Simplify unionBy

snippets/unionBy.md

@@ -8,7 +8,7 @@ Return the last set converted to an array.
 
 ```js
 const unionBy = (a, b, fn) => {
-  const s = new Set(a.map(v => fn(v)));
+  const s = new Set(a.map(fn));
   return Array.from(new Set([...a, ...b.filter(x => !s.has(fn(x)))]));
 };
 ```