Added uniqueSymmetricDifference and updated symmetricDifference

snippets/symmetricDifference.md

@@ -5,11 +5,14 @@ Returns the symmetric difference between two arrays.
 Create a `Set` from each array, then use `Array.filter()` on each of them to only keep values not contained in the other.
 
 ```js
-const symmetricDifference = (a, b) => [
-  ...new Set([...a.filter(v => !b.includes(v)), ...b.filter(v => !a.includes(v))])
-];
+const symmetricDifference = (a, b) => {
+  const sA = new Set(a),
+    sB = new Set(b);
+  return [...a.filter(x => !sB.has(x)), ...b.filter(x => !sA.has(x))];
+};
 ```
 
 ```js
-symmetricDifference([1, 2, 3], [1, 2, 4]); // [3,4]
+symmetricDifference([1, 2, 3], [1, 2, 4]); // [3, 4]
+symmetricDifference([1, 2, 2], [1, 3, 1]); // [2, 2, 3]
 ```

snippets/uniqueSymmetricDifference.md

@@ -0,0 +1,16 @@
+### uniqueSymmetricDifference
+
+Returns the unique symmetric difference between two arrays.
+
+Use `Array.filter()` and `Array.includes()` on each array to remove values contained in the other, then create a `Set` from the results, removing duplicate values.
+
+```js
+const uniqueSymmetricDifference = (a, b) => [
+  ...new Set([...a.filter(v => !b.includes(v)), ...b.filter(v => !a.includes(v))])
+];
+```
+
+```js
+uniqueSymmetricDifference([1, 2, 3], [1, 2, 4]); // [3, 4]
+uniqueSymmetricDifference([1, 2, 2], [1, 3, 1]); // [2, 3]
+```