How to find the index of the last matching item of an array in JavaScript

In this Article we will go through how to find the index of the last matching item of an array only using single line of code in JavaScript. This is a one-line JavaScript code snippet that uses one of the most popular ES6 features => Arrow Function.

Let's define this short function:

const lastIndex = (arr, predicate) => arr.reduce((prev, curr, index) => predicate(curr) ? index : prev, -1);

const lastIndex = (arr, predicate) => arr.map(item => predicate(item)).lastIndexOf(true);

lastIndex([1, 3, 5, 7, 9, 2, 4, 6, 8], i => i % 2 === 1);   // 4
lastIndex([1, 3, 5, 7, 9, 8, 6, 4, 2], i => i > 6);         // 5