Checking if a property exists in "window" fails if "window." is not prepended

Question

I want to check if a property exists in window, so I did this:

This code works:

if (window.foo) {
    alert("exists");
}
else {
    alert("doesn't exist");
}

Output:

doesn't exist

I thought the next code should work too because as far as I know when you define variables and you are not inside a function they are created as a property of the "window" object so this should be equivalent:

if (foo) { //it fails here "foo is not defined"
    alert("exists");
} else {
    alert("doesn't exist");
}

To my surprise it didn't work. My question is why it doesn't work if I don't prepend window?

Answer 1

The first version is property access, it can't find the property so it returns undefined. The second is trying to access undefined variable .

Use what elclanrs suggested:

if(typeof foo === "undefined"){
    alert("exists");
} else {
    alert("doesn't exist");
}

Why?

This is because of the GetValue specified in the language specification.

.3. If IsUnresolvableReference(V), throw a ReferenceError exception.

This is what happens in the if(foo) case, since foo was not defined before.

On the other hand, if its an object the following happens:

1. Let base be the result of calling GetBase(V).
2. Let O be ToObject(base).
3. Let desc be the result of calling the [[GetProperty]] internal method of O with property name P.
4. If desc is undefined, return undefined.

So window.foo returns the primitive language value undefined which is falsy.