Debug & Linting

Debug & Linting

Catch errors before delivery. Lint, type-check, browser verify.

When to Use

• After writing code → run checks
• Before delivery → verify quality
• Debugging → check console/​network

Process

LINT → TYPE → BROWSER → DELIVER

1. Run lint: npm run lint
2. Type check: npm run typecheck
3. Browser check: console + screenshot
4. Deliver when clean

Quick Commands

npm run lint          # ESLint check
npm run lint:fix      # Auto-fix
npm run typecheck     # TypeScript check
npm run format        # Prettier format
npm run check         # All checks

Common Fixes

TypeScript

"Type 'X' not assignable to 'Y'":
  → Fix type mismatch or add assertion

"'X' declared but never used":
  → Remove or prefix with _

"Object possibly 'undefined'":
  → Add null check: obj?.property
  → Add fallback: obj ?? default

React

"Missing dependencies in useEffect":
  → Add deps or wrap in useCallback

"Each child should have unique key":
  → Add key={item.id} to list items

"img must have alt prop":
  → Add alt text or alt=""

Formatting

"Prettier errors":
  → Run: npm run format

Browser Verification

# Check console errors
browser_console_messages: { onlyErrors: true }
→ Must be empty before delivery

# Check network
browser_network_requests
→ No failed (4xx, 5xx) requests

# Debug element styles
browser_evaluate: { function: "() => getComputedStyle(el)" }

Pre-Delivery Checklist

MUST PASS:
  - [ ] npm run lint → 0 errors
  - [ ] npm run typecheck → 0 errors
  - [ ] browser_console_messages → 0 errors
  - [ ] No failed network requests

CAN DELIVER WITH:
  - ESLint warnings (with explanation)
  - Console warnings (if understood)

Project Setup

# Add to existing Next.js project
npm install -D prettier eslint-config-prettier

# package.json scripts
{
  "lint": "eslint . --ext .ts,.tsx",
  "lint:fix": "eslint . --ext .ts,.tsx --fix",
  "format": "prettier --write .",
  "typecheck": "tsc --noEmit",
  "check": "npm run typecheck && npm run lint"
}

Debug Checklist

WHEN SOMETHING BREAKS:
  1. browser_console_messages → check errors
  2. browser_network_requests → failed requests?
  3. npm run typecheck → type errors?
  4. Add console.log → trace flow
  5. Isolate → comment out sections
  6. Fix → run checks → verify