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I have seen a piece of code that I can't figure out:

for (var m in client.actorTypes[d[5]]) {
   if (m !== 'update' && m !== 'destroy' && m !== 'remove') {
      this[m] = client.actorTypes[d[5]][m];
   }
}

but actorTypes is not a 2D array:

Game.prototype.BaseActor = function(rate) {
   this.updateRate   = rate;
   this.onCreate     = function(data, complete) {};
   this.onUpdate     = function(data) {};
   this.onEvent      = function(data) {};
   this.onInterleave = function() {};
   this.onDraw       = function() {};
   this.onDestroy    = function(complete) {};
};

Game.prototype.Actor = function(id, rate) {
   return this.$.actorTypes[id] = new this.BaseActor(rate);
};

I actually don't know what happens in this code. Can someone explain it to me? What is a this array, and how could actorTypes become a 2d array?

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3 Answers 3

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Arrays have nothing to do with it.

In Javascript, you can access object properties in one of two ways:

  1. theObject.theProperty
  2. theObject['theProperty']

Method 1 is only possible with a literal, valid variable name; method 2 allows you to use an arbitrary expression (e.g. in your case, a string variable).

Arrays are a special case of Javascript objects, which happen to have numerically-named properties. We use method 2 to access them because valid variable names may not start with (or be solely) numbers.

That doesn't mean that every time you see x[y], x must be an array, because that's not the case at all.

A clarifying example follows:

var o = {
   'a': {
      'b': 5
   }
};

var x = 'a', y = 'b';

console.log(o.a.b);       // Output: 5
console.log(o['a']['b']); // Output: 5
console.log(o[x][y]);     // Output: 5

All of those three are equivalent, and o is still not an array.

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6 Comments

aah so actually this[m]=xx means this.m=xx as actortypes[id][m]=xx means actortypes[id].m=xx ? am i right?
@deniz: Almost. this.m is the same as this['m'], but instead you're getting this[m] which is the property whose name is the same as the contents of the variable m, not actually 'm'. There's an extra level of indirection.
for ex m = "asd", so i actually define this["asd"] (this."asd") = xx ?
@deniz: Essentially, yes. Except that this."asd" is not valid syntax, which is why this["asd"] exists instead.
Thanks. How does "for in" work for classes(i mean functions) ? Does it get ALL of the elements including functions ?
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BaseActor and Actor are the methods attached to the Game object. The this inside Actor nothing but the reference to the object instance. So whenever a Game object will be instantiated, it will have these BaseActor and Actor methods attached to it. For e.g

var obj = new Game();

the return this.$.actorTypes[id] = new this.BaseActor(rate); inside the Actor will set the value of 'id' inside the actorTypes array object to the BaseActor object instance hence consisting of all the properties of that class.

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Arrays and objects in Javascript are very similar. You can use for( var i in object ) to loop through members of an object, like you would loop through an array with for( var i = 0; i < array.length; i++ ). Similarly, you can use array access on an object: object.property is the same as object['property'].

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6 Comments

Actually, arrays are rather just objects with numeric property names.
actually i have no problem about "for..in", i couldnt get this array and usage of actortypes
You can use for( var i in object ) to loop through members of an object as if they were an array. Except you should never loop through a Javascript Array in such a manner.
@LightnessRacesinOrbit No, you shouldn't. I didn't mean to imply that you should loop through arrays in that manner, just that you can loop through objects this way.
@RyanP: The "as if they were an array" suggests otherwise, but ok glad to hear it.
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