A bug has appeared that is difficult to reproduce — it reproduces in testing, but it's unclear how, and it is not logged. How would you debug it?

To debug a difficult-to-reproduce bug that is not logged, you can use the following approach:

1. Add extended logging — temporarily increase the logs, add more contextual information, such as variable states, call stack, event times.

2. Use browser debugging tools — set breakpoints, step through the code to understand under what conditions the bug occurs.

3. Record the sequence of actions — try to reproduce the bug manually, recording all steps to understand patterns.

4. Add a temporary UI indicator or tracing — for example, output the state to the console or on the screen to see what is happening.

5. Check asynchronous operations and application state — the bug may be related to race conditions or states that are hard to track.

6. Use session recording tools (e.g., Sentry, LogRocket) — to capture the application state at the moment of the error.

7. Review the test — the test might reproduce the bug under artificial conditions that do not match the real scenario.

Example of extended logging in JavaScript:

console.log('Current state:', { user, cart, step });

Thus, step by step, you can narrow down the search area and understand the cause of the bug.