Async Programming Patterns Skill

Master asynchronous programming - the foundation of Node.js performance and scalability.

Quick Start

Three pillars of async JavaScript:

1. Callbacks - Traditional pattern (error-first)
2. Promises - Modern chainable pattern
3. Async/Await - Synchronous-looking async code

Core Patterns

Callbacks → Promises → Async/Await Evolution

// 1. Callbacks (old way)
fs.readFile('file.txt', (err, data) => {
  if (err) throw err;
  console.log(data);
});

// 2. Promises (better)
fs.promises.readFile('file.txt')
  .then(data => console.log(data))
  .catch(err => console.error(err));

// 3. Async/Await (modern)
async function readFile() {
  try {
    const data = await fs.promises.readFile('file.txt');
    console.log(data);
  } catch (err) {
    console.error(err);
  }
}

Sequential vs Parallel

// ❌ Slow: Sequential (300ms total)
async function slow() {
  const a = await fetch('/api/a'); // 100ms
  const b = await fetch('/api/b'); // 100ms
  const c = await fetch('/api/c'); // 100ms
  return [a, b, c];
}

// ✅ Fast: Parallel (100ms total)
async function fast() {
  const [a, b, c] = await Promise.all([
    fetch('/api/a'),
    fetch('/api/b'),
    fetch('/api/c')
  ]);
  return [a, b, c];
}

Promise Methods

// Promise.all - Wait for all (fails if one fails)
const [users, posts] = await Promise.all([getUsers(), getPosts()]);

// Promise.allSettled - Wait for all (never fails)
const results = await Promise.allSettled([fetch1(), fetch2(), fetch3()]);

// Promise.race - First to complete
const fastest = await Promise.race([server1(), server2()]);

// Promise.any - First to succeed
const result = await Promise.any([tryAPI1(), tryAPI2()]);

Learning Path

Beginner (1-2 weeks)

• ✅ Understand event loop
• ✅ Master callbacks
• ✅ Learn Promises basics
• ✅ Practice async/await

Intermediate (3-4 weeks)

• ✅ Error handling patterns
• ✅ Sequential vs parallel execution
• ✅ Promise composition
• ✅ Event emitters

Advanced (5-6 weeks)

• ✅ Streams and backpressure
• ✅ Concurrency control
• ✅ Circuit breakers
• ✅ Performance optimization

Common Patterns

Error Handling

// Try/catch for async/await
async function safeOperation() {
  try {
    const result = await riskyOperation();
    return { success: true, data: result };
  } catch (error) {
    console.error('Operation failed:', error);
    return { success: false, error: error.message };
  }
}

// Process-level handlers
process.on('unhandledRejection', (reason, promise) => {
  console.error('Unhandled Rejection:', reason);
});

Retry with Exponential Backoff

async function retryWithBackoff(fn, maxRetries = 3) {
  for (let i = 0; i < maxRetries; i++) {
    try {
      return await fn();
    } catch (error) {
      if (i === maxRetries - 1) throw error;
      const delay = Math.pow(2, i) * 1000;
      await new Promise(resolve => setTimeout(resolve, delay));
    }
  }
}

Concurrency Limit

async function batchProcess(items, concurrency = 5) {
  const results = [];
  for (let i = 0; i < items.length; i += concurrency) {
    const batch = items.slice(i, i + concurrency);
    const batchResults = await Promise.all(
      batch.map(item => processItem(item))
    );
    results.push(...batchResults);
  }
  return results;
}

Streams

const { pipeline } = require('stream');
const fs = require('fs');

// Efficient file processing
pipeline(
  fs.createReadStream('input.txt'),
  transformStream,
  fs.createWriteStream('output.txt'),
  (err) => {
    if (err) console.error('Pipeline failed:', err);
    else console.log('Pipeline succeeded');
  }
);

Event Emitters

const EventEmitter = require('events');

class MyEmitter extends EventEmitter {}
const emitter = new MyEmitter();

emitter.on('event', (data) => {
  console.log('Event fired:', data);
});

emitter.emit('event', { id: 123 });

When to Use

Use async patterns when:

• Handling I/O operations (file, network, database)
• Building scalable Node.js applications
• Managing multiple concurrent operations
• Processing streams of data
• Creating event-driven architectures

Related Skills

• Express REST API (async route handlers)
• Database Integration (async queries)
• Testing & Debugging (test async code)
• Performance Optimization (async performance)

Resources

• MDN Promises
• Node.js Async Guide
• JavaScript.info Async