Kaizen

Incremental over revolutionary:

• Make smallest viable change that improves quality
• One improvement at a time
• Verify each change before next
• Build momentum through small wins

Always leave code better:

• Fix small issues as you encounter them
• Refactor while you work (within scope)
• Update outdated comments
• Remove dead code when you see it

Iterative refinement:

• First version: make it work
• Second pass: make it clear
• Third pass: make it efficient
• Don't try all three at once

// Iteration 2: Make it clear (refactor) const calculateTotal = (items: Item[]): number => { return items.reduce((total, item) => { return total + (item.price * item.quantity); }, 0); };

// Iteration 3: Make it robust (add validation) const calculateTotal = (items: Item[]): number => { if (!items?.length) return 0;

return items.reduce((total, item) => { if (item.price < 0 || item.quantity < 0) { throw new Error('Price and quantity must be non-negative'); } return total + (item.price * item.quantity); }, 0); };

Each step is complete, tested, and working
</Good>

<Bad>
```typescript
// Trying to do everything at once
const calculateTotal = (items: Item[]): number => {
  // Validate, optimize, add features, handle edge cases all together
  if (!items?.length) return 0;
  const validItems = items.filter(item => {
    if (item.price < 0) throw new Error('Negative price');
    if (item.quantity < 0) throw new Error('Negative quantity');
    return item.quantity > 0; // Also filtering zero quantities
  });
  // Plus caching, plus logging, plus currency conversion...
  return validItems.reduce(...); // Too many concerns at once
};

Overwhelming, error-prone, hard to verify </Bad>

In Practice

When implementing features:

1. Start with simplest version that works
2. Add one improvement (error handling, validation, etc.)
3. Test and verify
4. Repeat if time permits
5. Don't try to make it perfect immediately

When refactoring:

• Fix one smell at a time
• Commit after each improvement
• Keep tests passing throughout
• Stop when "good enough" (diminishing returns)

When reviewing code:

• Suggest incremental improvements (not rewrites)
• Prioritize: critical → important → nice-to-have
• Focus on highest-impact changes first
• Accept "better than before" even if not perfect

2. Poka-Yoke (Error Proofing)

Design systems that prevent errors at compile/design time, not runtime.

Principles

Make errors impossible:

• Type system catches mistakes
• Compiler enforces contracts
• Invalid states unrepresentable
• Errors caught early (left of production)

Design for safety:

• Fail fast and loudly
• Provide helpful error messages
• Make correct path obvious
• Make incorrect path difficult

Defense in layers:

1. Type system (compile time)
2. Validation (runtime, early)
3. Guards (preconditions)
4. Error boundaries (graceful degradation)

Type System Error Proofing

// Good: Only valid states possible type OrderStatus = 'pending' | 'processing' | 'shipped' | 'delivered'; type Order = {