An Introduction to JavaScript's "this"

http://justin.harmonize.fm/index.php/2009/09/an-introduction-to-javascripts-this/

by jmtulloss • 17 years ago

39 36 17 years ago

justin.harmonize.fm

mhansen• 17 years ago

The way I remember it: this is whatever is before the dot when you call the function.

E.g. for car.drive(); in the drive function, any reference to 'this' will get 'car'. If the function is called on it's own, as in 'drive()', 'this' will refer to the Global object (bad)

It's also possible to change 'this' by using the Function.prototype.apply() method, which allows you to pass a 'context' parameter that will become 'this'.

judofyr• 17 years ago

Is this supposed to be Ruby?

    class HotDog:
         condiments
         def getCondiments
              return @condiments // @ is a reference to the current instance of HotDog.
         end
    end

jmtulloss7• 17 years ago

sorry, that was typed up pretty quickly, and I'm not a rubyist... care to correct it?

jhancock3• 17 years ago

  class HotDog
         @condiments
         def getCondiments
              return @condiments // @condiments is an instance variable of an instance of HotDog
         end
    end

note the mod to both the ivar def and the comment. You also do not need the initial ivar "definition" as ruby defines them on the fly when they are first referenced.

Bluem004• 17 years ago

This article does a good job of explaining a concept I had trouble with when first using javascript. Now, I usually use the approach recommended in the article of setting a private variable (e.g. self, or my) to the current value of 'this' in a javascript class.

However, I think the article is slightly incorrect on one minor point. It states, "this wants to be window whenever possible, which is not always what you want." I believe the correct description is: the 'this' variable in a function called in response to an event on dom object is set to that dom object. For the example given in the article, this is set to the window object because setTimeout causes an event on window to fire, not because there's any inherent preference for the window object.

aboodman2• 17 years ago

There is indeed a preference for window (really the global) object. Try these:

|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.

aboodman2• 17 years ago

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.

jmtulloss7• 17 years ago

That was a bit inexact. It's kind of a hard point to generalize, I gave it another go.

Bluem004• 17 years ago

Aboodman seems to have a better handle on it than I do. I think what I said about the dom object is correct, but he point out that window is the default value of this for other function calls.

trjordan• 17 years ago

Does anybody have a mirror? It appears to be down for me.

_ck_• 17 years ago

http://addons.mozilla.org/en-US/firefox/addon/2570

oh and

http://google.com/search?q=cache:http%3A%2F%2Fjustin.harmoni...

kwamenum862• 17 years ago

The function: setTimeout(30, function() { myHotDog.getCondiments.apply(myHotDog); });

can actually just be: setTimeout(30, function() { myHotDog.getCondiments(); });

apply is definitely useful but it just makes the code more verbose in this case.

nixy• 17 years ago

Which JS engine allows you to call setTimeout like that? Isn't setTimeout(expression, timeInMs [, args]) the right way to go?

bouncingsoul• 17 years ago

That's correct, but the optional args parameter isn't supported in IE. So if you want to use setTimeout or setInterval to call a function with arguments you have to do it as an anonymous function with your function call inside.

You're right though that milliseconds should be the second argument, not the first.

https://developer.mozilla.org/En/Window.setTimeout

kwamenum862• 17 years ago

Yes my mistake. milliseconds is the first argument. I just copied the function text verbatim. My real point was that you don't need to use apply in this context,, as the author implied.

axod2• 17 years ago

It's not a quirk, it doesn't need "fixing". Get a good javascript book. Still surprising how many people try to learn js by feeling around in the dark or by googling.

jmtulloss7• 17 years ago

I absolutely agree. I tried to put "fixing" in quotes as much as possible. However, you have to see that for many programmers, it is unexpected behavior. In the eyes of many UI specialists, unexpected behavior is the definition of a quirk. JS fanatics have to be accepting of the fact that JS is not a regular language that follows "regular" rules.

axod2• 17 years ago

Sure, I agree... And if you go in #javascript on freenode for a few minutes, you'll see endless questions about why their event handler doesn't work. Endless answers "use a closure".

I just wish people would read a book on the language first, then the behavior would be pretty obvious and expected.

volida4• 17 years ago

This article is wrong. You need to use prototype when defining your functions.

Here is how to do it:

  <html>
  <head>
  <script>

  function HotDog () {
	this.name=arguments[0];
  } 

  HotDog.prototype.name=null;

  HotDog.prototype.getCondiments= function () { 
	document.getElementById('test').innerHTML='started...'  +this.name;
  };
  var myHotDog = new HotDog('test 001');

  window.onload= function() {
	setTimeout(30, myHotDog.getCondiments());
  };

  </script>

  </head>
  <body>
  <div id=test>loading...</div>
  </body>
  </html>

Bluem004• 17 years ago

While you can use prototype, why do you need to? What specifically do you mean by "the article is wrong?"

volida4• 17 years ago

if you dont use prototype the function will not belong το the new object you create, therefore you will not be able το call the function from settimeout, which was the problem the author assumed το have.

jmtulloss7• 17 years ago

I would never not use a prototype in production code, but prototypes are a different topic that I wanted to avoid for this article. I'm just trying to explain "this".

Also, your code isn't right. You're calling myHotDog.getCondiments instead of passing a reference to setTimeout.

volida4• 17 years ago

Sorry, I wasn't clear enough. I didn't mean what you are explaining is wrong.

diN0bot• 17 years ago

one neat way to define a class is like so:

  function MyClass(a, b) {
	// use this function as an instance constructor
	// eg, var m = new MyClass('foo', 'bar');
	this.a = a;
	this.b = b;
  }
  MyClass.prototype = {};
  _extend(MyClass.prototype, {
	instanceMethod: function() {
		...
	},
	...
  });

An Introduction to JavaScript's "this"

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