Why `this===window` gives me false?

Question

I've been testing the following code, but Firefox16 and Chrome22 gives me different outcomes.

console.log(this===window); //false in Firefox and true in Chrome
console.log(this.window===window); //true in both Firefox and Chrome
(function(){
    console.log(this===window); //false in Firefox and true in Chrome
    console.log(this.window===window); //true in both Firefox and Chrome
})();

As far as I can remember, Chrome's answer is right: Unless called with new, this is always same as the global object window, which leads to a pattern called scope safe constructors.

Answer 1

It is assumed that window is the global object, but until ES5, there was no specification to define that. It was just convention (i.e. part of "DOM 0"). In ES5 §15.1, there is:

in the HTML document object model the window property of the global object is the global object itself

Which I guess makes it a non–normative part of the ECMAScript standard at least.

For some time, there have been ambiguities between window and the global object, but for most uses, they were synonmuous. It was only certain cases where there were differences.

Regarding the statement:

Unless called with new, this is always same as the global object window

Not at all. A function's this keyword is set by how the function is called. In non–strict mode, if, on entering an execution context, no thisBinding is provided, then it's set to the global object. In strict mode, it is left as is (i.e. it might be undefined, null, 0, anything).

which leads to a pattern called scope safe constructors

I'm not sure what that means. A function's this has nothing to do with scope. When the new operator is used with a function call, then the function acts as a constructor and its this is set to a new object created as if by new Object(). That seems to be more a function of the new operator than a concept of "scope safe constructors".