Can JavaScript's function having an array.forEach return a value to the caller?

Question

For example:

  function findIndexOfFirstNonNegative(array) {
    array.forEach( (a, i) => { if (a >= 0) { return i; }});
    return null;
  }

it turns out the return i; will not return to the caller of the function findIndexOfFirstNonNegative(), but it will return from the fat arrow function and let the forEach go on (or even if it is a plain old function instead of fat arrow function, it is the same).

Is there a way to break out and return to the caller?

[how to stop Javascript forEach?](//stackoverflow.com/q/6260756), [How to short circuit Array.forEach like calling break?](//stackoverflow.com/q/2641347) — Tushar, Apr 26 '17 at 07:39
You using the wrong method if you wish to find an index of an item. `findIndex` may be better. See [here](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array#Iteration_methods) for full list. — ste2425, Apr 26 '17 at 07:41
You may want to use Array.some for more general purposes. https://developer.mozilla.org/fr/docs/Web/JavaScript/Reference/Objets_globaux/Array/some — FitzFish, Apr 26 '17 at 07:44
@Weedoze You can use it to break the loop by returning true. — FitzFish, Apr 26 '17 at 07:49
The term "fat arrow function" is obsolete. The correct term is "arrow function". — , Apr 26 '17 at 07:49
@torazaburo is it obsolete since when... since 2 weeks ago? (just kidding) — nonopolarity, Apr 26 '17 at 09:32

taile · Accepted Answer · 2017-04-26T07:50:18.110

4

You can use findIndex in Javascript instead

array.findIndex( a => a >= 0);

console.log([-5,-10, 0, 4].findIndex( a => a >= 0))

edited Apr 26 '17 at 07:50

answered Apr 26 '17 at 07:44

taile

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29

looks so beautiful... – nonopolarity Apr 26 '17 at 09:31
Glad to know :) Let accept it if it's the answer you're looking for – taile Apr 26 '17 at 10:13

score 1 · Answer 2 · answered Apr 26 '17 at 07:43

Use a classical loop. This will return to the function when the condition is met.

I added a console.log() inside the loop to show that it will stop after founding the non negative number

function findIndexOfFirstNonNegative(array) {
  for (let i = 0, l = array.length; i < l; i++) {
    console.log("Loop item : " + array[i] + " at index " + i);
    if (array[i] >= 0) return i;
  }
  return -1;
}

console.log("Index found : " + findIndexOfFirstNonNegative([-5, 2, 1]))

OR

You can directly use Array#findIndex()

function findIndexOfFirstNonNegative(array) {
  return array.findIndex(a=>a>=0);
}

console.log("Index found : " + findIndexOfFirstNonNegative([-5, 2, 1]))

Too bad the test is not for less-than-or-equal to zero, because then we could write `a=>a<=0`. — , Apr 26 '17 at 07:52
@torazaburo Or event better `a=>a<=a` but that would be useless — Weedoze, Apr 26 '17 at 07:57

Can JavaScript's function having an array.forEach return a value to the caller?

2 Answers2