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javascript/snippets/find-last-index.md

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+---
+title: Find last matching index
+type: snippet
+tags: [array]
+cover: taking-photos
+dateModified: 2020-10-22T20:23:47+03:00
+---
+
+Finds the index of the last element for which the provided function returns a truthy value.
+
+- Use `Array.prototype.map()` to map each element to an array with its index and value.
+- Use `Array.prototype.filter()` to remove elements for which `fn` returns falsy values
+- Use `Array.prototype.pop()` to get the last element in the filtered array.
+- Return `-1` if there are no matching elements.
+
+```js
+const findLastIndex = (arr, fn) =>
+  (arr
+    .map((val, i) => [i, val])
+    .filter(([i, val]) => fn(val, i, arr))
+    .pop() || [-1])[0];
+```
+
+```js
+findLastIndex([1, 2, 3, 4], n => n % 2 === 1); // 2 (index of the value 3)
+findLastIndex([1, 2, 3, 4], n => n === 5); // -1 (default value when not found)
+```