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  • Array findLast() and findLastIndex()

Array findLast() and findLastIndex()

# Sebastian Malton (7 years ago)

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# Jordan Harband (7 years ago)

indexOf and reduce are the only iteration methods that have "reverse" versions - I think to add any others, a clear case would have to be made that all of them shouldn't get the same treatment.

What's your use case?

# Sebastian Malton (7 years ago)

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# kdex (7 years ago)

I'm thinking that this may have performance benefits. The naïve approach would be something like:

function findLast(array, fn) {
	return array.reverse().find(fn);
}

Note that this requires a reversed copy of array to reside in memory before find can be run, which is not a particularly useful thing to do when all you want is to start the search from another direction.

Instead, you would have to do something like:

function findLast(array, predicate) {
	for (let i = array.length - 1; i >= 0; --i) {
		const x = array[i];
		if (predicate(x)) {
			return x;
		}
	}
}

which does the job, I guess. Apart from performance considerations, I would find it practical if all methods that work in some direction had a counterpart for the other direction, c.f. trim{Start,End}, pad{Start,End}, reduce{,Right}. It would be consistent, would it not?

# Sebastian Malton (7 years ago)

After some research iterators cannot be used because of lack of functions on them. However, for brevity and efficiency these sorts of functions should still exist.

Sebastian

Original Message   From: kdex at kdex.de Sent: August 4, 2017 3:10 PM To: es-discuss at mozilla.org Subject: Re: Array findLast() and findLastIndex()

I'm thinking that this may have performance benefits. The naïve approach would be something like:

function findLast(array, fn) {
return array.reverse().find(fn);
}

Note that this requires a reversed copy of array to reside in memory before find can be run, which is not a particularly useful thing to do when all you want is to start the search from another direction.

Instead, you would have to do something like:

function findLast(array, predicate) {
for (let i = array.length - 1; i >= 0; --i) {

const x = array[i];
if (predicate(x)) {
return x;
}
}
}

which does the job, I guess. Apart from performance considerations, I would find it practical if all methods that work in some direction had a counterpart for the other direction, c.f. trim{Start,End}, pad{Start,End}, reduce{,Right}. It would be consistent, would it not?