Ayende @ Rahien

Refunds available at head office

Useful lies about JavaScript Prototype Model

Note: This is not how it works, but it is close enough that it is a valid mental model for most scenarios.

When I think about JavaScript and its type system, I usually envision objects as hash tables that can carry values or functions. The prototype idea is just an associated hash table, and so on. The moment that I settled on this mental model, it was much easier to grok javascript.

Now, that is a vague explanation if I ever heard one, so let us speak in code:

public class Object
{
	private readonly Dictionary<string, Procedure> functions 
		= new Dictionary<string, Procedure>();
	private Object prototype;

	public Object Prototype
	{
		get
		{
			if(prototype==null)
				prototype = new Object();
			return prototype;
		}
		set { prototype = value; }
	}

	public Procedure this[string name]
	{
		get
		{
			if (functions.ContainsKey(name))
				return functions[name];
			if (prototype != null && prototype.functions.ContainsKey(name))
				return prototype[name];
			throw new InvalidOperationException("No function called " + name);
		}
		set { functions[name] = value; }
	}
}

This is a simple example of the matter, and here is how you can use it, if you ignore the fact that C# doesn't have any useful duck typing, then this code looks very much like the one that you would write in JavaScript:

Object obj = new Object();
obj["on_change"] = delegate { Console.WriteLine("changed"); };

obj.Prototype["on_load"] = delegate { Console.WriteLine("loaded from prototype"); };


obj["on_change"]();
obj["on_load"]();

//"overriding" the prototype method
obj["on_load"] = delegate{ Console.WriteLine("loaded from object"); };

obj["on_load"]();

obj["missing_method"]();//will throw

Comments

Ben Reichelt
08/22/2007 11:54 PM by
Ben Reichelt

I think the Prototype property should be static, right?

IIRC, in javascript if you modify the .prototype property, it applies to all instances of the class, rather than just that instance.

Comments have been closed on this topic.