javascript – JS – two array filters vs. one forEach?

I know that the Array prototype method filter is generally preferred over forEach

What makes you say that?

Filters and forEach are two different things; they are not interchangeable. There are situations where you’ll use filter, and the ones where you’ll use forEach.

I believe it is typically faster at the job as well

Don’t believe something is faster. Measure it; this way you’ll know.

So which should be expected to be faster? I imagine there may be […]

Again, don’t imagine. Performance is not about beliefs and what someone imagines. It’s about exact measurements and hard data. You have your both approaches, and that’s great. Now, you just have to run them side by side with the real data, and see which one outperforms the other.

Note the importance to use the real data. If the algorithm is expected to be used for the arrays containing about a dozen elements, don’t compare the measurements using the arrays containing a billion elements. Similarly, if you want to run the algorithm on an array containing billions of elements, don’t take measurements with an array containing a dozen elements, as the performance comparison would be perfectly irrelevant for your actual case.