ECMAScript proposal: .findLast() and .findLastIndex() search an array from end to end

Author: knaagar
Translator: Front-end Xiaozhi Source: dev

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find array element

Below are three ways to find an array element from start to end.

method one:

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

Method Two:

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

Method three:

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

The latest proposal introduces the findLast and findIndex methods, which are used as follows:

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

3. Simple implementation

Next, let's simply implement .findLast() and .findLastIndex() :

.findLast()

 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;
}

.findLastIndex()

 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;
}

polyfill

If you want to experience it in advance, you can introduce it in core-js.

Address: https://github.com/tc39/proposal-array-find-from-last#polyfill

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The bugs that may exist after the code is deployed cannot be known in real time. In order to solve these bugs afterwards, a lot of time is spent on log debugging. By the way, I recommend a useful bug monitoring tool , Fundebug .

Original: https://2aliy.com/2022/03/array-find-last.html

comminicate

If you have dreams and dry goods, you can search for [Great Move to the World] on WeChat and pay attention to this Shawanzhi who is still washing dishes in the early hours of the morning.

This article GitHub https://github.com/qq449245884/xiaozhi has been included, there are complete test sites, materials and my series of articles for interviews with first-line manufacturers.