Earlier quoted context omitted.
Using continuations. The function call itself and the local scope wrapped up into it can be varying degrees of expensive. When little state, because it’s inline or nearby for example, it might be optimized down to near what a for loop does. But if there’s a function call at all once it’s executed, that alone is slower than the for loop. Remember it’s a function call per element. If the function is further away with m…
If I modify that code to pre-initialise an array with the values 0 to N - 1, and then copy the value from that array to a new array (rather than using the loop index), then both map and forEach are faster than the for loop for me.
Like so?
const N = 1000000
let a = new Array(N), b = new Array(N)
for (let i = 0; i { for (var i = 0; i { b = a.map((x,i) => a[i]) })
test('copy forEach', _=> { a.forEach((v,i,a) => b[i] = a[i]) })
Are you using a different browser? I know that sometimes other browsers have very different results.
In any case, it's somewhat irrelevant if there are cases that optimize and cases that don't. When idiomatic functional code is sometimes up to 30x slower than a for loop, it can't be used in perf critical sections. Even if it's only Chrome and only certain cases. The forEach perf needs to be always reliably performant before I can use it without worry.