Rename js snippets

snippets/js/s/mapped-array-union.md

@@ -0,0 +1,28 @@
+---
+title: Mapped array union
+type: snippet
+language: javascript
+tags: [array]
+cover: lake-church
+excerpt: Returns every element from both arrays that exists at least once after applying the provided function.
+dateModified: 2020-10-22T20:24:44+03:00
+---
+
+Returns every element that exists in any of the two arrays at least once, after applying the provided function to each array element of both.
+
+- Create a `Set` by applying all `fn` to all values of `a`.
+- Create a `Set` from `a` and all elements in `b` whose value, after applying `fn` does not match a value in the previously created set.
+- Return the last set converted to an array.
+
+```js
+const unionBy = (a, b, fn) => {
+  const s = new Set(a.map(fn));
+  return Array.from(new Set([...a, ...b.filter(x => !s.has(fn(x)))]));
+};
+```
+
+```js
+unionBy([2.1], [1.2, 2.3], Math.floor); // [2.1, 1.2]
+unionBy([{ id: 1 }, { id: 2 }], [{ id: 2 }, { id: 3 }], x => x.id)
+// [{ id: 1 }, { id: 2 }, { id: 3 }]
+```