debug-pro
Systematic debugging methodology and language-specific debugging commands.
The 7-Step Debugging Protocol
- Reproduce — Get it to fail consistently. Document exact steps, inputs, and environment.
- Isolate — Narrow scope. Comment out code, use binary search, check recent commits with
git bisect. - Hypothesize — Form a specific, testable theory about the root cause.
- Instrument — Add targeted logging, breakpoints, or assertions.
- Verify — Confirm root cause. If hypothesis was wrong, return to step 3.
- Fix — Apply the minimal correct fix. Resist the urge to refactor while debugging.
- Regression Test — Write a test that catches this bug. Verify it passes.
Language-Specific Debugging
JavaScript / TypeScript
# Node.js debugger
node --inspect-brk app.js
# Chrome DevTools: chrome://inspect
# Console debugging
console.log(JSON.stringify(obj, null, 2))
console.trace('Call stack here')
console.time('perf'); /* code */ console.timeEnd('perf')
# Memory leaks
node --expose-gc --max-old-space-size=4096 app.js
Python
# Built-in debugger
python -m pdb script.py
# Breakpoint in code
breakpoint() # Python 3.7+
# Verbose tracing
python -X tracemalloc script.py
# Profile
python -m cProfile -s cumulative script.py
Swift
# LLDB debugging
lldb ./MyApp
(lldb) breakpoint set --name main
(lldb) run
(lldb) po myVariable
# Xcode: Product → Profile (Instruments)
CSS / Layout
/* Outline all elements */
* { outline: 1px solid red !important; }
/* Debug specific element */
.debug { background: rgba(255,0,0,0.1) !important; }
Network
# HTTP debugging
curl -v https://api.example.com/endpoint
curl -w "@curl-format.txt" -o /dev/null -s https://example.com
# DNS
dig example.com
nslookup example.com
# Ports
lsof -i :3000
netstat -tlnp
Git Bisect
git bisect start
git bisect bad # Current commit is broken
git bisect good abc1234 # Known good commit
# Git checks out middle commit — test it, then:
git bisect good # or git bisect bad
# Repeat until root cause commit is found
git bisect reset
Common Error Patterns
| Error | Likely Cause | Fix |
|---|---|---|
Cannot read property of undefined | Missing null check or wrong data shape | Add optional chaining (?.) or validate data |
ENOENT | File/directory doesn't exist | Check path, create directory, use existsSync |
CORS error | Backend missing CORS headers | Add CORS middleware with correct origins |
Module not found | Missing dependency or wrong import path | npm install, check tsconfig paths |
Hydration mismatch (React) | Server/client render different HTML | Ensure consistent rendering, use useEffect for client-only |
Segmentation fault | Memory corruption, null pointer | Check array bounds, pointer validity |
Connection refused | Service not running on expected port | Check if service is up, verify port/host |
Permission denied | File/network permission issue | Check chmod, firewall, sudo |
Quick Diagnostic Commands
# What's using this port?
lsof -i :PORT
# What's this process doing?
ps aux | grep PROCESS
# Watch file changes
fswatch -r ./src
# Disk space
df -h
# System resource usage
top -l 1 | head -10