Indexing is a little bit special in ECMAScript languages like AS3, AS2, and JavaScript. MXMLC will gleefully let you index just about anything, even if there isn’t a chance it’ll work.

Consider the normal indexing one might do instead of using the dot operator:

var obj:Object = {name:"Jackson", profession="coder"};
obj["name"]; // equivalent to

This is handy because you can index with any string, not a known property. In this example, a loop is able to handle many properties on object, not just the ones the programmer knows ahead of time:

// Make an object with 100x100 gray shapes
var grays:Object = {};
for (var i:int = 0; i < 256; ++i)
	var cur:Shape = new Shape(); | (i << 8) | (i << 16); // same in all channels, 0, 100, 100);;
	grays["gray " + i] = cur;

This is a great feature that would require a specially-created data structure (like a map or a hash) in other languages (like C or Java). Then, of course, there’s these little bits of strangeness I discovered this week:

var n:Number = 3;
n["hello"] = true;
3["hello"] = true;

Rather than giving a compiler error or warning, these throw an error at runtime:

ReferenceError: Error #1069

So, don’t think you can go adding on fields to basic types like Number or String like you can in other languages like Ruby. Instead, unfortunately, all I have to give you is another thing to watch out for. So watch out!