Does Javascript filter preserve order?

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.