20 Javascript interview questions with code answers.

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I am going to start a series for the JavaScript interview questions which may help junior to mid level developers.

Below are 20 common JavaScript ES6+ interview questions with examples.

1. What is let and const in JavaScript?

let and const are block-scoped declarations in JavaScript, used to declare variables. let allows you to reassign the value of the variable, while const creates a read-only reference to a value.

Example:

let message = "Hello, World!";
message = "Hello, JavaScript!";
console.log(message); // Output: "Hello, JavaScript!"
const PI = 3.14;
PI = 3.14159; // TypeError: Assignment to constant variable.

2. What is arrow function in JavaScript?

Arrow functions are a shorthand for anonymous functions in JavaScript. They are also known as “fat arrow” functions.

Example:

let add = (a, b) => a + b;
console.log(add(1, 2)); // Output: 3
let numbers = [1, 2, 3, 4, 5];
let doubledNumbers = numbers.map(num => num * 2);
console.log(doubledNumbers); // Output: [2, 4, 6, 8, 10]

3. What is default parameter in JavaScript?

Default parameters allow function parameters to be initialized with default values if…