debugging

# Debugging Skill

Systematic approaches to investigating and diagnosing bugs.

## Core Principle

**Understand before fixing.** A proper diagnosis leads to a proper fix.

## The Scientific Method for Debugging

### 1. Observe

**Gather all the facts:**

- What's the symptom? (What's happening that shouldn't?)
- When does it happen? (Always, sometimes, specific conditions?)
- Who's affected? (All users, some users, specific scenarios?)
- Error messages? (Exact text, stack traces, error codes?)
- Recent changes? (What changed before this started?)

**Evidence to collect:**

- Error messages and stack traces
- Application logs
- User reports
- Reproduction steps
- Environment details (browser, OS, versions)
- Network requests/responses
- Database query logs

### 2. Form Hypothesis

**Based on symptoms, what could cause this?**

**Common categories:**

- **Logic error:** Code does wrong thing
- **State management:** State gets out of sync
- **Async/timing:** Race condition, callback hell
- **Data issue:** Unexpected input format
- **Integration:** API change, service down
- **Environment:** Config, permissions, network
- **Resource:** Memory leak, connection pool exhausted

**Prioritize hypotheses:**

1. Most likely causes first
2. Easiest to test first (when equal likelihood)
3. Most impactful if true

### 3. Test Hypothesis

**Design experiment to prove/disprove:**

- Add logging to see values
- Add breakpoints to pause execution
- Modify input to isolate variable
- Disable feature to rule out
- Compare with working version

**Keep notes:**

```markdown
**Hypothesis:** Database query timeout
**Test:** Add query timing logs
**Result:** Query completes in 50ms
**Conclusion:** Not the database ❌

**Hypothesis:** Network latency
**Test:** Check network tab, add timing
**Result:** API call takes 5 seconds
**Conclusion:** Found the issue ✅
```

### 4. Analyze Results

**What did you learn?**

- Hypothesis confirmed or rejected?
- New questions raised?
- Unexpected findings