Rename js snippets

snippets/js/s/function-based-array-union.md

@@ -0,0 +1,28 @@
+---
+title: Array union based on function
+type: snippet
+language: javascript
+tags: [array]
+cover: orange-coffee-4
+dateModified: 2020-10-22T20:24:44+03:00
+---
+
+Returns every element that exists in any of the two arrays at least 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.prototype.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]
+```