About this challenge
Fix Mutation Bug is a medium debugging coding challenge. It tests your ability to read unfamiliar code, find the defect, and fix it without breaking the rest. Write your solution in the editor above, run it against the test cases, and get instant feedback — free, in your browser, no sign-up.
Show a hint & approach (spoiler)
Hint: Return a new array with the spread operator: `return [...arr, item]`.
Keep practicing
Browse all coding challenges, or build intuition for the underlying concepts with the algorithm visualizers and the developer cheatsheets.