js (v8) rest params and gc

Question

does rest params allocate array

function foo(a, b, ...rest) {
/*...*/
}

in example above its obvoius that array created, but what about this case

function foo(...rest) {
/*...*/
}

there rest technicly the same as arguments

function foo() {
   console.log.apply(null, arguments)
}

so, if i want avoid generate garbage could i prefer to use arguments or not ?
also, does arguments exists always or only when keyword used in function scope?
(if always, i assume thet using it is probably better, if not, is there any difference betwin rest and arguments?)

Answer 1

rest technicly the same as arguments

Not really. Rest parameters create a proper array, while the arguments object famously is not an array but needs to be converted to one. And in sloppy mode, it behaves even weirder, aliasing the variables that are declared as parameters.

Does arguments exists always or only when keyword used in function scope?

It is created only when any code in the function refers to it - this is a relatively trivial optimisation with big impact, not having to allocate a new object for every single function call. And in fact, this optimisation goes even further in V8, sometimes not even instantiating the object even code refers to it, see What's the deal with optimising arguments? and Crankshaft vs arguments object by Vyacheslav Egorov for details.

Is there any difference, should i prefer to use arguments?

In modern code, you should generally prefer rest parameters. They are cleaner (less weird behavior), more declarative (easier to understand), and are actual arrays (so that you can use the usual array methods). They are the idiomatic choice.