Why is "this" in an anonymous function undefined when using strict?

Question

Why is this in an anonymous function undefined when using javascript in strict mode? I understand why this could make sense, but I couldn't find any concrete answer.

Example:

(function () {
    "use strict";

    this.foo = "bar"; // *this* is undefined, why?
}());

Test in a fiddle: http://jsfiddle.net/Pyr5g/1/ Check out the logger (firebug).

Answer 1

It's because, until ECMAscript 262 edition 5, there was a big confusion if people who where using the constructor pattern, forgot to use the new keyword. If you forgot to use new when calling a constructor function in ES3, this referenced the global object (window in a browser) and you would clobber the global object with variables.

That was terrible behavior and so people at ECMA decided, just to set this to undefined.

Example:

function myConstructor() {
    this.a = 'foo';
    this.b = 'bar';
}

myInstance     = new myConstructor(); // all cool, all fine. a and b were created in a new local object
myBadInstance  = myConstructor(); // oh my gosh, we just created a, and b on the window object

The last line would throw an error in ES5 strict

"TypeError: this is undefined"

(which is a much better behavior)