JavaScript AbortController
The AbortController Object lets you cancel asynchronous operations.
It is commonly used to cancel fetch() requests.
Canceling unnecessary operations can improve performance and make applications more responsive.
Why Cancel a Request?
Sometimes an asynchronous operation is no longer needed.
Examples include:
- The user clicks a Cancel button.
- The user starts another search.
- The user leaves the page.
- A request takes too long.
In these situations, you can cancel the operation instead of waiting for it to finish.
Creating an AbortController
Create an AbortController with the new keyword.
Example
const controller = new AbortController();
The Abort Signal
An AbortController provides an AbortSignal.
The signal is passed to an asynchronous operation.
The operation receives the signal and stops.
Example
const controller = new AbortController();
const signal = controller.signal;
Canceling a Fetch Request
Pass the controller's signal to fetch().
Example
const controller = new AbortController();
fetch("largefile.txt", {
signal: controller.signal
});
Aborting a Request
Call abort() to cancel the operation.
Example
controller.abort();
Complete Example
Example
const controller = new AbortController();
// Async function to download a file
async function loadFile(file) {
try {
const response = await fetch(file, {
signal: controller.signal
});
myDisplayer(await response.text());
} catch(err) {
if (err.name == "AbortError") {
myDisplayer("Download canceled");
}
}
}
// Call the async function
loadFile("fetch.txt");
Try it Yourself »
Cancel with a Button
A request can be canceled when the user clicks a button.
Example
<button onclick='loadFile("fetch.txt")'>Start Download</button>
<button onclick="controller.abort()">Cancel Download</button>
<p id="demo"></p>
Try it Yourself »
Create a New Controller for Each Request
Once an AbortController has been aborted, it cannot be reused.
Create a new controller for each new request.
Cancel the Previous Request
Search applications often cancel an earlier request before starting a new one.
Example
let controller;
async function search(text) {
if (controller) {
controller.abort();
}
controller = new AbortController();
try {
const response = await fetch(
"/search?q=" + text,
{signal: controller.signal}
);
console.log(await response.text());
}
catch(err) {
if (err.name != "AbortError") {
console.log(err);
}
}
}
User types:
c
ca
cat
cats
↓
Only the last request finishes.
Cancel After a Timeout
You can combine timers with AbortController)=.
Example
const controller = new AbortController();
setTimeout(function(){
controller.abort();
},5000);
await fetch(url,{
signal: controller.signal
});
Common Mistakes
Forgetting to pass the signal.
Wrong:
fetch(url);
Right:
fetch(url);
Reusing a canceled controller.
Remember to create a new controller.
Treating AbortError as a Failure
An aborted request is often expected.
Handle it separately.
Example
if (err.name == "AbortError") {
return;
}
Browser Support
AbortController is supported by all modern browsers.
It is widely used together with the Fetch API.
AbortController Checklist
- Create a new controller for each new request
- Create it with new AbortController()
- Pass controller.signal to fetch()
- Call abort() to cancel the request
- Handle AbortError in a catch block
