30-seconds-of-code/snippets/findLastIndex.md

findLastIndex

Returns the index of the last element for which the provided function returns a truthy value.

Use Array.map() to map each element to an array with its index and value. Use Array.filter() to remove elements for which fn returns falsey values, Array.slice(-1) to get the last one.

const findLastIndex = (arr, fn) =>
  arr
    .map((val, i) => [i, val])
    .filter(val => fn(val[1], val[0], arr))
    .slice(-1)[0][0];

findLastIndex([1, 2, 3, 4], n => n % 2 === 1); // 2 (index of the value 3)