What's the difference between window.x and x?

Question

Let's say, "x" is a variable that never defined, so it should be undefined. In the following scenario:
1)

if(x){//do something}
//ReferenceError: x is not defined

2)

if(window.x){//do something}
//worked as expected

As far as I know, in browser environment here, x should be the same as window.x, could anyone help to point out the reason for the different behavior here?

Answer 1

x will only be the same as window.x if it was declared (in the global scope, naturally). That can be done with an explicit var statement in the global scope, or a simple assignment without var in any scope (which is considered an implicit global declaration):

var a; // declares global a
function foo() {
    b = 10; // declares (implict) global b
}

Both a and b will also be available as window.a and window.b in web browsers.

This also creates a global variable on browsers:

window.c = 20; // can be accessed as just c

Now, trying to access a non-existing variable throws a ReferenceError, while trying to access a non-existing object property just returns undefined.

Interesting fact: globals created with var can't be removed with the delete operator, while implicit globals and those created as properties of the global object can.