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javascript/snippets/sorted-last-index-by.md

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+---
+title: Last insertion index in sorted array based on function
+type: snippet
+tags: [array]
+cover: hard-disk
+excerpt: Finds the highest insertion index to maintain array sorting order based on the provided iterator function.
+dateModified: 2020-10-22T20:24:30+03:00
+---
+
+Finds the highest index at which a value should be inserted into an array in order to maintain its sort order, based on a provided iterator function.
+
+- Loosely check if the array is sorted in descending order.
+- Use `Array.prototype.map()` to apply the iterator function to all elements of the array.
+- Use `Array.prototype.reverse()` and `Array.prototype.findIndex()` to find the appropriate last index where the element should be inserted, based on the provided iterator function.
+
+```js
+const sortedLastIndexBy = (arr, n, fn) => {
+  const isDescending = fn(arr[0]) > fn(arr[arr.length - 1]);
+  const val = fn(n);
+  const index = arr
+    .map(fn)
+    .reverse()
+    .findIndex(el => (isDescending ? val <= el : val >= el));
+  return index === -1 ? 0 : arr.length - index;
+};
+```
+
+```js
+sortedLastIndexBy([{ x: 4 }, { x: 5 }], { x: 4 }, o => o.x); // 1
+```