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