ran npm run tdd & generated TDD

test/symmetricDifference/symmetricDifference.js

@@ -0,0 +1,5 @@
+module.exports = (a, b) => {
+const sA = new Set(a),
+sB = new Set(b);
+return [...a.filter(x => !sB.has(x)), ...b.filter(x => !sA.has(x))];
+};