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snippets/sorted-index-by.md

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+---
+title: Insertion index in sorted array based on function
+tags: array,math
+cover: digital-nomad-15
+firstSeen: 2018-01-26T13:39:09+02:00
+lastUpdated: 2020-10-22T20:24:30+03:00
+---
+
+Finds the lowest index at which a value should be inserted into an array in order to maintain its sorting order, based on the provided iterator function.
+
+- Loosely check if the array is sorted in descending order.
+- Use `Array.prototype.findIndex()` to find the appropriate index where the element should be inserted, based on the iterator function `fn`.
+
+```js
+const sortedIndexBy = (arr, n, fn) => {
+  const isDescending = fn(arr[0]) > fn(arr[arr.length - 1]);
+  const val = fn(n);
+  const index = arr.findIndex(el =>
+    isDescending ? val >= fn(el) : val <= fn(el)
+  );
+  return index === -1 ? arr.length : index;
+};
+```
+
+```js
+sortedIndexBy([{ x: 4 }, { x: 5 }], { x: 4 }, o => o.x); // 0
+```