Question

Is it (in a performant way) possible to define “fallback” methods in JavaScript?

For example

function MyObject () {
   /* what do i have to add here to have my defaultMethod? */
}

var obj = new MyObject ();
obj.doesntExistInMyObject (); // I want defaultMethod to be called
obj.doesntExistEither (); // I want defaultMethod to be called, too

I.e.: I want defaultMethod to be called whenever I write obj.calledMethod (); and obj.calledMethod == undefined, but I do not want to check for undefined in the calling code.

Answer 1

JavaScript doesn’t currently have that feature. It may well in the next version, though, via proxies. So until then, to do this you’d have to do something fairly ugly, like:

MyObject.prototype.ex = function(fname) {
    var f    = this[fname],
        args = Array.prototype.slice.call(arguments, 1);
    if (typeof f === "function") {
        return f.apply(this, args);
    }
    return this.defaultMethod.apply(this, args);
};

…and use it like this:

var obj = new MyObject();
obj.ex("doesntExistInMyObject", "arg", "arg");

(ex for “execute”, since call is too easily confused with Function#call.)