debugging
Guide for debugging with targeted log injection and runtime analysis
What this skill does
# Debugging Skill
Guide for debugging with targeted log injection and runtime analysis.
## When to Activate
Suggest `/debug-tools:debug` when users describe:
- "X is not working"
- "Getting error Y when doing Z"
- "Something broke after [change]"
- Silent failures or unexpected behavior
## Log Format
```javascript
console.log("[DEBUG] [file:line] description", { values })
```
- `[DEBUG]` - Prefix for grep and cleanup
- `[file:line]` - Location for navigation
- `description` - What this log checks
- `{ values }` - Relevant data (no sensitive info)
## Log Patterns
### React/Next.js
```javascript
// Lifecycle
console.log("[DEBUG] [Component.tsx:10] mount", { props })
// Effect
useEffect(() => {
console.log("[DEBUG] [Component.tsx:15] effect run", { deps })
return () => console.log("[DEBUG] [Component.tsx:17] cleanup")
}, [deps])
// State
console.log("[DEBUG] [Component.tsx:25] before setState", { current: state })
```
### Node.js/Express
```javascript
// Request
console.log("[DEBUG] [route.ts:10] request", { method: req.method, path: req.path })
// Error
console.log("[DEBUG] [service.ts:30] caught error", { name: err.name, message: err.message })
```
### API Calls
```javascript
console.log("[DEBUG] [api.ts:10] fetch start", { url, method })
console.log("[DEBUG] [api.ts:15] fetch done", { status: res.status, ok: res.ok })
```
## Common Bug Patterns
| Pattern | Symptom | Check |
| -------------- | ------------------------------------- | --------------------------------------- |
| Null access | "Cannot read property X of undefined" | Optional chaining, defaults |
| Race condition | Works sometimes, fails randomly | Async ordering, state timing |
| Stale closure | Using old values in callbacks | useCallback deps, event bindings |
| API mismatch | Data not displaying | Response shape, null handling |
| Silent error | Nothing happens | Empty catch blocks, missing error state |
## Confidence Scoring
| Score | Meaning | Action |
| ----- | --------------------- | ------------------------ |
| >= 70 | High - clear evidence | Report as probable cause |
| 50-69 | Medium - possible | Suggest logs to confirm |
| < 50 | Low - speculation | Do not report |
## Cleanup
After debugging, all `[DEBUG]` logs are removed automatically.
Manual check:
```bash
grep -rn '\[DEBUG\]' . --include='*.ts' --include='*.tsx' --include='*.js' --include='*.jsx'
```
## MCP Integration
| MCP | Provides |
| --------------- | ---------------------------------------------- |
| Console Ninja | Runtime values, test status, coverage |
| Chrome DevTools | Network inspection, browser console, DOM state |
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