most asked javascript interview questions

Here are some of the most commonly asked JavaScript interview questions and answers:

1. What is JavaScript and its uses?

JavaScript is a high-level, dynamic, and interpreted programming language that is primarily used for client-side scripting on the web. It's used for creating interactive web pages, web applications, and mobile applications.

2. What is the difference between null and undefined in JavaScript?

null represents a deliberate absence of any object value.
undefined represents an uninitialized variable or a non-existent property.

Example:

let x = null; // x is explicitly set to null
let y; // y is undefined (not initialized)

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

== checks for equality in value only.
=== checks for equality in both value and type.

Example:

console.log(5 == '5'); // true
console.log(5 === '5'); // false

4. What is a closure in JavaScript?

A closure is a function that has access to its own scope and the scope of its outer functions. It's used to create private variables and functions.

Example:

function outer() {
  let x = 10;
  function inner() {
    console.log(x); // has access to outer's scope
  }
  return inner;
}
const innerFunc = outer();
innerFunc(); // outputs 10

5. How do you create a new object in JavaScript?

There are several ways to create a new object in JavaScript:

Using the Object constructor: let obj = new Object();
Using object literals: let obj = {};
Using the Object.create() method: let obj = Object.create(null);

6. What is the difference between var, let, and const in JavaScript?

var is function-scoped and can be redeclared.
let is block-scoped and cannot be redeclared.
const is block-scoped and cannot be redeclared or reassigned.

Example:

var x = 10;
let y = 20;
const z = 30;

console.log(x); // 10
console.log(y); // 20
console.log(z); // 30

x = 40; // no error
y = 50; // no error
z = 60; // error (cannot reassign const)

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

The this keyword refers to the current execution context of a function. It's used to access properties and methods of an object.

Example:

function Person(name) {
  this.name = name;
  this.sayHello = function() {
    console.log(`Hello, my name is ${this.name}`);
  };
}
const person = new Person('John');
person.sayHello(); // outputs "Hello, my name is John"

8. How do you handle errors in JavaScript?

JavaScript provides a built-in try-catch block to handle errors.

Example:

try {
  let x = 10 / 0; // will throw an error
} catch (error) {
  console.log(`Error: ${error.message}`);
}

9. What is the difference between synchronous and asynchronous programming in JavaScript?

Synchronous programming executes code in a linear sequence.
Asynchronous programming executes code in parallel, using callbacks or promises.

Example:

// synchronous
console.log('1');
console.log('2');
console.log('3');

// asynchronous (using callbacks)
setTimeout(() => {
  console.log('1');
}, 1000);
console.log('2');
console.log('3');

10. What is a promise in JavaScript?

A promise is an object that represents the eventual completion (or failure) of an asynchronous operation.

Example:

const promise = new Promise((resolve, reject) => {
  setTimeout(() => {
    resolve('Hello, world!');
  }, 2000);
});
promise.then((message) => {
  console.log(message); // outputs "Hello, world!" after 2 seconds
});

11. How do you use the map() method in JavaScript?

The map() method creates a new array with the results of applying a provided function to every element in the original array.

Example:

const numbers = [1, 2, 3, 4, 5];
const doubledNumbers = numbers.map((number) => number * 2);
console.log(doubledNumbers); // outputs [2, 4, 6, 8, 10]

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most asked javascript questions along with detailed explanations