The ISSUE tag is reserved for critical flaws with the tests. 718 people have solved this Kata in JS before just fine, so, when in doubt, please use the QUESTION tag instead. As for your question, the code inside your loop needs correction. Instead of focusing on the complicated test that fails, focus on the other more simple test that the solution fails on: f: abcd s: dabcd returns abcd for you when it should return abcdabcd. This is a byproduct of the method you use to calculate indexInSecond.