Principle: Small, focused functions

Large functions that do many things are hard to understand, test, and reuse.

Guidelines for function size and focus:

A function should do one thing, do it well, and do it only.

SIGNS A FUNCTION IS TOO LARGE:
- More than ~20 lines (signal, not rule)
- Multiple levels of indentation
- Comments explaining sections (each section = a function)
- Variables used only in one section
- Hard to name (does too many things)
- Hard to test (too many paths)

REFACTORING:

// Before: 60-line function
function processOrder(order) {
  // Validate (10 lines)
  // Calculate totals (15 lines)
  // Apply discounts (10 lines)
  // Save to database (10 lines)
  // Send confirmation (15 lines)
}

// After: Composed from focused functions
function processOrder(order) {
  validateOrder(order);
  const totals = calculateTotals(order);
  const finalTotals = applyDiscounts(totals, order.coupon);
  const savedOrder = saveOrder(order, finalTotals);
  sendConfirmation(savedOrder);
}

When it's OK to be long:

• Simple sequential steps (setup code, config)
• A single switch/match with many cases
• When extraction would add more complexity

The test: Can you describe what the function does without using 'and'? If not, it does too many things.

Robert C. Martin: 'Functions should do one thing. They should do it well. They should do it only.' Small functions compose into readable, maintainable programs.

Code organization and readability