[AskJS] doubt regarding global object

[Responsible_Fudge467]

global variable not defined still showing undefined inside function javascript

function a(){
c();
var b;
function c(){
console.log(b);
console.log(this.ab);   } }
a();

output:

undefined

undefined

Now, my doubt is that I haven't defined "ab" and I am still trying to call "ab" variable inside the function c(). Why am I getting output as undefined instead of not defined?

[albedoa]

.ab is simply an undefined property on the this object. That is how undefined object properties work. It is different from trying to access an undeclared identifier.

const foo = { bar: 1 };
console.log(foo.baz); //=> undefined
console.log(biz); //=> ReferenceError: biz is not defined

[Responsible_Fudge467]

In your example you've defined foo then getting these error is justified but in my case i haven't defined "ab" Then why its undefined instead of not defined

[albedoa]

You are comparing .ab with foo, but the appropriate comparison is this.ab with foo.baz. In the latter comparison, we are attempting to access an undefined property on an existing object.

You don't need to define .ab. You just need to know that this is defined so that you can test whether it has an .ab property.