[mikeryan]

While your example reads a bit better, you’re now iterating through your list twice.

[fwip]

Most code isn't performance-critical.

This pattern will probably be dominated by the time taken to filter and map, not loop iteration.

[saagarjha]

Does JavaScript have adapters to covert lists into generators or lazy iterators?

[hn_throwaway_99]

Unfortunately not in a way that can optimize these methods. This is one reason the Java Streams API, which has a similar purpose (e.g. map, filter, flatMap, reduce, etc.), is a better design IMO because you can chain together method calls without creating (and iterating) the full array at each step.

[londt8]

Technically there is no iteration in filter/map/reduce at all.

[Stratoscope]

That's incorrect. All of those functions are implemented using iteration.

Consider this excerpt from the .map() polyfill on MDN:

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Refe...

    // 7. Let k be 0
    k = 0;

    // 8. Repeat, while k < len
    while (k < len) {
      // [loop body omitted]

      // d. Increase k by 1.
      k++;
    }