JavaScript Promises and Async/Await: Common Pitfalls and Fixes

JavaScript's asynchronous programming model is powerful but can be tricky to master. While Promises and async/await have made asynchronous code more manageable, developers still encounter common pitfalls that lead to bugs, performance issues, and unreadable code. This post explores these challenges and provides practical solutions to write more robust asynchronous JavaScript.

Introduction

Asynchronous programming is fundamental to JavaScript, whether you're making API calls, reading files, or handling user events. Promises (introduced in ES6) and async/await (ES2017) revolutionized how we write async code, but they come with their own set of gotchas. Understanding these common mistakes and their solutions will help you write cleaner, more maintainable asynchronous code.

1. Unhandled Promise Rejections

One of the most common mistakes is failing to handle Promise rejections properly, which can lead to silent failures in your application.

The Problem:

function fetchData() {
  return fetch('https://api.example.com/data')
    .then(response => response.json());
}

// No error handling
fetchData().then(data => console.log(data));

The Fix: Always include a .catch() handler or use try/catch with async/await:

// Promise version
function fetchData() {
  return fetch('https://api.example.com/data')
    .then(response => response.json())
    .catch(error => {
      console.error('Fetch failed:', error);
      throw error; // Re-throw if you want calling code to handle it
    });
}

// Async/await version
async function fetchData() {
  try {
    const response = await fetch('https://api.example.com/data');
    return await response.json();
  } catch (error) {
    console.error('Fetch failed:', error);
    throw error;
  }
}

Best Practice: Consider adding a global unhandled rejection handler as a safety net:

process.on('unhandledRejection', (reason, promise) => {
  console.error('Unhandled Rejection at:', promise, 'reason:', reason);
});

2. Promise Creation Mistakes

Developers often misunderstand when and how to create Promises, leading to anti-patterns.

Common Anti-patterns:

1. Creating unnecessary Promises (Promise constructor antipattern)
2. Not returning Promises from .then() chains
3. Mixing Promise construction styles

Example of Anti-pattern:

// Unnecessary Promise wrapper
function getUser(id) {
  return new Promise((resolve) => {
    fetch(`/users/${id}`)
      .then(res => res.json())
      .then(resolve);
  });
}

The Fix:

// Proper implementation
function getUser(id) {
  return fetch(`/users/${id}`).then(res => res.json());
}

// Or with async/await
async function getUser(id) {
  const response = await fetch(`/users/${id}`);
  return response.json();
}

Key Insight: Only use new Promise() when you need to convert callback-based APIs to Promises. Most modern APIs (like fetch) already return Promises.

3. Async/Await Misuse

While async/await simplifies Promise handling, it's often misused in ways that hurt performance or readability.

Common Mistakes:

1. Unnecessary sequential awaits
2. Forgetting that async functions always return Promises
3. Ignoring error boundaries

Problematic Code:

async function processItems(items) {
  const results = [];
  for (const item of items) {
    // Each await happens sequentially
    const result = await processItem(item);
    results.push(result);
  }
  return results;
}

Improved Version:

async function processItems(items) {
  // Process in parallel
  const promises = items.map(item => processItem(item));
  return Promise.all(promises);
}

Important Note: Be mindful of error handling with Promise.all. If any Promise rejects, the whole batch fails. Use Promise.allSettled() if you need partial results.

4. Memory Leaks in Promise Chains

Long-running Promise chains can cause memory leaks if not managed properly, especially in Node.js applications.

The Problem:

function createChain() {
  let promise = Promise.resolve();
  
  for (let i = 0; i < 100000; i++) {
    promise = promise.then(() => doSomething(i));
  }
  
  return promise;
}

This creates a massive Promise chain that keeps all intermediate Promises in memory until the chain completes.

The Fix:

async function optimizedChain() {
  for (let i = 0; i < 100000; i++) {
    await doSomething(i);
    // Garbage collector can clean up previous iterations
  }
}

Alternative Solution: For truly massive operations, consider breaking the work into batches with setImmediate or similar techniques to prevent blocking the event loop.

Conclusion

Mastering Promises and async/await requires understanding not just the syntax but also the underlying patterns and pitfalls. Key takeaways:

1. Always handle Promise rejections explicitly
2. Avoid unnecessary Promise construction
3. Use parallel execution when possible
4. Be mindful of memory usage in long Promise chains
5. Remember that async functions always return Promises

By being aware of these common pitfalls and applying the fixes we've discussed, you'll write more robust, performant asynchronous JavaScript code. The async/await syntax makes asynchronous code appear synchronous, but it's crucial to remember that under the hood, you're still working with Promises and the event loop.

For further learning, explore Promise combinators like Promise.allSettled(), Promise.any(), and patterns like cancellation tokens for more advanced Promise management.