object oriented with java script

Question

below is my code. I want that Player1,2 will be objects of proto RPSPlayer and that each one would have separate 'history' array. But now, somehow every time i run Player1.play() and Player2.play() they both updates the same 'history' array and shares it. Thanks!

function RPSPlayer() {}
    RPSPlayer.prototype.numOfWins = 0;
    RPSPlayer.prototype.history = [];
    RPSPlayer.prototype.myCurrPick;
    RPSPlayer.prototype.getW = function () {return this.numOfWins;};
    RPSPlayer.prototype.choose = function (item) {
        this.history.push(item);
 };

Player1 = new RPSPlayer();
Player1.play = function(){
    var randomnumber=Math.floor(Math.random()*3);
    if(randomnumber==0){this.currPick = "rock";}
    else if(randomnumber==1){this.currPick = "paper";}
    else{this.currPick = "scissors";}
    this.choose(this.currPick);
    return this.currPick;
};

Player2 = new RPSPlayer();
Player2.play = function(){
    this.choose("rock");
    this.currPick = "rock";
    return "rock";
};

Felix Kling · Accepted Answer · 2011-11-19 00:53:56Z

2

You have to initialize instance properties in the constructor:

function RPSPlayer() {
    this.history = [];
}

Every value assigned to a property of RPSPlayer.prototype is shared among all instances (which is one of the purposes of the prototype).

answered Nov 19, 2011 at 0:53

Felix Kling

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1 Comment

Sanich Over a year ago

How is it possible that Player1 and Player2 had different 'numOfWins'? (in my first implementation)

Alex Wayne · Accepted Answer · 2011-11-19 01:03:45Z

2

Anything on the prototype of a constructor is shared with ALL instantiations. For functions, this is awesome. It's very efficient to use the same functions for multiple objects. But for data is less awesome because objects no longer have a fresh copy for each one.

It's especially confusing because the prototype is where you put "instance" methods, right? But just realize that all instantiation share the same function objects, and the functions are just called with a different "this".

Solve this by initializing instance level properties in your constructor.

var Foo = function() {
    this.someProperty = someValue;
};

Check this snippet out, which demonstrates the difference.

var Foo = function() {
    this.instanceArray = [];
};
Foo.prototype.sharedArray = [];

var a = new Foo();
a.bar = function() {
    this.instanceArray.push('a');
    this.sharedArray.push('a');
};
a.bar();

var b = new Foo();
b.bar = function() {
    console.log(this.instanceArray[0]); // undefined
    console.log(this.sharedArray[0]);   // "a"
};
b.bar();

http://jsfiddle.net/nf3Jm/3/

edited Nov 19, 2011 at 1:03

answered Nov 19, 2011 at 0:57

Alex Wayne

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3 Comments

Sanich Over a year ago

Thanks! But How is it possible that Player1 and Player2 had different 'numOfWins'? (in my first implementation)

Alex Wayne Over a year ago

If you do instance.property = value you set property on the instance, sort of writing over the prototype value. No matter what was there before, this will always set a property only on that instance. This is because when an object doesn't have a property itself, it looks up the prototype to see if there is one there. Assigning a property set that on the instance object, and it never crawls up to the prototype.

Alex Wayne Over a year ago

Oh, and lastly do not modify the prototype from the constructor itself. Either create your prototype methods outside the constructor, or use this.foo = function(){} from within the constructor.

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