如何使用RequireJS实现工厂设计模式？

Question

I have something like this...

define(['ClassA', 'ClassB', 'ClassC'], 
  function(ClassA, ClassB, ClassC) 
  {
    return {
      build: function(className) {
        var obj;
        switch(className)
        {
            case 'ClassA': obj = new ClassA(); break;
            case 'ClassB': obj = new ClassB(); break;
            case 'ClassC': obj = new ClassC(); break;
        }
        return obj;
    }
  }
}

This seems ok but is there a better way to write it? I tried replacing switch to

return new arguments[className]();    // doesn't work

The closest I can get to is to use a map:

var classes = {
    ClassA: ClassA,
    ClassB: ClassB,
    ClassC: ClassC
}
return new classes[className]();

Is there a better way?

Answer 1

There's really no problem with your object there.
It's fast, efficient and easy to follow.
The only suggestion I'd have is to create a var to hold the value, or to test more-explicitly if className is a suitable string, and is in fact in your list:

Either:

var construct = classes[className];
if (construct) { return new construct(); }
else { /* handle the case where the class doesn't exist */ }

Or:

return classes[className] && (new classes[className]()) || null;

The second checks for classes[className] and should return a new instance if it exists (JS returns the value on the very right of AND)...
OR it will return null .

Maybe null isn't what you want.
But the point is that you should be prepared to handle somebody passing: "Bob" to your factory, despite the fact that there is no Bob class.

Answer 2

Using Function Modules as Factories :

define(function(require){
    var classes = {
        'ClassA' : require('ClassA'),
        'ClassB' : require('ClassB'),
        'ClassC' : require('ClassC')
    };

    return function(className){
        try {
            return new classes[className];
        } catch(error) {
            throw new Error('Unknown className Specified.');
        }
    }
});