How do I return the response from an asynchronous function call?
I’m trying to return a value from a function that makes an asynchronous request, but I keep getting undefined instead of the actual response. I’ve tried returning the value from the callback and assigning it to a local variable, but neither approach works.
For example, with jQuery’s ajax function:
function foo() {
var result;
$.ajax({
url: '...',
success: function(response) {
result = response;
}
});
return result; // Always returns undefined
}
The same issue occurs with Node.js fs.readFile:
function foo() {
var result;
fs.readFile("path/to/file", function(err, data) {
result = data;
});
return result; // Always returns undefined
}
And with promises using fetch:
function foo() {
var result;
fetch(url).then(function(response) {
result = response;
});
return result; // Always returns undefined
}
What’s the proper way to return the response from an asynchronous operation in JavaScript?
Async functions in JavaScript always return promises, which is why you’re getting undefined when trying to return values directly from asynchronous operations. The key is to properly handle the asynchronous nature of these operations by using callbacks, promises, or async/await syntax to ensure you’re working with the value after it has been resolved.
Contents
- Understanding the Asynchronous Problem
- Callback-Based Solutions
- Promise-Based Solutions
- Async/Await Solutions
- Best Practices and Recommendations
Understanding the Asynchronous Problem
The fundamental issue in your examples is timing. When you call an asynchronous function like $.ajax(), fs.readFile(), or fetch(), JavaScript doesn’t wait for the operation to complete before moving to the next line of code. Instead, it continues execution, and the callback function (whether it’s a traditional callback or a promise handler) is executed later when the asynchronous operation finishes.
This is why return result always returns undefined - the callback function hasn’t had a chance to execute and assign the value to result yet.
Key Insight: “Async functions always return a promise.” - MDN Web Docs
The word “async” before a function means one simple thing: a function always returns a promise, as explained in the JavaScript.info documentation.
Callback-Based Solutions
For traditional callback-based APIs like jQuery’s ajax or Node.js fs.readFile, you need to use a callback pattern:
// jQuery ajax with callback
function foo(callback) {
$.ajax({
url: '...',
success: function(response) {
callback(response); // Pass the result to the callback
}
});
}
// Usage
foo(function(result) {
console.log(result); // This will get the actual response
});
Alternatively, you can convert callback-based functions to return promises:
// Convert callback to promise using Promise constructor
function readFilePromise(path) {
return new Promise((resolve, reject) => {
fs.readFile(path, (err, data) => {
if (err) reject(err);
else resolve(data);
});
});
}
// Usage
readFilePromise("path/to/file")
.then(result => {
console.log(result); // This will get the file content
});
Promise-Based Solutions
For APIs that already return promises like fetch(), you should return the promise itself:
function foo() {
return fetch(url); // Return the promise directly
}
// Usage
foo()
.then(response => {
console.log(response); // This will get the fetch response
});
Modern JavaScript provides cleaner syntax using async/await:
async function foo() {
const response = await fetch(url);
return response; // Return the resolved value
}
// Usage
foo().then(result => {
console.log(result); // This will get the fetch response
});
Important: “The purpose of async/await is to simplify the syntax necessary to consume promise-based APIs.” - MDN Web Docs
Async/Await Solutions
The most modern and readable approach is using async/await syntax:
// Using async/await with fetch
async function getData() {
const response = await fetch(url);
const data = await response.json();
return data; // This will be wrapped in a promise
}
// Usage
getData().then(data => {
console.log(data); // This gets the actual response data
});
// Or with async function calling another async function
async function main() {
const data = await getData();
console.log(data); // This gets the actual response data
}
As Ben Nadel explains, the return value of an async/await function is implicitly wrapped in a Promise.resolve() call in JavaScript and TypeScript.
// Example of implicit promise wrapping
async function getVal() {
return await doSomethingAsync(); // Return value is wrapped in a promise
}
// Equivalent to:
function getVal() {
return Promise.resolve(doSomethingAsync());
}
Best Practices and Recommendations
1. Always Return Promises from Async Functions
When working with async operations, your functions should return promises:
// Good
async function fetchData() {
const response = await fetch(url);
return response.json();
}
// Better - explicit promise handling
function fetchData() {
return fetch(url)
.then(response => response.json());
}
2. Use Async/Await for Readability
Modern JavaScript code is more readable with async/await:
// Instead of nested callbacks
function complexOperation() {
return fetch(url)
.then(response => response.json())
.then(data => {
return processData(data)
.then(processed => {
return saveData(processed)
.then(() => processed);
});
});
}
// Use async/await
async function complexOperation() {
const response = await fetch(url);
const data = await response.json();
const processed = await processData(data);
await saveData(processed);
return processed;
}
3. Handle Errors Properly
Always handle errors in asynchronous operations:
// Promise error handling
fetch(url)
.then(response => response.json())
.then(data => console.log(data))
.catch(error => console.error('Error:', error));
// Async/await error handling
async function getData() {
try {
const response = await fetch(url);
const data = await response.json();
return data;
} catch (error) {
console.error('Error:', error);
throw error; // Re-throw if needed
}
}
4. Avoid Mixing Patterns
Be consistent with your asynchronous patterns. Don’t mix callbacks with promises in the same function:
// Bad - mixing patterns
function badExample(callback) {
fetch(url)
.then(response => {
callback(response); // Using callback with promise
});
}
// Good - consistent promise approach
function goodExample() {
return fetch(url);
}
Sources
- async function - JavaScript | MDN
- Async/await - JavaScript.info
- Returning Promises From Async / Await Functions In JavaScript - Ben Nadel
- How to return the result of an asynchronous function in JavaScript - Flavio Copes
- How do I return the response from an asynchronous call? - Stack Overflow
- async function implicitly returns promise? - Stack Overflow
- Async / Await - An Idiot’s Guide
- How to use promises - MDN Learn
Conclusion
Returning values from asynchronous functions in JavaScript requires understanding the fundamental asynchronous nature of the language. Here are the key takeaways:
- Async functions always return promises - This is the fundamental rule that explains why you’re getting undefined values
- Don’t try to return values directly - Instead, return the promise or use callback patterns
- Use async/await for cleaner code - Modern JavaScript provides async/await syntax that makes asynchronous code look synchronous
- Handle errors properly - Always include error handling in your asynchronous operations
- Be consistent with patterns - Choose either callbacks, promises, or async/await and stick with it for better code maintainability
The proper way to return the response from an asynchronous operation depends on your specific use case, but the most modern and recommended approach is using async/await syntax, which provides the best balance of readability and functionality.