ran npm run tdd & generated TDD

test/join/join.js

@@ -0,0 +1,8 @@
+module.exports = (arr, separator = ',', end = separator) =>
+arr.reduce(
+(acc, val, i) =>
+i == arr.length - 2
+? acc + val + end
+: i == arr.length - 1 ? acc + val : acc + val + separator,
+''
+);