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 convert the arguments Object into a real Array (as much as JS has real arrays anyway) and it allows for whatever array methods you have in your Array prototypes to be applied to it, etc etc.

I remember reading somewhere that accessing the arguments Object directly can be significantly slower than an Array clone or than the obvious choice of named arguments. Is there any truth to that and under what circumstances / browsers does it incur a performance penalty to do so? Any articles on the subject you know of?

update interesting find from http://bonsaiden.github.com/JavaScript-Garden/#function.arguments that invalidates what I read previously... Hoping the question gets some more answers from the likes of @Ivo Wetzel who wrote this.

At the bottom of that section it says:

Performance myths and truths

The arguments object is always created with the only two exceptions being the cases where it is declared as a name inside of a function or one of its formal parameters. It does not matter whether it is used or not.

this goes in conflict with http://www.jspatterns.com/arguments-considered-harmful/, which states:

However, it's not a good idea to use arguments for the reasons of :

• performance
• security

The arguments object is not automatically created every time the function is called, the JavaScript engine will only create it on-demand, if it's used. And that creation is not free in terms of performance. The difference between using arguments vs. not using it could be anywhere between 1.5 times to 4 times slower, depending on the browser

clearly, can't both be correct, so which one is it?

ECMA die-hard Dmitrty Soshnikov said:

Which exactly “JavaScript engine” is meant? Where did you get this exact info? Although, it can be true in some implementations (yep, it’s the good optimization as all needed info about the context is available on parsing the code, so there’s no need to create arguments object if it was not found on parsing), but as you know ECMA-262-3 statements, that arguments object is created each time on entering the execution context.

Answer 1

Here's some q&d testing. Using predefined arguments seems to be the fastest, but it's not always feasible to do this. If the arity of the function is unknown beforehand (so, if a function can or must receive a variable amount of arguments), I think calling Array.prototype.slice once would be the most efficient way, because in that case the performance loss of using the arguments object is the most minimal.