Debugging Strategies

Transform debugging from frustrating guesswork into systematic problem-solving with proven strategies, powerful tools, and methodical approaches.

When to Use This Skill

Tracking down elusive bugs
Investigating performance issues
Understanding unfamiliar codebases
Debugging production issues
Analyzing crash dumps and stack traces
Profiling application performance
Investigating memory leaks
Debugging distributed systems

Core Principles

1. The Scientific Method

1. Observe: What's the actual behavior? 2. Hypothesize: What could be causing it? 3. Experiment: Test your hypothesis 4. Analyze: Did it prove/disprove your theory? 5. Repeat: Until you find the root cause

2. Debugging Mindset

Don't Assume:

"It can't be X" - Yes it can
"I didn't change Y" - Check anyway
"It works on my machine" - Find out why

Do:

Reproduce consistently
Isolate the problem
Keep detailed notes
Question everything
Take breaks when stuck

3. Rubber Duck Debugging

Explain your code and problem out loud (to a rubber duck, colleague, or yourself). Often reveals the issue.

Systematic Debugging Process

Phase 1: Reproduce

## Reproduction Checklist

1. **Can you reproduce it?**
   - Always? Sometimes? Randomly?
   - Specific conditions needed?
   - Can others reproduce it?

2. **Create minimal reproduction**
   - Simplify to smallest example
   - Remove unrelated code
   - Isolate the problem

3. **Document steps**
   - Write down exact steps
   - Note environment details
   - Capture error messages

Phase 2: Gather Information

## Information Collection

1. **Error Messages**
   - Full stack trace
   - Error codes
   - Console/log output

2. **Environment**
   - OS version
   - Language/runtime version
   - Dependencies versions
   - Environment variables

3. **Recent Changes**
   - Git history
   - Deployment timeline
   - Configuration changes

4. **Scope**
   - Affects all users or specific ones?
   - All browsers or specific ones?
   - Production only or also dev?

Phase 3: Form Hypothesis

## Hypothesis Formation

Based on gathered info, ask:

1. **What changed?**
   - Recent code changes
   - Dependency updates
   - Infrastructure changes

2. **What's different?**
   - Working vs broken environment
   - Working vs broken user
   - Before vs after

3. **Where could this fail?**
   - Input validation
   - Business logic
   - Data layer
   - External services

Phase 4: Test & Verify

## Testing Strategies

1. **Binary Search**
   - Comment out half the code
   - Narrow down problematic section
   - Repeat until found

2. **Add Logging**
   - Strategic console.log/print
   - Track variable values
   - Trace execution flow

3. **Isolate Components**
   - Test each piece separately
   - Mock dependencies
   - Remove complexity

4. **Compare Working vs Broken**
   - Diff configurations
   - Diff environments
   - Diff data

Before selecting debugging tools or performing bisect, differential, trace, or memory analysis, read references/tools-and-techniques.md.

Debugging Patterns by Issue Type

Pattern 1: Intermittent Bugs

## Strategies for Flaky Bugs

1. **Add extensive logging**
   - Log timing information
   - Log all state transitions
   - Log external interactions

2. **Look for race conditions**
   - Concurrent access to shared state
   - Async operations completing out of order
   - Missing synchronization

3. **Check timing dependencies**
   - setTimeout/setInterval
   - Promise resolution order
   - Animation frame timing

4. **Stress test**
   - Run many times
   - Vary timing
   - Simulate load

Pattern 2: Performance Issues

## Performance Debugging

1. **Profile first**
   - Don't optimize blindly
   - Measure before and after
   - Find bottlenecks

2. **Common culprits**
   - N+1 queries
   - Unnecessary re-renders
   - Large data processing
   - Synchronous I/O

3. **Tools**
   - Browser DevTools Performance tab
   - Lighthouse
   - Python: cProfile, line_profiler
   - Node: clinic.js, 0x

Pattern 3: Production Bugs

## Production Debugging

1. **Gather evidence**
   - Error tracking (Sentry, Bugsnag)
   - Application logs
   - User reports
   - Metrics/monitoring

2. **Reproduce locally**
   - Use production data (anonymized)
   - Match environment
   - Follow exact steps

3. **Safe investigation**
   - Don't change production
   - Use feature flags
   - Add monitoring/logging
   - Test fixes in staging

Best Practices

Reproduce First: Can't fix what you can't reproduce
Isolate the Problem: Remove complexity until minimal case
Read Error Messages: They're usually helpful
Check Recent Changes: Most bugs are recent
Use Version Control: Git bisect, blame, history
Take Breaks: Fresh eyes see better
Document Findings: Help future you
Fix Root Cause: Not just symptoms

Common Debugging Mistakes

Making Multiple Changes: Change one thing at a time
Not Reading Error Messages: Read the full stack trace
Assuming It's Complex: Often it's simple
Debug Logging in Prod: Remove before shipping
Not Using Debugger: console.log isn't always best
Giving Up Too Soon: Persistence pays off
Not Testing the Fix: Verify it actually works

Quick Debugging Checklist

## When Stuck, Check:

- [ ] Spelling errors (typos in variable names)
- [ ] Case sensitivity (fileName vs filename)
- [ ] Null/undefined values
- [ ] Array index off-by-one
- [ ] Async timing (race conditions)
- [ ] Scope issues (closure, hoisting)
- [ ] Type mismatches
- [ ] Missing dependencies
- [ ] Environment variables
- [ ] File paths (absolute vs relative)
- [ ] Cache issues (clear cache)
- [ ] Stale data (refresh database)