javascript functions and arguments object, is there a cost involved

Question

It is common place to see code like that around the web and in frameworks:

var args = Array.prototype.slice.call(arguments);

In doing so, you

Answer 1

The arguments has two problems: one is that it's not a real array. The second one is that it can only include all of the arguments, including the ones that were explicitly declared. So for example:

function f(x, y) {
    // arguments also include x and y
}

This is probably the most common problem, that you want to have the rest of the arguments, without the ones that you already have in x and y, so you would like to have something like that:

var rest = arguments.slice(2);

but you can't because it doesn't have the slice method, so you have to apply the Array.prototype.slice manually.

I must say that I haven't seen converting all of the arguments to a real array just for the sake of performance, only as a convenience to call Array methods. I'd have to do some profiling to know what is actually faster, and it may also depend faster for what, but my guess would be that there's not much of a difference if you don't want to call the Array methods in which case you have no choice but to convert it to a real array or apply the methods manually using call or apply.

The good news is that in new versions of ECMAScript (Harmony?) we'll be able to write just this:

function f(x, y, ...rest) {
   // ...
}

and we'll be able to forget all of those ugly workarounds.