Systematic Debugging

Systematic Debugging

The Fundamental Rule

NO FIXES WITHOUT ROOT CAUSE INVESTIGATION FIRST.

Quick patches create technical debt. Symptoms mask deeper issues. Every fix must address the actual root cause.

The Four Phases

Phase 1: Root Cause Investigation

Before touching code, gather evidence:

1. Read the error carefully

   Error: Cannot read properties of undefined (reading 'map')
       at ProductList (src/components/ProductList.tsx:15:24)
       at renderWithHooks (node_modules/react-dom/...)
   

   • What exactly failed?
   • Where in the stack trace?
   • What data was involved?

2. Reproduce consistently

   # Document exact reproduction steps
   1. Navigate to /products
   2. Click "Filter by category"
   3. Select "Electronics"
   4. Error appears
   

3. Check recent changes

   git log --oneline -10
   git diff HEAD~3 -- src/components/ProductList.tsx
   

4. Add instrumentation at boundaries

   // Before the failing line
   console.log('products:', JSON.stringify(products, null, 2));
   console.log('type:', typeof products);
   console.log('isArray:', Array.isArray(products));
   

Phase 2: Pattern Analysis

Find working examples and compare:

// Working component
const WorkingList = ({ items }) => {
  if (!items || items.length === 0) return <Empty />;
  return items.map(item => <Item key={item.id} {...item} />);
};

// Broken component - what's different?
const BrokenList = ({ items }) => {
  return items.map(item => <Item key={item.id} {...item} />);
  // Missing: null check before .map()
};

Key questions:

• What does working code do that broken code doesn't?
• What assumptions does broken code make?
• Where do the data flows diverge?

Phase 3: Hypothesis and Testing

Apply the scientific method:

1. Form specific hypothesis

   "The error occurs because products is undefined on initial render before the API response arrives."

2. Test with minimal change

   // Test hypothesis: add early return
   if (!products) return <Loading />;
   

3. One variable at a time

   • Don't bundle multiple fixes
   • Change one thing, test, observe
   • Document what you learned

Phase 4: Implementation

1. Write failing test FIRST

   it('should handle undefined products gracefully', () => {
     render(<ProductList products={undefined} />);
     expect(screen.getByText('Loading...')).toBeInTheDocument();
   });
   

2. Implement root cause fix

   // Fix at the source, not symptoms
   const ProductList = ({ products }: Props) => {
     if (!products) return <Loading />;
     if (products.length === 0) return <Empty />;
     return (
       <ul>
         {products.map(product => (
           <ProductItem key={product.id} product={product} />
         ))}
       </ul>
     );
   };
   

3. Verify without bundling

   • Only the fix, nothing else
   • Run full test suite
   • Confirm original issue resolved

Red Flags - Stop and Reassess

The Three-Fix Rule

If you've tried 3 fixes without success:

1. STOP - More fixes won't help
2. Step back - Is the architecture flawed?
3. Document - What did each attempt reveal?
4. Reconsider - Is this the right approach at all?

Debugging Toolkit

Console Methods

console.log()      // Basic output
console.table()    // Tabular data
console.trace()    // Stack trace
console.time()     // Performance timing
console.group()    // Grouped logs

React DevTools

• Component props inspection
• State timeline
• Profiler for performance

Network Tab

• Request/response payloads
• Timing analysis
• Failed requests

Breakpoints

// Conditional breakpoint
debugger; // Only when condition met

// Logpoint (VS Code)
// Right-click line → Add Logpoint

FrankX-Specific Patterns

Next.js Server Components

// Common issue: using client hooks in server components
// Phase 1: Check if component has 'use client'
// Phase 2: Compare with working client components
// Phase 3: Add 'use client' or restructure

API Route Debugging

// Add request logging
export async function POST(request: Request) {
  console.log('=== API DEBUG ===');
  const body = await request.json();
  console.log('Body:', JSON.stringify(body, null, 2));
  console.log('Headers:', Object.fromEntries(request.headers));
  // ... rest of handler
}

Hydration Mismatches

// Phase 1: Check for Date.now(), Math.random(), or browser APIs
// Phase 2: Find server vs client render differences
// Phase 3: Use useEffect for client-only code
// Phase 4: Verify with 'suppressHydrationWarning' as last resort

When to Use This Skill

• Any bug that isn't immediately obvious
• Bugs that "shouldn't be possible"
• Issues that keep coming back
• Problems others have already attempted to fix

Related Skills

• test-driven-development - Write test for bug first
• webapp-testing - E2E debugging with Playwright
• verification-before-completion - Ensure fix actually works