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proposal-array-find-from-last

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

Status

This is a stage 4 proposal.

Motivation

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

The proposal has a major concerns: Semantical. Which means clearly 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 some value 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 element from 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:

  1. 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.

Scenarios

Core features

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

This would behave the same as Array.prototype.find and Array.prototype.findIndex but would iterate from the last to the first.

eg:

const array = [{ 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 }

// findIndex
array.length - 1 - [...array].reverse().findIndex(n => n.value % 2 === 1); // 2
array.length - 1 - [...array].reverse().findIndex(n => n.value === 42); // should be -1, but 4

// ======== In the proposal =========== 
// find
array.findLast(n => n.value % 2 === 1); // { value: 3 }

// findIndex
array.findLastIndex(n => n.value % 2 === 1); // 2
array.findLastIndex(n => n.value === 42); // -1

Slides

Polyfill

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