Add unionWith, unionBy

snippets/unionWith.md

@@ -0,0 +1,16 @@
+### unionWith
+
+Returns every element that exists in any of the two arrays once, using a provided comparator function.
+
+Create a `Set` with all values of `a` and values in `b` for which the comparator finds no matches in `a`, using `Array.findIndex()`.
+
+```js
+const unionWith = (a, b, comp) =>
+  Array.from(
+    new Set([...a, ...b.filter(x => a.findIndex(y => comp(x, y)) === -1)])
+  );
+```
+
+```js
+unionWith([1, 1.2, 1.5, 3, 0], [1.9, 3, 0, 3.9], (a, b) => Math.round(a) === Math.round(b)); // [1, 1.2, 1.5, 3, 0, 3.9]
+```