Announcing real-cancellable-promise

Hi! I'm Sam, a senior software developer at Interface Technologies.

Today I'm announcing the public release of real-cancellable-promise, a simple but robust cancellable promise library for JavaScript and TypeScript.

real-cancellable-promise solves two key problems that I've encountered in every React app I've ever written:

Problem 1: setState after unmount

Update: This warning has been removed in React 18! 😁

If you try to update your component's state after it has unmounted, you'll get

Warning: Can't perform a React state update on an unmounted component. This is a no-op, but it indicates a memory leak in your application. To fix, cancel all subscriptions and asynchronous tasks in a useEffect cleanup function.

This can happen, for example, if your component starts an API call but the user navigates away before the API call completes. React tells you to "cancel all asynchronous tasks" but doesn't tell you how to do it. That's where real-cancellable-promise comes in.

The CancellablePromise class from real-cancellable-promise is just like a normal promise, except it has a cancel method. You can use the cancel method as the cleanup function in a useEffect to cancel your API call and prevent the setState after unmount warning.

useEffect(() => {
    const cancellablePromise = listBlogPosts()
        .then(setPosts)
        .catch(console.error)

    return cancellablePromise.cancel
}, [])

Problem 2: Queries with variable parameters

API calls often have parameters that can change. A searchUsers API method might take in a search string and return users whose name matches that string. You can implement a React UI for this like:

function searchUsers(searchTerm: string): Promise<User[]> {
    // call the API
}

export function UserList() {
    const [searchTerm, setSearchTerm] = useState('')
    const [users, setUsers] = useState<User[]>([])

    useEffect(() => {
        searchUsers(searchTerm)
            .then(setUsers)
            .catch(console.error)
    }, [searchTerm])

    return <div>...</div>
}

But there are two issues here:

1. If the API calls complete in a different order than they were initiated in, your UI shows the wrong data.
2. If the search term changes while an API call is in progress, the in-progress API call is allowed to complete even though its result is now irrelevant. This wastes bandwidth and server resources.

(Also in a real app you would definitely want to debounce searchTerm, but that's another topic.)

real-cancellable-promise resolves both issues by allowing you to cancel the in-progress API call when the search term changes:

useEffect(() => {
    const cancellablePromise = searchUsers(searchTerm)
        .then(setUsers)
        .catch(console.error)

    return cancellablePromise.cancel
}, [searchTerm])

But I'm using React Query!

The useQuery hook from React Query has many advantages over making API calls in a useEffect like I showed in the previous example. React Query already handles API calls returning in the wrong order, but isn't able to abort the HTTP request without your help. real-cancellable-promise has you covered here — React Query will automatically call the cancel method of CancellablePromise when the query key changes. (Reference)

How do I get started?

Head on over to the README on GitHub for instructions on integrating your HTTP library with real-cancellable-promise and for more detailed examples.

Not just for React

I built CancellablePromise to solve problems I encountered in React development, but the library is not tied to React in any way. real-cancellable-promise is also tested in Node.js and React Native and should provide value in frontend applications built with other frameworks like Vue and Angular.

The story behind the code

While this is the initial public release of the library, older versions of CancellablePromise have been used in production at Interface Technologies for over 3 years! It's one of the foundational components in our family of packages that enable us to deliver stable and user-friendly React apps quickly.

Previous implementations of CancellablePromise were designed specifically to work with async-await and didn't have good support for traditional Promise callbacks via then, catch, and finally. The new CancellablePromise supports everything that normal Promises do, and the nice thing is that your promise stays cancellable no matter what you throw at it:

const cancellablePromise = asyncOperation1()
    .then(asyncOperation2)
    .then(asyncOperation3)
    .catch(asyncErrorHandler)
    .finally(cleanup)

cancellablePromise.cancel() // Cancels ALL the async operations

Prior art

There are other libraries that enable Promise cancellation in JavaScript, namely p-cancelable and make-cancellable-promise.

make-cancellable-promise is limited in that it doesn't provide the facility to cancel the underlying asynchronous operation (often an HTTP call) when cancel is called. It simply prevents your callbacks from running after cancellation occurs.

p-cancelable does let you cancel the underlying operation via the onCancel callback, but the library's API is limited compared to real-cancellable-promise in that

• then, catch, or finally return a normal, non-cancellable Promise and,
• There is no support for returning a cancellable Promise from Promise.all, Promise.race, and Promise.allSettled. real-cancellable-promise provides these via CancellablePromise.all, CancellablePromise.race, and CancellablePromise.allSettled.

Stability

real-cancellable-promise has been tested extensively and is ready for production! The new CancellablePromise will be rolling out to one of our production apps next week, and our other apps will be updated soon after.

Issues

Please post any issues you encounter in the GitHub repository.