A thing you can do with javascript's function.length

gistfile1.js

// Implementation of the router pattern using function.length
var add_two_things = function(a, b) {
    return a + b
}

var add_three_things = function(a, b, c) {
    return a + b + c
}

var get_router = function() {
    // The user doesn't need to know which function they need to add 
    // 2 things vs. 3 things: they just want a single function they can pass 2 or 3 
    // numbers to...
    var arg_map, router, i;
    arg_map = {}
    for (i = 0; i < arguments.length; i++) {
        arg_map[arguments[i].length] = arguments[i];
    }
    router = function() {
        var num_args, args;
        num_args = arguments.length
        if (arg_map[num_args]) {
            // Convert arguments to an array.
            args = Array.prototype.slice.call(arguments, 0);
            return arg_map[num_args].apply(window, args);
        }
    }
    return router;
}

add_router = get_router(add_two_things, add_three_things)
alert(add_router(2,3));
alert(add_router(2,3,4));