ECMAScript proposal: searching Arrays from end to start via .findLast() and .findLastIndex()

This blog post describes the ECMAScript proposal “Array find from last” by Wenlu Wang and Daniel Rosenwasser.

Table of contents:

Topic of this blog post: methods of Arrays and of Typed Arrays  #

In this blog post, we explore methods of Arrays. But everything we learn also applies to Typed Arrays.

Searching Arrays  #

The following three methods search Arrays from start to end:

> ['a', 'b', 'a'].indexOf('a')
0
> ['a', 'b', 'a'].indexOf('c')
-1

> ['a1', 'b', 'a2'].find(x => x.startsWith('a'))
'a1'
> ['a1', 'b', 'a2'].find(x => x.startsWith('c'))
undefined

> ['a1', 'b', 'a2'].findIndex(x => x.startsWith('a'))
0
> ['a1', 'b', 'a2'].findIndex(x => x.startsWith('c'))
-1

So far, only .indexOf() has a version that searches from end to start:

> ['a', 'b', 'a'].lastIndexOf('a')
2

The proposal introduces such versions for .find() and .findIndex():

> ['a1', 'b', 'a2'].findLast(x => x.startsWith('a'))
'a2'

> ['a1', 'b', 'a2'].findLastIndex(x => x.startsWith('a'))
2

Simple implementations  #

The following code shows simple implementations (fewer checks than the actual implementations) of .findLast() and .findLastIndex():

function findLast(arr, callback, thisArg) {
  for (let index = arr.length-1; index >= 0; index--) {
    const value = arr[index];
    if (callback.call(thisArg, value, index, arr)) {
      return value;
    }
  }
  return undefined;
}

function findLastIndex(arr, callback, thisArg) {
  for (let index = arr.length-1; index >= 0; index--) {
    const value = arr[index];
    if (callback.call(thisArg, value, index, arr)) {
      return index;
    }
  }
  return -1;
}

Availability  #

The proposal mentions 2 polyfills that are currently available.

Additionally libraries such as Lodash and Ramda also provide the operations findLast and findLastIndex for Arrays.

Further reading  #

• Section “Searching elements: .find(), .findIndex()” in “JavaScript for impatient programmers”
• Chapter “Arrays” in “JavaScript for impatient programmers”
• Chapter “Typed Arrays: handling binary data” in “JavaScript for impatient programmers”