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+### sortedIndex
+
+Returns the lowest index at which value should be inserted into array in order to maintain its sort order.
+
+Check if the array is sorted in descending order (loosely).
+Use `Array.findIndex()` to find the appropriate index where the element should be inserted.
+
+```js
+const sortedIndex = (arr, n) => {
+  const isDescending = arr[0] > arr[arr.length - 1];
+  const index = arr.findIndex(el => isDescending ? n >= el : n <= el);
+  return index === -1 ? arr.length : index;
+};
+```
+
+```js
+sortedIndex([5,3,2,1], 4); // 1
+sortedIndex([30,50],40); // 1
+```