null and undefined may seem to be the same value, but they actually have subtle differences in JavaScript.

JavaScript primitive values

In JavaScript, there are six primitive values. Both null and undefined are two of them. The primitive values are:

• Boolean
• Null
• Undefined
• Number
• String
• Symbol

All other values besides these are objects (objects, functions, arrays, etc.).

Falsy values

In JavaScript, there are six falsy values. Both null and undefined are two of the six. The list of falsy values are:

• false
• 0 (zero)
• "" (empty string)
• null
• undefined
• NaN (Not A Number)

All other values in JavaScript are considered truthy.

What is null?

null is an empty or non-existent value. It literally means nothing.

const a = null;

console.log(a); // null

Null must be assigned. If const a was not assigned to null, it would be undefined and not null when we console.log() it.

What is undefined?

Undefined usually means that a variable has been declared, but not defined.

let b;

console.log(b); // undefined

We can also declare a variable to be undefined.

let c = undefined;

console.log(c); // undefined

If we try looking up the non-existent properties in an object, we will receive undefined.

const user = {};

console.log(user.name); // undefined

Equality checks

null does not strictly equal undefined because they are not of the same type. As we saw above, they are two different types of primitive values in JavaScript.

console.log(null !== undefined); // false

However, null loosely equals undefined.

console.log(null == undefined); // true

Triple Equals

Triple equals === tests for strict equality. This means that both the type and the value that we are comparing have to be the same.

Double Equals

Double equals == tests for loose equality and preforms type coercion. It compares two values after converting them to a common type.

Function parameters

Consider the following function that prints out the name that is passed to it as a parameter.

function sayHello(name = 'John') {
  console.log(`Hi ${name}!`);
}

Passing undefined will cause the function to use the default value of the parameter.

sayHello(undefined); // Hi John!

Passing null will not make use the default value of the parameter.

sayHello(null); // Hi null!