addMethod function from Secrets of the Javascript Ninja

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Javascript

Hi, guys, this is my first time here and I've been reading the book by John Resig - Secrets of the Javascript Ninja. I have come across the addMethod function that takes in 3 parameters and helps with function overloading. I am familiar with function overloading but i don't understand how this function works, it is small but complex.

function addMethod(object, name, fn)
{
    var old = object[name];
    object[name] = function()
    {
        if( fn.length == arguments.length )
        {
            return fn.apply(this, arguments);
        }
        else if
        {
            return old.apply(this, arguments);
        }
    }
}

// object - object to be acted on
// name - name of the function, i presume
// fn - callback function, i presume

Now I don't understand how this function is working, it can't work without the if and else if statements, i tried and i get 'undefined' in the console.

Please explain how the function works.