Node.js Async/Await
Introduction to Async/Await
Async/await is a modern way to handle asynchronous operations in Node.js, building on top of Promises to create even more readable code.
Introduced in Node.js 7.6 and standardized in ES2017, async/await allows you to write asynchronous code that looks and behaves more like synchronous code.
Async/await is basically Promises with a more readable syntax. This makes your code cleaner and more maintainable.
Async/await makes asynchronous code look and more feel like synchronous code. It does not block the main thread, but is easy to follow and understand.
Syntax and Usage
The syntax consists of two keywords:
async: Used to declare an asynchronous function that returns a Promiseawait: Used to pause execution until a Promise is resolved, can only be used inside async functions
Example: Basic Async/Await
async function getData() {
console.log('Starting...');
const result = await someAsyncOperation();
console.log(`Result: ${result}`);
return result;
}
function someAsyncOperation() {
return new Promise(resolve => {
setTimeout(() => resolve('Operation completed'), 1000);
});
}
// Call the async function
getData().then(data => console.log('Final data:', data));
Run example »
Example: Reading a File with Async/Await
const fs = require('fs').promises;
async function readFile() {
try {
const data = await fs.readFile('myfile.txt', 'utf8');
console.log(data);
} catch (error) {
console.error('Error reading file:', error);
}
}
readFile();
Run example »
Error Handling with Try/Catch
One of the advantages of async/await is that you can use traditional try/catch blocks for error handling, making your code more readable.
Example: Error Handling with Async/Await
async function fetchUserData() {
try {
const response = await fetch('https://api.example.com/users/1');
if (!response.ok) {
throw new Error(`HTTP error: ${response.status}`);
}
const user = await response.json();
console.log('User data:', user);
return user;
} catch (error) {
console.error('Error fetching user data:', error);
throw error; // Re-throw the error if needed
}
}
You can also mix async/await with Promise .catch() for different scenarios:
// Using catch with an async function
fetchUserData().catch(error => {
console.log('Caught outside of async function:', error.message);
});
Run example »
Running Promises in Parallel
Although async/await makes code look synchronous, sometimes you need to run operations in parallel for better performance.
Example: Sequential vs Parallel Operations
// Helper function to simulate an API call
function fetchData(id) {
return new Promise(resolve => {
setTimeout(() => resolve(`Data for ID ${id}`), 1000);
});
}
// Sequential operation - takes ~3 seconds
async function fetchSequential() {
console.time('sequential');
const data1 = await fetchData(1);
const data2 = await fetchData(2);
const data3 = await fetchData(3);
console.timeEnd('sequential');
return [data1, data2, data3];
}
// Parallel operation - takes ~1 second
async function fetchParallel() {
console.time('parallel');
const results = await Promise.all([
fetchData(1),
fetchData(2),
fetchData(3)
]);
console.timeEnd('parallel');
return results;
}
// Demo
async function runDemo() {
console.log('Running sequentially...');
const seqResults = await fetchSequential();
console.log(seqResults);
console.log('\nRunning in parallel...');
const parResults = await fetchParallel();
console.log(parResults);
}
runDemo();
Run example »
Async/Await vs Promises vs Callbacks
Let's see how the same task is handled with different asynchronous patterns:
With Callbacks
function getUser(userId, callback) {
setTimeout(() => {
callback(null, { id: userId, name: 'John' });
}, 1000);
}
function getUserPosts(user, callback) {
setTimeout(() => {
callback(null, ['Post 1', 'Post 2']);
}, 1000);
}
// Using callbacks
getUser(1, (error, user) => {
if (error) {
console.error(error);
return;
}
console.log('User:', user);
getUserPosts(user, (error, posts) => {
if (error) {
console.error(error);
return;
}
console.log('Posts:', posts);
});
});
Try it Yourself »
With Promises
function getUserPromise(userId) {
return new Promise(resolve => {
setTimeout(() => {
resolve({ id: userId, name: 'John' });
}, 1000);
});
}
function getUserPostsPromise(user) {
return new Promise(resolve => {
setTimeout(() => {
resolve(['Post 1', 'Post 2']);
}, 1000);
});
}
// Using promises
getUserPromise(1)
.then(user => {
console.log('User:', user);
return getUserPostsPromise(user);
})
.then(posts => {
console.log('Posts:', posts);
})
.catch(error => {
console.error(error);
});
Try it Yourself »
With Async/Await
// Using async/await
async function getUserAndPosts() {
try {
const user = await getUserPromise(1);
console.log('User:', user);
const posts = await getUserPostsPromise(user);
console.log('Posts:', posts);
} catch (error) {
console.error(error);
}
}
getUserAndPosts();
Try it Yourself »
| Pattern | Pros | Cons |
|---|---|---|
| Callbacks | - Simple to understand - Widely supported |
- Callback hell - Error handling is complex - Hard to reason about |
| Promises | - Chaining with .then() - Better error handling - Composable |
- Still requires nesting for complex flows - Not as readable as async/await |
| Async/Await | - Clean, synchronous-like code - Easy error handling with try/catch - Easier debugging |
- Requires understanding of Promises - Easy to accidentally block execution |
Best Practices
When working with async/await in Node.js, follow these best practices:
- Remember that async functions always return Promises
async function myFunction() {
return 'Hello';
}
// This returns a Promise that resolves to 'Hello', not the string 'Hello' directly
const result = myFunction();
console.log(result); // Promise { 'Hello' }
// You need to await it or use .then()
myFunction().then(message => console.log(message)); // Hello - Use Promise.all for concurrent operations
When operations can run in parallel, use
Promise.allto improve performance. - Always handle errors
Use try/catch blocks or chain a
.catch()to the async function call. - Avoid mixing async/await with callbacks
Convert callback-based functions to Promises using util.promisify or custom wrappers.
const util = require('util');
const fs = require('fs');
// Convert callback-based function to Promise-based
const readFile = util.promisify(fs.readFile);
async function readFileContents() {
const data = await readFile('file.txt', 'utf8');
return data;
} - Create clean async functions
Keep async functions focused on a single responsibility.
Best Practice: Be aware of the "top-level await" feature available in ECMAScript modules (ESM) in Node.js 14.8.0 and above, which allows using await outside of async functions at the module level.
