Common patterns:

- AttributeError: Check object types

- KeyError: Check dictionary keys

- IndexError: Check list bounds

- TypeError: Check function arguments

- ImportError: Check module installation


**JavaScript Stack Traces:**
```javascript
// Analyze JavaScript errors
// Look for:
// - Error type (TypeError, ReferenceError, etc.)
// - File and line numbers
// - Function call stack
// - Async operation context

// Common patterns:
// - "Cannot read property 'x' of undefined"
// - "x is not a function"
// - "Maximum call stack size exceeded"

2. Log Analysis

Parse Application Logs:

# Find errors in logs
grep -i "error\|exception\|fatal\|critical" logs/*.log

# Find warnings
grep -i "warn\|warning" logs/*.log

# Analyze by timestamp
grep "2026-01-22" logs/*.log | grep -i "error"

# Count error types
grep -i "error" logs/*.log | cut -d: -f2 | sort | uniq -c | sort -rn

# Find patterns before crashes
grep -B10 "fatal\|crash" logs/*.log

Structured Log Analysis:

# JSON logs
cat logs/app.log | jq 'select(.level == "error")'
cat logs/app.log | jq 'select(.statusCode >= 500)'

# Parse specific fields
cat logs/app.log | jq '.timestamp, .message, .error'

# Group by error type
cat logs/app.log | jq -r '.error.type' | sort | uniq -c | sort -rn

3. Common Bug Patterns

Null/Undefined Issues:

# Find potential null reference errors
grep -r "\..*\." . --include="*.js" --include="*.py"

# Check for null checks
grep -r "if.*is None\|if.*== None" . --include="*.py"
grep -r "if.*=== null\|if.*!== null" . --include="*.js"

Race Conditions:

# Find potential race conditions
grep -r "threading\|Thread\|async\|await" . --include="*.py" --include="*.js"

# Check for proper locking
grep -r "lock\|mutex\|semaphore" . --include="*.py" --include="*.js"

# Find shared state
grep -r "global\|shared\|static" . --include="*.py" --include="*.js"

Memory Leaks:

# Find potential memory leaks (Python)
grep -r "global\|cache" . --include="*.py"

# JavaScript event listeners without cleanup
grep -r "addEventListener" . --include="*.js" | grep -v "removeEventListener"

# Unclosed file handles
grep -r "open(" . --include="*.py" | grep -v "with\|close()"

Off-by-One Errors:

# Check loop boundaries
grep -r "range(.*len\|for.*in.*length" . --include="*.py" --include="*.js"

# Array access patterns
grep -r "\[.*-\s*1\]\|\[.*+\s*1\]" . --include="*.py" --include="*.js"

4. Debugging Techniques

Add Debug Logging:

# Python debug logging
import logging
logging.basicConfig(level=logging.DEBUG)

def problematic_function(arg):
    logging.debug(f"Input: {arg}, Type: {type(arg)}")
    # ... rest of function
    logging.debug(f"Output: {result}")
    return result

// JavaScript debug logging
function problematicFunction(arg) {
    console.log('Input:', arg, 'Type:', typeof arg);
    console.trace('Call stack');
    // ... rest of function
    console.log('Output:', result);
    return result;
}

Binary Search Debugging:

# Comment out half the code to isolate the issue
# Use git bisect to find the commit that introduced the bug
git bisect start
git bisect bad  # Current commit is bad
git bisect good <commit-hash>  # Known good commit
# Test each commit until bug is found
git bisect reset

Rubber Duck Debugging:

# Explain the code step by step:
1. What is the expected input?
2. What transformations happen?
3. What is the expected output?
4. Where does actual differ from expected?

5. Interactive Debugging

Python Debugger (pdb):

# Add breakpoint
import pdb; pdb.set_trace()

# Or Python 3.7+
breakpoint()

# pdb commands:
# n - next line
# s - step into
# c - continue
# p variable - print variable
# pp variable - pretty print
# l - list code
# w - where (stack trace)
# q - quit

Node.js Debugger:

# Start with debugger
node inspect app.js

# Or use Chrome DevTools
node --inspect app.js
node --inspect-brk app.js  # Break on first line

# Debugger commands:
# cont, c - Continue
# next, n - Next line
# step, s - Step in
# out, o - Step out
# watch('expr') - Watch expression

Browser DevTools:

// Add breakpoint
debugger;

// Console debugging
console.log('Value:', value);
console.table(arrayOfObjects);
console.time('label');
// ... code ...
console.timeEnd('label');

// Conditional breakpoints in DevTools:
// Right-click line number > Add conditional breakpoint
// e.g., user.id === 123

6. Performance Debugging

Find Performance Bottlenecks:

# Python profiling
import cProfile
import pstats

cProfile.run('slow_function()', 'output.prof')
stats = pstats.Stats('output.prof')
stats.sort_stats('cumulative')
stats.print_stats(20)

# Line profiler
# pip install line_profiler
@profile
def slow_function():
    # Code here
    pass

# Run: kernprof -l -v script.py

// Node.js profiling
node --prof app.js
node --prof-process isolate-*.log

// Or use Chrome DevTools Performance tab

Memory Profiling:

# Python memory profiler
from memory_profiler import profile

@profile
def memory_heavy_function():
    # Code here
    pass

# Run: python -m memory_profiler script.py

# Node.js memory profiling
node --inspect --expose-gc app.js
# Use Chrome DevTools Memory tab to take heap snapshots

7. Database Debugging

SQL Query Debugging:

-- Add EXPLAIN to understand query execution
EXPLAIN SELECT * FROM users WHERE email = 'test@example.com';

-- Check for missing indexes
EXPLAIN ANALYZE SELECT ...;

-- Find slow queries
SELECT * FROM pg_stat_statements ORDER BY total_time DESC LIMIT 10;

ORM Debugging:

# Django debug toolbar
# settings.py
DEBUG = True
INSTALLED_APPS += ['debug_toolbar']

# SQLAlchemy echo
engine = create_engine('sqlite:///db.db', echo=True)

# Sequelize logging
const sequelize = new Sequelize({
    logging: console.log
});

8. Network Debugging

HTTP Request Debugging:

# cURL with verbose output
curl -v https://api.example.com/endpoint

# Check DNS resolution
nslookup api.example.com
dig api.example.com

# Test connectivity
ping api.example.com
telnet api.example.com 443

# Trace route
traceroute api.example.com

API Debugging:

# Request/Response logging
curl -v -X POST https://api.example.com/users \
  -H "Content-Type: application/json" \
  -d '{"name":"test"}'

# Check SSL/TLS
openssl s_client -connect api.example.com:443

# Monitor network traffic
tcpdump -i any port 443

9. Error Investigation Workflow

Step 1: Reproduce the Error

Consistent reproduction steps
Minimal test case
Document environment (OS, versions, etc.)

Step 2: Gather Information

# System information
uname -a
python --version
node --version

# Environment variables
env | grep -i app

# Dependencies
pip list
npm list --depth=0

# Recent changes
git log --oneline -10
git diff HEAD~1

Step 3: Isolate the Issue

Binary search through code
Remove dependencies one by one
Test with minimal configuration
Check if issue exists in different environments

Step 4: Analyze

Read stack traces carefully
Check logs around the error time
Review recent code changes
Search for similar issues online

Step 5: Form Hypothesis

What could cause this error?
What are the assumptions?
What can be tested?

Step 6: Test Hypothesis

Add logging/debugging statements
Create unit test that reproduces issue
Test edge cases

Step 7: Fix and Verify

Implement fix
Verify fix resolves issue
Add regression test
Check for similar issues elsewhere

10. Common Debugging Commands

Process Debugging:

# Find running processes
ps aux | grep python
ps aux | grep node

# Check process details
top -p <pid>
htop -p <pid>

# Monitor system resources
vmstat 1
iostat -x 1

# Check open files
lsof -p <pid>

# Trace system calls
strace -p <pid>
strace -c python script.py  # Count syscalls

Container Debugging:

# Docker logs
docker logs <container-id>
docker logs -f <container-id>  # Follow

# Execute in container
docker exec -it <container-id> bash

# Inspect container
docker inspect <container-id>

# Container stats
docker stats <container-id>

11. Debugging Checklist

When debugging, verify:

When to Use This Skill

Use /debug when:

Application crashes or throws errors
Unexpected behavior occurs
Performance issues need investigation
Stack traces need interpretation
Logs need analysis
Root cause needs identification
Debugging strategy needed
Troubleshooting production issues

The skill will help analyze errors, suggest debugging approaches, and guide you to the root cause.

debug

Debug Skill

1. Stack Trace Analysis