Question

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?

Answer 1

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.