Add filterNonUniqueBy

snippets/filterNonUniqueBy.md

@@ -0,0 +1,24 @@
+### filterNonUniqueBy
+
+Filters out the non-unique values in an array, based on a provided comparator function.
+
+Use `Array.filter()` and `Array.every()` for an array containing only the unique values, based on the comparator function, `fn`.
+The comparator function takes four arguments: the values of the two elements being compared and their indexes.
+
+```js
+const filterNonUniqueBy = (arr, fn) =>
+	arr.filter((v, i) => arr.every((x, j) => i == j == fn(v, x, i, j)));
+```
+
+```js
+filterNonUniqueBy(
+	[
+		{ id: 0, value: 'a' },
+		{ id: 1, value: 'b' },
+		{ id: 2, value: 'c' },
+		{ id: 1, value: 'd' },
+		{ id: 0, value: 'e' },
+	],
+	(a, b) => a.id != b.id
+); // [{id: 2, value: 'c'}]
+```