‘undefined’ that mean the variable hasn’t been set

If you don’t declare variable:

console.log(a)

You will get error:

Uncaught ReferenceError: a is not defined

That’s because that initial execution context creation, that creation phase, when it went through it didn’t find a var a so it never set up the memory space. So when it went to execute this code it said hey, I don’t have a in memory at all. That’s it.