Add set operations. Union, intersect and difference

snippets/array-intersection.md

@@ -0,0 +1,8 @@
+### Array intersection (Common values between two arrays)
+
+Create a `Set` from `b`, then use `Array.filter()` on `a` to only keep values contained in `b`.
+
+```js
+const intersection = (a, b) => { const s = new Set(b); return a.filter(x => s.has(x)); }
+// intersection([1,2,3], [4,3,2]) -> [2,3]
+```