Reproducing a Bug
Before you can fix a bug, you need to make it happen on demand. A bug you can't reproduce is a bug you can't confidently fix. Reproduction is the foundation of systematic debugging.
Why Reproduction Matters
Think of debugging like a scientific experiment. Scientists need to reproduce results to verify their findings. Similarly, you need to reproduce bugs to:
- Confirm the bug actually exists (not a one-time glitch)
- Understand what triggers it
- Verify your fix actually works
- Prevent the bug from returning
If you change code without reproducing the bug first, you're just guessing. You might "fix" something that wasn't broken, or think you fixed it when you didn't.
Finding the Trigger
Start by asking: What inputs or conditions cause this bug?
Consider:
- What data was being processed?
- What buttons were clicked or commands run?
- What was the state of the system?
- Does it happen every time or only sometimes?
A bug that happens every time with the same inputs is much easier to fix than one that appears randomly. If it's intermittent, look for patterns — does it happen with large files? After running for a while? With specific user accounts?
Documenting Reproduction Steps
Write down the exact steps to trigger the bug. Be specific:
Vague: "The app crashes when I upload files."
Specific:
- Log in as a regular user (not admin)
- Navigate to the Upload page
- Select a PNG file larger than 5MB
- Click "Upload"
- Error: "TypeError: Cannot read property 'size' of undefined"
Specific steps help you test consistently and help others understand the problem if you need assistance.
Minimum Steps to Reproduce
Once you can trigger the bug, try to find the minimum steps needed. Remove anything unnecessary:
- Does it happen with any file, or only large PNGs?
- Does the user type matter?
- Does it happen on a fresh login or only after other actions?
Fewer steps mean a simpler problem to understand. Sometimes this process alone reveals the cause — you discover the bug only happens under specific conditions you hadn't noticed.
The Crime Scene Analogy
Reproducing a bug is like reconstructing a crime scene. You gather evidence, recreate the conditions, and observe what happens. Same conditions should produce the same result. If they don't, you're missing something about what actually triggers the problem.
Once you can reliably reproduce the bug, you're ready to locate exactly where in the code things go wrong.