This might be controversial. As far as I know, the async/await feature is favored by many developers. For those unfamiliar, simply put, it’s a native syntax that makes it more elegant for developers to handle Promises.
async/await syntax:
async function doSomethingCool() {
let someValue = await getSomePromise();
console.log(someValue + '!!!');
}
Promise syntax:
function doSomethingCool() {
getSomePromise.then(someValue => someValue + '!!!');
}
In the above example, assume getSomePromise is a function that returns a Promise.
Looking Back#
Many years ago, C# introduced the async/await feature, which excited many people and was touted as ‘making asynchronous programming simpler and more understandable’ (at least that’s what my colleagues thought), because its ‘simplicity’ made many people like it. Now the async/await feature has landed in JavaScript, and I’ve heard the same voices. But I have doubts about the benefits it brings.
The Advantages of Async/Await#
1. Cleaner Syntax#
// Before: Promise chains
function fetchUserData() {
return fetch('/api/user')
.then(response => response.json())
.then(user => {
return fetch(`/api/posts/${user.id}`)
.then(response => response.json())
.then(posts => {
return { user, posts };
});
});
}
// After: async/await
async function fetchUserData() {
const response = await fetch('/api/user');
const user = await response.json();
const postsResponse = await fetch(`/api/posts/${user.id}`);
const posts = await postsResponse.json();
return { user, posts };
}
2. Better Error Handling#
// Before: Promise error handling
function fetchData() {
return fetch('/api/data')
.then(response => {
if (!response.ok) {
throw new Error('Network response was not ok');
}
return response.json();
})
.catch(error => {
console.error('Error:', error);
throw error;
});
}
// After: async/await with try/catch
async function fetchData() {
try {
const response = await fetch('/api/data');
if (!response.ok) {
throw new Error('Network response was not ok');
}
return await response.json();
} catch (error) {
console.error('Error:', error);
throw error;
}
}
3. Sequential vs Parallel Execution#
// Sequential execution (slower)
async function sequentialFetch() {
const user = await fetch('/api/user');
const posts = await fetch('/api/posts');
const comments = await fetch('/api/comments');
return { user, posts, comments };
}
// Parallel execution (faster)
async function parallelFetch() {
const [user, posts, comments] = await Promise.all([
fetch('/api/user'),
fetch('/api/posts'),
fetch('/api/comments')
]);
return { user, posts, comments };
}
4. Easier Debugging#
// Before: Hard to debug Promise chains
function complexOperation() {
return fetch('/api/data')
.then(response => response.json())
.then(data => {
// Hard to set breakpoints here
return processData(data);
})
.then(result => {
// And here
return saveResult(result);
});
}
// After: Easy to debug with async/await
async function complexOperation() {
const response = await fetch('/api/data');
const data = await response.json();
// Easy to set breakpoints and inspect variables
const processedData = processData(data);
const result = await saveResult(processedData);
return result;
}
The Disadvantages of Async/Await#
1. Performance Overhead#
// async/await creates additional overhead
async function overheadExample() {
// Each await creates a microtask
const result1 = await operation1();
const result2 = await operation2();
const result3 = await operation3();
return result1 + result2 + result3;
}
// Promise.all is more efficient for parallel operations
function efficientExample() {
return Promise.all([
operation1(),
operation2(),
operation3()
]).then(results => {
return results.reduce((sum, result) => sum + result, 0);
});
}
2. Hidden Complexity#
// This looks simple but hides complexity
async function simpleLooking() {
const user = await fetchUser();
const posts = await fetchPosts(user.id);
return posts;
}
// But this is what's actually happening
function whatActuallyHappens() {
return fetchUser()
.then(user => {
return fetchPosts(user.id);
})
.then(posts => {
return posts;
});
}
3. Error Propagation Issues#
// async/await can hide error sources
async function errorExample() {
try {
const result = await someOperation();
return result;
} catch (error) {
// Where did this error come from?
console.error('Something went wrong:', error);
throw error;
}
}
// Promises make error sources clearer
function clearerErrorExample() {
return someOperation()
.then(result => {
return result;
})
.catch(error => {
// Error source is clearer in the chain
console.error('Error in someOperation:', error);
throw error;
});
}
4. Memory Usage#
// async/await can lead to higher memory usage
async function memoryIntensive() {
const results = [];
for (let i = 0; i < 1000; i++) {
// Each iteration creates a new Promise
const result = await fetchData(i);
results.push(result);
}
return results;
}
// More memory efficient with Promise.all
function memoryEfficient() {
const promises = [];
for (let i = 0; i < 1000; i++) {
promises.push(fetchData(i));
}
return Promise.all(promises);
}
Common Pitfalls#
1. Forgetting await#
// Bug: Missing await
async function buggyFunction() {
const result = fetchData(); // Should be await fetchData()
return result; // Returns a Promise, not the actual data
}
// Fixed
async function fixedFunction() {
const result = await fetchData();
return result;
}
2. Unnecessary await#
// Unnecessary await
async function unnecessaryAwait() {
const result = await Promise.resolve(42);
return result;
}
// Better
function betterVersion() {
return Promise.resolve(42);
}
3. Mixing async/await with Promises#
// Confusing mix
async function confusingMix() {
const result = await fetchData()
.then(data => processData(data))
.catch(error => handleError(error));
return result;
}
// Clearer with pure async/await
async function clearerVersion() {
try {
const data = await fetchData();
const result = processData(data);
return result;
} catch (error) {
return handleError(error);
}
}
Best Practices#
1. Use async/await for Sequential Operations#
async function sequentialOperations() {
const user = await authenticateUser();
const permissions = await getUserPermissions(user.id);
const data = await fetchUserData(user.id, permissions);
return data;
}
2. Use Promise.all for Parallel Operations#
async function parallelOperations() {
const [user, posts, comments] = await Promise.all([
fetchUser(),
fetchPosts(),
fetchComments()
]);
return { user, posts, comments };
}
3. Handle Errors Appropriately#
async function properErrorHandling() {
try {
const result = await riskyOperation();
return result;
} catch (error) {
if (error instanceof ValidationError) {
// Handle validation errors
return { error: 'Invalid input' };
} else if (error instanceof NetworkError) {
// Handle network errors
return { error: 'Network issue' };
} else {
// Handle unexpected errors
throw error;
}
}
}
4. Use async/await with Generators#
async function* asyncGenerator() {
for (let i = 0; i < 10; i++) {
const result = await fetchData(i);
yield result;
}
}
// Usage
async function useAsyncGenerator() {
for await (const data of asyncGenerator()) {
console.log(data);
}
}
Performance Considerations#
1. Avoid Unnecessary Awaits#
// Bad: Unnecessary await
async function badPerformance() {
const result = await Promise.resolve(42);
return result;
}
// Good: Direct return
function goodPerformance() {
return Promise.resolve(42);
}
2. Use Promise.all for Independent Operations#
// Bad: Sequential when parallel is possible
async function badParallel() {
const a = await fetchA();
const b = await fetchB();
const c = await fetchC();
return { a, b, c };
}
// Good: Parallel execution
async function goodParallel() {
const [a, b, c] = await Promise.all([
fetchA(),
fetchB(),
fetchC()
]);
return { a, b, c };
}
3. Consider Memory Usage#
// Bad: High memory usage
async function highMemory() {
const results = [];
for (let i = 0; i < 10000; i++) {
const result = await fetchData(i);
results.push(result);
}
return results;
}
// Good: Lower memory usage
async function lowMemory() {
const results = [];
const batchSize = 100;
for (let i = 0; i < 10000; i += batchSize) {
const batch = await Promise.all(
Array.from({ length: batchSize }, (_, j) => fetchData(i + j))
);
results.push(...batch);
}
return results;
}
Conclusion#
Async/await is a powerful feature that makes asynchronous code more readable and maintainable, but it’s not without its drawbacks:
Advantages:#
- Cleaner syntax and better readability
- Easier error handling with try/catch
- Better debugging experience
- Sequential execution when needed
Disadvantages:#
- Performance overhead in some cases
- Hidden complexity that can be misleading
- Error propagation issues
- Memory usage concerns
Best Practices:#
- Use async/await for sequential operations
- Use Promise.all for parallel operations
- Handle errors appropriately
- Consider performance implications
- Avoid unnecessary awaits
- Be mindful of memory usage
The key is to use async/await judiciously, understanding when it adds value and when it might introduce unnecessary complexity or performance issues. Like any tool, it’s most effective when used appropriately for the specific use case.