Add sortedLastIndex

snippets/sortedLastIndex.md

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+### sortedLastIndex
+
+Returns the highest 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.map()` to map each element to an array with its index and value.
+Use `Array.filter()` to find all possible positions where the element could be inserted, `Array.slice(-1)` to get the last one.
+
+```js
+const sortedLastIndex = (arr, n) => {
+  const isDescending = arr[0] > arr[arr.length - 1];
+  const index = arr
+    .map((val, i) => [i, val])
+    .filter(el => (isDescending ? n >= el[1] : n >= el[1]))
+    .slice(-1)[0][0];
+  return index === -1 ? arr.length : index;
+};
+```
+
+```js
+sortedLastIndex([10, 20, 30, 30, 40], 30); // 3
+```