Update sortedLastIndex, add sortedIndexBy, sortedLastIndexBy

snippets/sortedIndexBy.md

@@ -0,0 +1,19 @@
+### sortedIndexBy
+
+Returns the lowest 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.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
+```