Add findLastIndex, update findLast

snippets/findLastIndex.md

@@ -0,0 +1,18 @@
+### 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.
+
+```js
+const findLastIndex = (arr, fn) =>
+  arr
+    .map((val, i) => [i, val])
+    .filter(val => fn(val[1], val[0], arr))
+    .slice(-1)[0][0];
+```
+
+```js
+findLastIndex([1, 2, 3, 4], n => n % 2 === 1); // 2 (index of the value 3)
+```