Update sortedLastIndex, add sortedIndexBy, sortedLastIndexBy

snippets/sortedLastIndexBy.md

@@ -0,0 +1,22 @@
+### sortedLastIndexBy
+
+Returns the highest index at which value should be inserted into array in order to maintain its sort order, based on a provided iterator function.
+
+Check if the array is sorted in descending order (loosely).
+Use `Array.reverse()` and `Array.findIndex()` to find the appropriate last index where the element should be inserted, based on the iterator function `fn`..
+
+```js
+const sortedLastIndexBy = (arr, n, fn) => {
+  const isDescending = fn(arr[0]) > fn(arr[arr.length - 1]);
+  const val = fn(n);
+  const index = arr
+    .map((val, i) => [i, fn(val)])
+    .reverse()
+    .findIndex(el => (isDescending ? val <= el[1] : val >= el[1]));
+  return index === -1 ? 0 : arr.length - index;
+};
+```
+
+```js
+sortedLastIndexBy([{ 'x': 4 }, { 'x': 5 }], { 'x': 4 }, o => o.x); // 1
+```