|this| is basically a magical variable that by default is set to the global object, but can also be set by the caller of a function one of two ways:
a) by setting a reference to the function on an object, and calling it with object or bracket notation:
var someObj = {};
someObj.f = theFunction;
someObj.f(); // now |this| inside theFunction will
// point to someObj
someObj["f"](); // same thing
b) by using apply:
theFunction.apply(someObj);
You are right that in the case of event handlers, it is common in the DOM for event handlers to get their |this| set to the DOM node that fired the event. Since the default value of |this| is the global object, it isn't possible in the case of setTimeout for us to tell whether the caller (the host) explicitly set it, or if the function is being called without any value set for |this|.
Basically, |this| is an extra parameter to any function. It is controlled by the caller, and as such, should either be documented as part of the signature of the function, or else should not be relied on by the implementation of the function.
The common misunderstanding with people new to JS is that they expect |this| to be controlled by the callee, like it is in lots of other popular languages, and are surprised when calling convention can change its value.
BTW, imo, the behavior of |this| is a really unfortunate design flaw of JS. The amount of time that must have been spent learning and teaching this edge case over and over to every person new to the language is ... well, it's big.
The behavior of 'this' is from DOM bindings and not from the language per se. Javascript can set 'this' of any calling context to any object with 'apply' or 'call'. The convention for on* handlers is that 'this' is set to the DOM element when the handlers are called. I only learned to appreciate this, when I wrote my own little JS DOM framework a la JQuery or Dojo.
I believe actually that 'this' points to the global object.
So if there is no obvious thing for this to point at, it'll point at the global object. Its just implementation detail that in browsers it points to the window object.
Do you know why it was designed this way? Was 'new' added to javascript after the behavior of 'this' was already well defined and the creators did not feel comfortable changing it?
actually, no. As I understand it, the language was supposed to be some sort of weird conglomeration between traditional object oriented languages and dynamic, lisp-ish languages. The problem was that the creators had about 30 seconds to get this done and they could make things look more "normal" by not explicitly supporting every feature they envisioned.
"new", I believe, is actually an ancient relic of the prototypal class system. That's another topic entirely, but it's incredibly cool. Check out Douglas Crockford's articles on inheritance if you want closure on the issue. He has written way more than I have on the topic.
"new", I believe, is actually an ancient relic of the prototypal class system.
I believe it is quite the opposite. It was a failed attempt to make the prototype-based object system look like class-based one. I vaguely remember reading this opinion in Crockford's "Javascript:The Good Parts" book.
<script> alert(this == window); function foo() { alert(this == window); } foo(); </script>
|this| is basically a magical variable that by default is set to the global object, but can also be set by the caller of a function one of two ways:
a) by setting a reference to the function on an object, and calling it with object or bracket notation: var someObj = {}; someObj.f = theFunction; someObj.f(); // now |this| inside theFunction will // point to someObj someObj["f"](); // same thing
b) by using apply: theFunction.apply(someObj);
You are right that in the case of event handlers, it is common in the DOM for event handlers to get their |this| set to the DOM node that fired the event. Since the default value of |this| is the global object, it isn't possible in the case of setTimeout for us to tell whether the caller (the host) explicitly set it, or if the function is being called without any value set for |this|.
Basically, |this| is an extra parameter to any function. It is controlled by the caller, and as such, should either be documented as part of the signature of the function, or else should not be relied on by the implementation of the function.
The common misunderstanding with people new to JS is that they expect |this| to be controlled by the callee, like it is in lots of other popular languages, and are surprised when calling convention can change its value.