SebastianVerified account

Ok, let's see if we can find someone with a practical use case.https://twitter.com/bmeurer/status/938270217364439041 …

Short answer, it supports subclassing of arrays. Inherited methods should not produce hard-coded Array instances.

Like all polymorphic operations, should be specialized and in-lined when used in hot loops.

I remember the story behind it from the ES6 days. I personally never understood why these things had to be made that complex nevertheless. Subclassing of builtins adds a lot of complexity and trying to keep the generic methods working is a great source of security bugs.

Also in my personal experience, just trying to explain how Array[@@species] works today and how that affects Array builtins constantly confuses developers. The intuitive behavior seems to be the ES5 behavior that Array#map just creates an Array always.

That being said, I'm not the best measure for these things. So there might be valid, real use cases for Array[Symbol.species] (which will be terribly slow in all engines as @jfbastien points out correctly).

Sounds like you're saying engines don't know how to optimize hot polymorphic JS code.

patches welcome