Fwd: Feature proposal

Dmitry Shulgin shulhindvst at gmail.com
Wed Jul 18 09:17:30 UTC 2018

---------- Forwarded message ----------
From: Dmitry Shulgin <shulhindvst at gmail.com>
Date: 2018-07-05 10:36 GMT+03:00
Subject: Feature proposal
To: es-discuss at mozilla.org

I came across a task of finding an index of the last element in array that
satisfies the condition, so i reversed array and found it by *findIndex *
I was thinking:
Why do we have *findIndex*` method, but no *findLastIndex*?
It seems logical to follow *[index|lastIndex]Of* pair, doesn't it?
Reversing array in this case seems too complicated to me.

So, i would like to suggest new method like


function isPrime(element, index, array) {
  var start = 2;
  while (start <= Math.sqrt(element)) {
    if (element % start++ < 1) {
      return false;
  return element > 1;

console.log([4, 6, 8, 12].findIndex(isPrime)); // -1, not found
console.log([7, 4, 6, 7, 12].findIndexLast(isPrime)); // 3

Would be glad to implement, if it makes sense.

Thx for replies.

P.S. Small issue on GitHub was closed due to offtop (not an issue, as i
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