Debugging

Debugging — CodeSensei Teaching Module

What is Debugging?

• Analogy: Debugging is detective work. Something isn't working, and you need to find out why. Every error message is a clue.
• Key insight: Errors are NOT failures. They're the computer telling you exactly what went wrong and where. Learning to read errors is the single most valuable programming skill.

How to Read Error Messages

Teach users to look for these three things in EVERY error:

1. What happened — the error type (TypeError, SyntaxError, 404, etc.)
2. Where it happened — the file name and line number
3. Why it happened — the description message

Example:

TypeError: Cannot read property 'name' of undefined
    at UserCard (src/components/UserCard.jsx:12)

Translation: "On line 12 of UserCard.jsx, you tried to get .name from something that doesn't exist yet."

Common Error Types (teach as encountered)

Frontend Errors

• "Cannot read property X of undefined" — you're trying to use data that hasn't loaded yet. Like trying to read a book that hasn't been delivered.
• "X is not defined" — you're using a variable/function that doesn't exist. Likely a typo or missing import.
• "X is not a function" — you're trying to call something as a function, but it's not one.
• White screen of death — usually a component crashed during rendering. Check the browser console.

Backend Errors

• 404 Not Found — the URL/route doesn't exist. Like going to a room number that doesn't exist in the building.
• 500 Internal Server Error — something broke on the server. Check the server logs.
• 401 Unauthorized — you're not logged in (or your token expired).
• 403 Forbidden — you're logged in but don't have permission.
• CORS error — your frontend is trying to talk to a backend that doesn't trust it. Like calling a secure building and not being on the approved list.

Database Errors

• "relation X does not exist" — the table hasn't been created yet. Run your migrations.
• "unique constraint violation" — you're trying to add a duplicate entry (like two users with the same email).
• "null value in column X violates not-null constraint" — you forgot to include a required field.

The Debugging Mindset

1. Don't panic — errors are normal. Every developer sees them constantly.
2. Read the error — the answer is usually right there in the message.
3. Google the error — copy the exact error message. Someone has had this problem before.
4. Check what changed — what did you (or Claude) do right before it broke?
5. Isolate the problem — narrow down which part is broken.

Teaching Opportunity

Every error is a learning moment. When Claude fixes an error, explain:

• What the error meant
• Why it happened
• How it was fixed
• How to prevent it next time

This is where real engineering intuition is built.