diff --git a/snippets/sortedIndexBy.md b/snippets/sortedIndexBy.md
new file mode 100644
index 000000000..97335dc58
--- /dev/null
+++ b/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
+```
diff --git a/snippets/sortedLastIndex.md b/snippets/sortedLastIndex.md
index ca6e327ba..57d2499a3 100644
--- a/snippets/sortedLastIndex.md
+++ b/snippets/sortedLastIndex.md
@@ -4,16 +4,16 @@ Returns the highest index at which value should be inserted into array in 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.
+Use `Array.reverse()` and `Array.findIndex()` to find the appropriate last index where the element should be inserted.
 
 ```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;
+    .reverse()
+    .findIndex(el => (isDescending ? n <= el[1] : n >= el[1]));
+  return index === -1 ? 0 : arr.length - index - 1;
 };
 ```
 
diff --git a/snippets/sortedLastIndexBy.md b/snippets/sortedLastIndexBy.md
new file mode 100644
index 000000000..02ad7ce60
--- /dev/null
+++ b/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
+```
diff --git a/tag_database b/tag_database
index c94034010..2ddb8058d 100644
--- a/tag_database
+++ b/tag_database
@@ -206,7 +206,9 @@ size:object,array,string
 sleep:function,promise
 sortCharactersInString:string
 sortedIndex:array,math
+sortedIndexBy:array,math,function
 sortedLastIndex:array,math
+sortedLastIndexBy:array,math,function
 splitLines:string
 spreadOver:adapter
 standardDeviation:math,array