What is the difference between null and undefined in JavaScript?

Question

I want to know what the difference is between null and undefined in JavaScript.

Answer 1

For the undefined type, there is one and only one value: undefined.

For the null type, there is one and only one value: null.

So for both of them, the label is both its type and its value.

The difference between them. For example:

• null is an empty value
• undefined is a missing value

Or:

• undefined hasn't had a value yet
• null had a value and doesn't anymore

Actually, null is a special keyword, not an identifier, and thus you cannot treat it as a variable to assign to.

However, undefined is an identifier. In both non-strict mode and strict mode, however, you can create a local variable of the name undefined. But this is one terrible idea!

function foo() {
    undefined = 2; // bad idea!
}

foo();

function foo() {
    "use strict";
    undefined = 2; // TypeError!
}

foo();