What happens when you assign undefined in JavaScript?

Question

Consider the following code:

var x = undefined;

It's a self-contradicting line of code. Is x defined or not? Do implementations of JavaScript remove the variable x from memory, or do they assign it the value undefined?

Answer 1

There's a difference between a variable being undeclared and being undefined:

var x;     //x is equal to *undefined*
alert(y);  //error, y is undeclared

This isn't self-contradicting, but it is redundant:

var x = undefined;

Think of undefined as simply the value a variable has when it hasn't been initialized – or the value an object property has when it hasn't been initialized or declared.