Find last index of element inside array by certain condition

Question

Let's say I have an array of objects:

[   { 'a': 'something',     'b':12 },   { 'a': 'something',     'b':12 },   { 'a': 'somethingElse', 'b':12 },   { 'a': 'something',     'b':12 },   { 'a': 'somethingElse', 'b':12 } ] 

What would be the cleanest way to get the last index of an element where a has the value 'something' - in this case index 3? Is there any way to avoid loops?

Answer 1

Here's a reusable typescript version which mirrors the signature of the ES2015 findIndex function:

/** * Returns the index of the last element in the array where predicate is true, and -1 * otherwise. * @param array The source array to search in * @param predicate find calls predicate once for each element of the array, in descending * order, until it finds one where predicate returns true. If such an element is found, * findLastIndex immediately returns that element index. Otherwise, findLastIndex returns -1. */ export function findLastIndex<T>(array: Array<T>, predicate: (value: T, index: number, obj: T[]) => boolean): number {     let l = array.length;     while (l--) {         if (predicate(array[l], l, array))             return l;     }     return -1; } 

Answer 2

You can use findIndex to get index. This will give you first index, so you will have to reverse the array.

var d = [{'a': "something", 'b':12}, {'a': "something", 'b':12}, {'a': "somethingElse", 'b':12}, {'a': "something", 'b':12}, {'a': "somethingElse", 'b':12}]    function findLastIndex(array, searchKey, searchValue) {    var index = array.slice().reverse().findIndex(x => x[searchKey] === searchValue);    var count = array.length - 1    var finalIndex = index >= 0 ? count - index : index;    console.log(finalIndex)    return finalIndex;  }    findLastIndex(d, 'a', 'something')  findLastIndex(d, 'a', 'nothing')