diff --git a/snippets/unionBy.md b/snippets/unionBy.md
index 3680c6d1c..91c9e6662 100644
--- a/snippets/unionBy.md
+++ b/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)))]));
 };
 ```
diff --git a/test/unionBy/unionBy.js b/test/unionBy/unionBy.js
index 86d52180c..e03f9998b 100644
--- a/test/unionBy/unionBy.js
+++ b/test/unionBy/unionBy.js
@@ -1,5 +1,5 @@
 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)))]));
 };
 module.exports = unionBy;