Question

I recently read somewhere (I am really sorry I can’t provide the source) that you can use this.varname to access globals in replacement to window.varname to save 2 chars

var myVar = "global";
function myFunc() {
    var myVar = "notGlobal";
    alert( this.myVar ); //global
}

It seems to work, but I want to know if:

• it is safe to use in old browsers
• it is cross-browser compatible
• it will fail under some weird circumstances

Answer 1

I don’t think I’d do it, but it’s completely cross-browser compatible if this is referring to the global object (window). Whether it is will depend on how the function in question was called (at global scope, this does indeed refer to the global object), and whether the code in question is in “strict mode” or not. (In strict mode, this does not refer to the global object. Kudos and upvotes to Esailija for pointing that out.)

In non-strict code:

So at global scope:

console.log(this === window); // true if not in strict mode

And similarly, if you have a function you call directly:

function foo() {
   console.log(this === window);
}

foo(); // Logs "true"

But, in JavaScript, this is set entirely by how a function is called. So we could call foo setting this to something else:

var obj = {};
foo.call(obj); // Now it logs "false", `this` === `obj` during the call

Similarly:

var obj = {};
obj.f = foo;
obj.f(); // Also logs "false", `this` === `obj` during the call

So in conclusion, at global scope (not in any function call), yes, this is reliably pointing to the global object, and if you control how the function gets called and you call it without setting this to anything else (via call or apply, or by using it from an object property a’la obj.f above), then, again, it will reliably refer to the global object. This is covered by sections 10.4.1 (Entering Global Code) and 10.4.3 (Entering Function Code) of the specification. This is how it’s been from the very beginning, I believe; certainly for the last 15 years, so you’re unlikely to find a non-compliant environment.

More reading:

• Mythical methods
• You must remember this