JavaScript Interview Question #13

What is null, and how is it different from undefined?

JavaScript Fundamentals, Types, Scope & Functions Junior Beginner

Quick Interview Answer

undefined usually represents a value that has not been assigned or is missing, while null is normally used intentionally to represent the absence of a value.

Detailed Explanation

undefined usually represents a value that has not been assigned or is missing, while null is normally used intentionally to represent the absence of a value.

For example, a declared variable without an assigned value is undefined. A developer might explicitly assign null when a value has been checked and intentionally cleared. Although null == undefined is true because of loose equality rules, null === undefined is false because they are different types.

Code Example

let firstName;
const selectedUser = null;

console.log(firstName);               // undefined
console.log(selectedUser);            // null
console.log(null == undefined);       // true
console.log(null === undefined);      // false