use-promised-state v1.0.0

use-promised-state

What?

Like useState(), but it doesn't complain when used after unmount.

yarn add use-promised-state

import usePromisedState from 'use-promised-state'

function MyComponent(props) {
  let [state, setState] = usePromisedState(props.initialValue)

  // ...
}

Why?

When working with Promises in React, you'll sometimes come across errors like this one:

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.

While there are a number of situations where you do have a memory leak, there's another common situation where this occurs: *you've called setState() from a promise handler.

For example:

const [username, setUsername] = useState('')
const [availability, setAvailability] = useState({})

const handleChange = event => {
  const username = event.target.value
  setUsername(username)
  if (username !== availability.username) {
    getUsernameAvailability(username).then(
      // If the component is unmounted before the username's
      // availability is known, then calling `setAvailability`
      // will result in the above error.
      isAvailable => setAvailability({ username, isAvailable })
    )
  }
}

Because JavaScript promises are not cancellable, there's no way to avoid the fact that the handler will eventually be called -- you haven't actually got a memory leak. But you do have an error, and fixing that error has taken a lot of code -- until now.

The usePromisedState() hook is just like useState(). The only difference is that it won't emit an error on the first time that you try to set the state after unmount.

const [availability, setAvailability] = usePromisedState({})

The usePromisedState() hook is a signal that yes, I know that calling setState after unmount doesn't do anything, and I don't care. It makes working with promises easier, and it'll still emit an error if you keep trying to set state after unmount, because that probably does indicate a memory leak.