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Answer 1

+4 A:

According to this article, there are three ways to define a class in JavaScript:

1 Using a function

Example:

 function Apple (type) {
     this.type = type;
     this.color = "red";
     this.getInfo = getAppleInfo;
 }

 function getAppleInfo() {
     return this.color + ' ' + this.type + ' apple';
 }


 var apple = new Apple('macintosh');
 apple.color = "reddish";
 alert(apple.getInfo());

2 Using JSON

 var apple = {
     type: "macintosh",
     color: "red",
     getInfo: function () {
         return this.color + ' ' + this.type + ' apple';
     }
 }


 apple.color = "reddish";
 alert(apple.getInfo());

3 Singleton using a function

 var apple = new function() {
     this.type = "macintosh";
     this.color = "red";
     this.getInfo = function () {
         return this.color + ' ' + this.type + ' apple';
     };
 }


 apple.color = "reddish";
 alert(apple.getInfo());

Jan Kuboschek 2010-06-13 23:23:23

Yeah, I seen this article, but when I tried using the 3nd style it didn't work at all. That's the one I need to use cause the 2rd is a per variable class.

William 2010-06-13 23:29:06

what variables are you talking about? be more specific. you are being way too vague.

meder 2010-06-13 23:30:07

@William please clarify - you want to use the 3rd or the 2nd? :)

Jan Kuboschek 2010-06-13 23:31:55

Your second example is not JSON. There is no such thing as a function in JSON.

friedo 2010-06-13 23:34:14

Answer 2

+2 A:

The var makes a private variable, do prefix with this instead in your constructor:

        function Player () {
            this.speed = 5;
            this.x = 50;
            this.y = 50;
            var pri = 'private';
            this.update = function() {
                  if(keys[37]) this.x -= this.speed;
                  if(keys[39]) this.x += this.speed;
            }
        }

        var player = new Player;
        alert( player.speed ) // should alert 5
        alert( player.pri ) // should fail or say undefined

You can also do...

      var player = {
           speed: 5,
           x:50,
           y:50,
           update: function() {
               // code
           }
      }

And then get rid of new Player and the Player constructor.

meder 2010-06-13 23:31:11

"The var makes a private variable" - you mean local?

Jan Kuboschek 2010-06-14 00:06:07

It's both local and private.

meder 2010-06-14 00:07:44

Answer 3

+1 A:

You're using Player like a constructor, but it is not set up like one.

Rather than using var inside the constructor function, like var speed = 5; you need to use

this.speed=5;

Then it wil return an instance of Player. As it is, you're just setting some variables and returning nothing in particular.

Now, as far as learning JS object creation and inheritance, I suggest checking out Douglas Crockford's resources. As you may know, it's not intended to be class-based like Java, PHP, Python and so on. JavaScript has prototypal inheritance based on cloning objects that already exist.

Crockford discusses doing class-based inheritance in JS in this older article. The problem is, you're not using JS to it's best trying to do that. This treatise may be interesting where he explains one way of cloning objects. That is the Object.beget method, which is good, but has limits as well. The best way is the 'functional' method. Sorry for the PowerPoint links, but you should read this: http://www.crockford.com/codecamp/The%20Good%20Parts%20ppt/4%20prototypal.ppt and this: http://www.crockford.com/codecamp/The%20Good%20Parts%20ppt/5%20functional.ppt

http://video.yahoo.com/watch/630959/2974197 is one version of a video where Crockford discusses the ins and outs of JS. http://www.youtube.com/watch?v=hQVTIJBZook is another of the same.

I really recommend getting the book JavaScript: The Good Parts for a thorough run down of pragmatic advanced JavaScript.

Alex JL 2010-06-13 23:38:44

To add to that: [JavaScript. The Definitive Guide](http://www.amazon.com/JavaScript-Definitive-Guide-David-Flanagan/dp/0596101996/ref=dp_ob_title_bk) is perhaps better suited for someone who doesn't know JavaScript's OOP already.

Marcel Korpel 2010-06-13 23:53:07

That book is quite good and thorough, also. The difference is that it covers the DOM and browsers thoroughly, while The Good Parts is strictly about the language itself.

Alex JL 2010-06-14 16:59:19

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How do I create a class in Javascript?

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