What is the difference between null and undefined?

What You'll Learn

Difference between null and undefined
When each is used
Checking for both

Comparison

Aspect	undefined	null
Type	undefined	object
Meaning	Not assigned	Intentionally empty
Default	Variable declaration	Must be assigned

Examples

code.jsJavaScript

// undefined - automatically assigned
let x;
console.log(x); // undefined

function test(a) {
  console.log(a); // undefined if not passed
}

const obj = {};
console.log(obj.prop); // undefined

// null - intentionally assigned
let y = null;

function findUser(id) {
  // Return null if not found
  return users.find(u => u.id === id) || null;
}

Type Checking

code.jsJavaScript

typeof undefined // 'undefined'
typeof null      // 'object' (historical bug)

// Equality
null == undefined  // true
null === undefined // false

// Check for both
if (value == null) {
  // Catches both null and undefined
}

// Modern approach
value ?? 'default' // Nullish coalescing

Nullish Coalescing (??)

code.jsJavaScript

const a = null ?? 'default';    // 'default'
const b = undefined ?? 'default'; // 'default'
const c = 0 ?? 'default';       // 0
const d = '' ?? 'default';      // ''