How do I cancel an HTTP fetch() request?

Question

There is a new API for making requests from JavaScript: fetch(). Is there any built in mechanism for canceling these requests in-flight?

Answer 1

TL/DR:

fetch now supports a signal parameter as of 20 September 2017, but not all browsers seem support this at the moment.

2020 UPDATE: Most major browsers (Edge, Firefox, Chrome, Safari, Opera, and a few others) support the feature, which has become part of the DOM living standard. (as of 5 March 2020)

This is a change we will be seeing very soon though, and so you should be able to cancel a request by using an AbortControllers AbortSignal.

Long Version

How to:

The way it works is this:

Step 1: You create an AbortController (For now I just used this)

const controller = new AbortController()

Step 2: You get the AbortControllers signal like this:

const signal = controller.signal

Step 3: You pass the signal to fetch like so:

fetch(urlToFetch, {
    method: 'get',
    signal: signal, // <------ This is our AbortSignal
})

Step 4: Just abort whenever you need to:

controller.abort();

Here's an example of how it would work (works on Firefox 57+):

<script>
    // Create an instance.
    const controller = new AbortController()
    const signal = controller.signal

    /*
    // Register a listenr.
    signal.addEventListener("abort", () => {
        console.log("aborted!")
    })
    */


    function beginFetching() {
        console.log('Now fetching');
        var urlToFetch = "https://httpbin.org/delay/3";

        fetch(urlToFetch, {
                method: 'get',
                signal: signal,
            })
            .then(function(response) {
                console.log(`Fetch complete. (Not aborted)`);
            }).catch(function(err) {
                console.error(` Err: ${err}`);
            });
    }


    function abortFetching() {
        console.log('Now aborting');
        // Abort.
        controller.abort()
    }

</script>



<h1>Example of fetch abort</h1>
<hr>
<button onclick="beginFetching();">
    Begin
</button>
<button onclick="abortFetching();">
    Abort
</button>

Sources:

• The final version of AbortController has been added to the DOM specification
• The corresponding PR for the fetch specification is now merged.
• Browser bugs tracking the implementation of AbortController is available here: Firefox: #1378342, Chromium: #750599, WebKit: #174980, Edge: #13009916.