On a more focused note, the final function doesn’t account for null, which is (sadly) typeof ‘object’. Also, you can use for..in for both arrays and objects.

function deepCopy(o) {
    var copy = o,k;
 
    if (o && typeof o === 'object') {
        copy = Object.prototype.toString.call(o) === '[object Array]' ? [] : {};
        for (k in o) {
            copy[k] = deepCopy(o[k]);
        }
    }
 
    return copy;
}

This will handle sparse arrays and cases of arrays treated as objects (e.g. var a = []; a.dontDoThis = ‘you should use an Object’)

Functions generally don’t contain data or anything unique for that matter, so there’s nothing to copy; their functionality cannot be changed in any way once the function has been created. So you can essentially copy a function (although not really “copy”) just by re-assigning it to a different variable:

var fn = function(){
    alert(5);
};
 
var fn2 = fn;
// Even though fn === fn2 it doesn't matter because
// a function cannot be changed.
// All you can do with a function is run it.

But note that, because functions are objects, unique properties can be added and if that happens then you have to treat it like a regular object when copying.

Thanks for your posts, always useful.

Thanks for your comment! 🙂

On a side note, I’ve noticed that you always use arguments.callee rather than ever referring to the name of the function. I like your style – low coupling. Good stuff.