Debug runtime error

The app crashes on page load with a runtime error. Inspect the stack trace, identify the failing file and line, add a defensive guard and a clear error message, then re-run to confirm the page loads. Commit the fix with a one-line description referencing the stack trace top line.

Before I push a runtime error fix, make it easier for the reviewer to accept. Highlight the stack trace top file and line, show the minimal guard you propose, and call out any assumptions (null value, undefined API response) that would make the guard wrong so the reviewer can challenge them.

I added a component and now users hit TypeError: cannot read property 'map' of undefined in renderList. I don't know whether data failed to load or I passed the wrong prop. The component is used in three places. Which is most likely, how do I quickly confirm it, and what minimal code change will make the UI resilient without hiding a real bug?

Across sprints we keep shipping code that assumes API arrays always exist and it blows up in production when responses change. I need a pattern to stop these runtime errors at source. Recommend one coding practice, one automated check for CI, and a short PR review question that catches dangerous assumptions about external data.