4

== performs loose comparison with type coercion, while === performs strict comparison without type coercion.

Question 5

What is hoisting?

Answer 5

Hoisting is a JavaScript behavior where variable and function declarations are moved to the top of their containing scope.

Question 6

What is a closure?

Answer 6

A closure is a function that retains access to its lexical scope even when executed outside of that scope.

Question 7

What is the event loop?

Answer 7

The event loop is a mechanism in JavaScript that allows non-blocking, asynchronous code execution.

Question 8

When is it a good idea to use a class?

Answer 8

When you need to create objects that share common behavior or properties, especially in object-oriented programming.

Question 9

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

Answer 9

var is function-scoped and can be re-declared; let and const are block-scoped. let allows reassignment, const does not.

Question 10

What is event delegation in JavaScript?

Answer 10

Event delegation is attaching a single event listener to a parent element to handle events from child elements.

Question 11

What is a closure in JavaScript?

Answer 11

A closure retains access to variables from its lexical scope after that scope has finished executing.

Question 12

What is the event loop and how does it work?

Answer 12

The event loop checks the call stack and processes tasks in the callback queue when the stack is empty.

Question 13

Explain JavaScript’s asynchronous programming model.

Answer 13

JavaScript uses Promises and async/await for asynchronous operations, making code look synchronous.

Question 14

What is the difference between null and undefined?

Answer 14

null represents intentional absence of a value; undefined means a variable is declared but not assigned.

Question 15

Explain the concept of hoisting in JavaScript.

Answer 15

Hoisting moves variable and function declarations to the top of their scope, but only declarations are hoisted.

Question 16

What are higher-order functions in JavaScript?

Answer 16

Higher-order functions take other functions as arguments or return functions.

Question 17

Give an example of a higher-order function.

Answer 17

Example: function sayHi(fn, name) { console.log(fn(name)); }

Question 18

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

Answer 18

var: Declares a variable with function scope (or global scope if declared outside a function). It is hoisted to the top of its scope and can be re-assigned.

let: Declares a variable with block scope (within curly braces). It is hoisted but not initialized, so it has a “temporal dead zone” until assigned.

const: Declares a variable with block scope that cannot be re-assigned after initialization. It is also hoisted, but it must be initialized at the time of declaration

Question 19

What are closures in JavaScript, and how do they work?

Answer 19

A closure is a function that retains access to its lexical scope, even when the function is executed outside that scope. Closures are created when a function is defined inside another function and the inner function has access to the outer function’s variables.

Question 20

What is the event loop in JavaScript, and how does it work?

Answer 20

The event loop is the mechanism that allows JavaScript to execute asynchronous code. JavaScript is single-threaded, and the event loop enables non-blocking I/O operations. When an asynchronous operation (e.g., setTimeout(), network requests) is triggered, the callback is added to the event queue. The event loop continuously checks the call stack and moves items from the event queue to the stack when it’s empty.

Question 21

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

Answer 21

== (Loose equality): Compares two values for equality, with type coercion. It will try to convert values to the same type before comparison.

=== (Strict equality): Compares both the value and the type without type coercion.

Question 22

What are promises in JavaScript, and how do you use them?

Answer 22

A Promise represents the eventual completion (or failure) of an asynchronous operation and its resulting value. It has three states: pending, fulfilled, and rejected.

Question 23

What is the this keyword in JavaScript, and how does it behave in different contexts?

Answer 23

Global scope: this refers to the global object (in browsers, it’s window).

Object method: this refers to the object the method is called on.

Arrow functions: this refers to the lexical scope (it doesn’t create its own this).

Question 24

Explain the concept of “hoisting” in JavaScript

Answer 24

Hoisting is JavaScript’s behavior of moving declarations (not initializations) to the top of their containing scope during compilation. This applies to both variables and functions.

Question 25

What is a callback function, and how is it used in JavaScript?

Answer 25

A callback function is a function passed as an argument to another function, which is executed after the completion of some operation (typically asynchronous).

Question 26

What is an IIFE (Immediately Invoked Function Expression), and when would you use it?

Answer 26

An IIFE is a function that is defined and invoked immediately after its creation. It's often used to create a private scope, avoiding polluting the global namespace.

Question 27

What are higher-order functions in JavaScript?

Answer 27

A higher-order function is a function that takes one or more functions as arguments or returns a function as a result.

Question 28

What is the difference between null and undefined in JavaScript?

Answer 28

undefined: A variable that has been declared but not assigned a value is undefined. null: An explicit assignment indicating "no value" or "empty value."

Question 29

What are arrow functions, and how do they differ from regular functions?

Answer 29

Arrow functions have a more concise syntax and lexical scoping for this, meaning they inherit this from the surrounding context.

Question 30

What is the spread operator (...) in JavaScript, and how is it used?

Answer 30

The spread operator allows you to unpack values from arrays or objects. It's used to copy, merge, or spread elements.

Question 31

What is destructuring in JavaScript?

Answer 31

Destructuring allows you to unpack values from arrays or properties from objects into distinct variables.

Question 32

What is the bind() method in JavaScript?

Answer 32

The bind() method creates a new function that, when called, has its this keyword set to the provided value, with a given sequence of arguments preceding any provided to the function when it’s invoked.

Question 33

What are template literals in JavaScript?

Answer 33

Template literals (or template strings) allow for multi-line strings and embedding expressions inside strings.

Question 34

What is the setTimeout() function, and how is it used?

Answer 34

setTimeout() is used to execute a function or a piece of code after a specified delay (in milliseconds).

Question 35

What is JSON.stringify() and JSON.parse()?

Answer 35

JSON.stringify(): Converts a JavaScript object into a JSON string. JSON.parse(): Converts a JSON string into a JavaScript object.

Question 36

What are async/await in JavaScript?

Answer 36

async/await is a modern syntax for working with promises. async is used to declare an asynchronous function, and await is used inside the function to pause execution until a promise is resolved

Question 37

What is the difference between synchronous and asynchronous code in JavaScript?

Answer 37

Synchronous code: Executes one line after another, blocking further execution until the current task completes. Asynchronous code: Executes tasks independently, allowing other tasks to run while waiting for an operation (like fetching data).