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

I want to add a very subtle difference between null and undefined which is good to know when you are trying to learn Vanilla JavaScript(JS) from ground up:

• null is a reserved keyword in JS while undefined is a variable on the global object of the run-time environment you're in.

While writing code, this difference is not identifiable as both null and undefined are always used in RHS of a JavaScript statement. But when you use them in LHS of an expression then you can observe this difference easily. So JS interpreter interprets the below code as error:

var null = 'foo'

It gives below error:

Uncaught SyntaxError: Unexpected token null

While below code runs successfully although I won't recommend doing so in real life:

var undefined = 'bar'

This works because undefined is a variable on the global object (browser window object in case of client-side JS)