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Proposal for.findLast() and.findLastIndex() methods on array and typed array.

This is astage 4 proposal.

Finding an element in an array is a very common programming pattern.

The proposal has a major concerns:Semantical. Which meansclearly representing the operation i want.

And with the changes. There's a sugar here: Performance. Avoid obvious overhead. And may improve the constant factors in the time complexity.Even there's not an order of magnitude change. But it's may useful in some performance-sensitive scenarios. eg: React render function.

ECMAScript currently supports{Array, %TypedArray%}.prototype.indexOf and{Array, %TypedArray%}.prototype.lastIndexOf to find an index of somevalue in the array.

There is also{Array, %TypedArray%}.prototype.find and{Array, %TypedArray%}.prototype.findIndex to find an element or its index in the array that satisfies a provided condition.

However, the language does not provide a method to find an elementfrom the last to the first of an array with a condition function.

[...[]].reverse().find() is a workaround but there are two issues:

1. unnecessary mutation (by reverse).
2. unnecessary copy (to avoid mutation)

For.findIndex(), you are required to perform additional steps after calling the method (re-calculate the index and handle the-1) to calculate the result of[...arr].reverse().findIndex().

Therefore there is a third issue:

3. complex index calculation

So, perhaps we need something directly and effectively. In this proposal, they are{Array, %TypedArray%}.prototype.findLast and{Array, %TypedArray%}.prototype.findLastIndex.

• You know find from last may have better performance (The target element on the tail of the array, could append withpush orconcat in a queue or stack, eg: recently matched time point in a timeline).
• You care about the order of the elements (May have duplicate item in the array, eg: last odd in the list of numbers).
• Etc.

Add{Array, %TypedArray%}.prototype.findLast and{Array, %TypedArray%}.prototype.findLastIndex.

This would behave the same asArray.prototype.find andArray.prototype.findIndex but would iterate from the last to the first.

eg:

constarray=[{value:1},{value:2},{value:3},{value:4}];array.find(n=>n.value%2===1);// { value: 1 }array.findIndex(n=>n.value%2===1);// 0// ======== Before the proposal ===========// find[...array].reverse().find(n=>n.value%2===1);// { value: 3 }// findIndexarray.length-1-[...array].reverse().findIndex(n=>n.value%2===1);// 2array.length-1-[...array].reverse().findIndex(n=>n.value===42);// should be -1, but 4// ======== In the proposal ===========// findarray.findLast(n=>n.value%2===1);// { value: 3 }// findIndexarray.findLastIndex(n=>n.value%2===1);// 2array.findLastIndex(n=>n.value===42);// -1

• For stage 1:https://drive.google.com/file/d/1nzO9cjy4YlRa8h6ntTJ4Is8mx--sTuL_/view
• For stage 2:https://drive.google.com/file/d/1rhER8TZ5GsHDzl8nLvo8qSIQCUXPw3AQ/view
• For stage 3:https://kingwl.github.io/proposal-array-find-from-last-looking-for-stage-3-sides
• For stage 4:https://github.com/DanielRosenwasser/findLast-and-findLastIndex-for-Stage-4/blob/10cfbc10e155641e3260c68c4d5902ee28de116a/findLast%20%26%20findLastIndex%20for%20Stage%204%20(TC39%20June%202022).pdf

• core-js: you can find it in theECMAScript proposals section
• es-shims:array.prototype.findlast /array.prototype.findlastindex

• lodash.findLast
• lodash.findLastIndex
• ramda.findLast
• ramda.findLastIndex
• TypeScript internal findLast
• TypeScript internal findLastIndex
• @extra-array/find-right

Champions:

• @Kingwl (Wenlu Wang, KWL)
• @DanielRosenwasser (Daniel Rosenwasser, DRR)