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'll explain undefined, null and Uncaught ReferenceError:

1 - Uncaught ReferenceError : variable has not been declared in your script, there is no reference to this varaible
2 - undefined: Variable declared but does not initialised
3 - null : Variable declared and is an empty value