Home Grown Script Loader

I was wondering how RequireJS and AMD worked. So I created this snip. It might not be correct but it worked for me.

load('foo', function (foo){
        var f = new foo();
        f.a = 100;
        f.b = 200;
        alert(f.add());
    });

    function define(name, callback) {
        modules[name] = callback;
    }

    function load(file, callback) {
        var script = document.createElement('script');
        script.type = 'text/javascript';
        script.async = true;
        script.src = file + ".js";
        (document.getElementsByTagName('head')[0] || document.getElementsByTagName('body')[0]).appendChild(script);
        script.onload = function() {
            callback(modules[file]);
        };
    }

    var modules = [];

// foo.js
define("foo", function() {
    var foo = {
        a: 1,
        b: 2,
        add: function () {
            return this.a + this.b;
        }
    };
    return foo;
});