Javascript call object methods from array?

Question

I am trying to loop through an array of gameobjects and call their update methods. Gameobjects can have different update implementations (eg: update of enemy is different from update of friend), so I created an prototype inheritance chain. But I can't get it to work: while looping through all objects I don't seem to be able to call their update methods: the compiler says they don't exist. So my question is: is it possible in Javascript to loop trough an array of objects that share the same base class and call a method on them that can be overwritten by different sub-classes?

This is what I have so far, don't know where I went wrong...:

//base gameobject class 
function GameObject(name) {
    this.name = name
};
GameObject.prototype.update = function(deltaTime) {
    throw new Error("can't call abstract method!")
};

//enemy inherits from gameobject
function Enemy() {
    GameObject.apply(this, arguments)
};
Enemy.prototype = new GameObject();
Enemy.prototype.constructor = Enemy;
Enemy.prototype.update = function(deltaTime) {
    alert("In update of Enemy " + this.name);
};


var gameobjects = new Array();

// add enemy to array
gameobjects[gameobjects.length] = new Enemy("weirdenemy");

// this doesn't work: says 'gameobject doesn't have update method'
for (gameobject in gameobjects) {
    gameobject.update(1); // doesn't work!!
}

thefourtheye · Accepted Answer · 2014-12-13 15:08:42Z

1

Its not a problem with your Inheritance chain, but with this construct

for(gameobject in gameobjects){
    gameobject.update(1); // doesn't work!!
}

When you iterate an Array with for..in, the variable will have the index values only. So, gameobject will have 0, 1.. like that, in every iteration. It is not recommended to use for..in to iterate an Array.

You might want to use, Array.prototype.forEach, like this

gameobjects.forEach(function(gameObject) {
    gameObject.update(1);
});

edited Dec 13, 2014 at 15:08

answered Dec 13, 2014 at 15:03

thefourtheye

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1 Comment

svanheste Over a year ago

Thank you, it works now. I was looking for the problem in the wrong place.

Pointy · Accepted Answer · 2014-12-13 15:03:34Z

0

When you iterate through an array with for ... in, the values of the loop variable will be the keys to the array, not the values.

You really shouldn't iterate through arrays that way anyway:

for (var i = 0; i < gameobjects.length; ++i)
    gameobjects[i].update(1);

answered Dec 13, 2014 at 15:03

Pointy

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Comments

null · Accepted Answer · 2014-12-13 15:05:44Z

0

Try this, it works for me: =)

 gameobjects.forEach(function(gameobject){
        gameobject.update(1); // doesn't work!!
    });

answered Dec 13, 2014 at 15:05

null

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