Use Array.prototype.findLastIndex. This function will be removed in std@0.117.0.

Returns the index of the last element in the given array matching the given predicate

Example:

import { findLastIndex } from "https://deno.land/std@0.116.0/collections/mod.ts";
import { assertEquals } from "https://deno.land/std@0.116.0/testing/asserts.ts";

const numbers = [ 4, 2, 7 ]
const lastIndexNumber = findLastIndex(numbers, it => it % 2 === 0)

assertEquals(lastIndexNumber, 1)