Does Javascript filter preserve order?

Question

I looked at the documentation and while looking at examples it looks like filter preserves order of the original list (though it returns a new one). Can I rely on that?

Ecmascript spec

MDN

Neither of these reference preserving order. Should I just assume I can't rely on preserved order?

Answer 1

Yes. From the spec,

• Let selected be the result of calling the [[Call]] internal method of callbackfn with T as the this value and argument list containing kValue, k, and O.

• If ToBoolean(selected) is true, then

  • Call the [[DefineOwnProperty]] internal method of A with arguments ToString(to), Property Descriptor {[[Value]]: kValue, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}, and false.
  • Increase to by 1.

So the items in the returned array have the same order than in the original one.

Answer 2

Yes, the .filter() method returns a new array, without the filtered elements in the same order as initially.

The order of the elements is one of the main feature of a array.