Question

This is making me angry:
If I don’t do this:

var a;

And then do this:

if(a){
    // doing stuff
}

It spits out this error:

ReferenceError: a is not defined

So how can I check if a variable is defined!

Please don’t tell me to use typeof.

EDIT:

a is defined by a library sometimes. I want to check if the library defined it so if not I define it so I can still use it in my code!

Answer 1

If you want to ensure the a is declared, and not causing any reference error problem. You may add a re-declaration statement.

var a = a;

In this case, if a is defined, nothing changes, otherwise it’s undefined.

NOTICE: this only works when a is in global scope, check out comments for more details.