Feature proposal

[T.J. Crowder]

On Wed, Jul 18, 2018 at 5:44 PM, Cyril Auburtin <cyril.auburtin at gmail.com>
wrote:
> sorry you get 1, and need again to subtract the array length, `arr.length
-1
> - 1` to get the final result 3

So you can't just use the passed-in third argument, and need:

```js
let a = [7, 4, 6, 7, 12];
console.log(a.length - 1 - a.findIndex((_, i) => isPrime(a[a.length-1-i])));
```

Strikes me as a reasonable argument for adding `findLastIndex`. ;-) (And
perhaps `findLast`, since the same trick with `find` just won't work at
all.)

Prior art (looking for instance at the libs that inspired the array
additions in ES5 and ES2015):
* Lodash implements both `findLastIndex` and `findLast`).
* Underscore, PrototypeJS, and MooTools don't as far as I can see from the
docs.

-- T.J. Crowder
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