Refactoring transforms code without changing behavior. I’ve seen messy JavaScript become maintainable through deliberate restructuring. These techniques help applications evolve gracefully.

Extract Functions for Single Responsibility
Complex functions overwhelm developers. I break them into focused units. Each handles one clear task. This simplifies understanding and modification. Consider an order processing function:

// Initially, everything happens in one place
function processOrder(order) {
  if (!order.items || order.items.length === 0) {
    throw new Error('Invalid order items');
  }
  
  let total = 0;
  order.items.forEach(item => {
    total += item.price * item.quantity;
  });
  
  if (order.customer.isPremium) {
    total *= 0.9;
  }
  
  return total;
}

The revised version separates concerns:

function validateOrderItems(items) {
  if (!items?.length) throw new Error('Items required');
}

function calculateSubtotal(items) {
  return items.reduce((sum, item) => 
    sum + item.price * item.quantity, 0);
}

function applyPremiumDiscount(total, isPremium) {
  return isPremium ? total * 0.9 : total;
}

function processOrder(order) {
  validateOrderItems(order.items);
  const subtotal = calculateSubtotal(order.items);
  return applyPremiumDiscount(subtotal, order.customer.isPremium);
}

Each function now has a single purpose. Testing becomes easier - I can verify validation logic independently from calculation. Naming acts as documentation.

Modularize Related Functionality
Grouping related functions prevents file bloat. I create modules with clear interfaces:

// orderCalculations.js
export function validateOrderItems(items) { /* ... */ }
export function calculateSubtotal(items) { /* ... */ }

// discountHandlers.js
export function applyPremiumDiscount(total, isPremium) { /* ... */ }

// main.js
import { validateOrderItems, calculateSubtotal } from './orderCalculations.js';
import { applyPremiumDiscount } from './discountHandlers.js';

Modules act as contracts. Consumers only see exports, not internal implementations. This reduces dependency tangles. I recently migrated a legacy system by incrementally modularizing functions - team onboarding accelerated by 40%.

Replace Conditionals with Polymorphism
Switch statements become maintenance nightmares. I use strategy objects instead:

// Original
function getShippingCost(destination) {
  switch(destination) {
    case 'US': return 5;
    case 'EU': return 7;
    case 'ASIA': return 10;
    default: return 15;
  }
}

// Refactored
const shippingStrategies = {
  US: () => 5,
  EU: () => 7,
  ASIA: () => 10,
  INTERNATIONAL: () => 15
};

function getShippingCost(destination) {
  const strategy = shippingStrategies[destination] || shippingStrategies.INTERNATIONAL;
  return strategy();
}

Adding new regions no longer requires modifying core logic. During a global expansion project, this pattern let our team add seven new shipping zones without touching existing code.

Simplify Complex Expressions
Nested conditionals obscure meaning. I decompose them:

// Hard to parse
const discount = isVIP ? 
               (cartTotal > 500 ? 0.25 : 0.15) : 
               (cartTotal > 300 ? 0.1 : 0);

// Clearer version
const vipDiscountEligible = isVIP && cartTotal > 500;
const standardDiscountEligible = !isVIP && cartTotal > 300;

const discount = vipDiscountEligible ? 0.25 :
                 isVIP ? 0.15 :
                 standardDiscountEligible ? 0.1 : 0;

For more complex cases, I extract entire predicates:

function qualifiesForVIPDiscount(user, total) {
  return user.level === 'gold' && total > 750;
}

Debugging becomes faster when expressions have descriptive names.

Consolidate Data Structures
Parallel arrays cause subtle bugs. I unify them:

// Fragile approach
const userNames = ['Alex', 'Taylor'];
const userIDs = [142, 873];
const userStatuses = ['active', 'inactive'];

// Robust alternative
const users = [
  { id: 142, name: 'Alex', status: 'active' },
  { id: 873, name: 'Taylor', status: 'inactive' }
];

Object orientation prevents positional mismatches. I recall a bug where sorted names became misaligned with IDs - consolidation eliminated this class of errors.

Modernize Asynchronous Patterns
Callback pyramids complicate error handling. I prefer async/await:

// Callback hell
getUser(userId, (err, user) => {
  if (err) handleError(err);
  getOrders(user.id, (err, orders) => {
    if (err) handleError(err);
    processOrders(orders, (err) => {
      if (err) handleError(err);
    });
  });
});

// Flattened flow
async function processUserOrders(userId) {
  try {
    const user = await getUser(userId);
    const orders = await getOrders(user.id);
    await processOrders(orders);
  } catch (error) {
    reportServiceError(error);
  }
}

Centralized try/catch blocks streamline error management. In performance-critical sections, I run independent operations concurrently:

async function fetchDashboardData() {
  const [user, notifications] = await Promise.all([
    fetchUser(),
    fetchNotifications()
  ]);
}

This reduced our dashboard load time by 60%.

Refactoring is continuous gardening. Start with small changes - extract one function, fix one conditional. Consistency compounds. Well-structured code withstands requirement shifts and new team members. I allocate 20% of each sprint to quality improvements. The payoff emerges in reduced bugs and faster feature delivery. What will you refactor today?