The React useEffect
hook is great for performing side effects in functional components. One common example of this is fetching data. If you're not careful to clean up your effect, however, you can end up with a race condition! In this post, we'll make sure we appropriately clean up our effects so we don't have this race condition issue.
Setup
In our example app, we are going to fake-load people's profile data when their names are clicked. To help visualize the race condition, we'll create a fakeFetch
function that implements a random delay between 0 and 5 seconds.
const fakeFetch = person => {
return new Promise(res => {
setTimeout(() => res(`${person}'s data`), Math.random() * 5000);
});
};
Initial Implementation
Our initial implementation will use buttons to set the current profile. We reach for the useState
hook to implement this, maintaining the following states:
-
person
, the person selected by the user -
data
, the data loaded from our fake fetch based on the selected person -
loading
, whether data is currently being loaded
We additional use the useEffect
hook, which performs our fake fetch whenever person
changes.
import React, { Fragment, useState, useEffect } from 'react';
const fakeFetch = person => {
return new Promise(res => {
setTimeout(() => res(`${person}'s data`), Math.random() * 5000);
});
};
const App = () => {
const [data, setData] = useState('');
const [loading, setLoading] = useState(false);
const [person, setPerson] = useState(null);
useEffect(() => {
setLoading(true);
fakeFetch(person).then(data => {
setData(data);
setLoading(false);
});
}, [person]);
return (
<Fragment>
<button onClick={() => setPerson('Nick')}>Nick's Profile</button>
<button onClick={() => setPerson('Deb')}>Deb's Profile</button>
<button onClick={() => setPerson('Joe')}>Joe's Profile</button>
{person && (
<Fragment>
<h1>{person}</h1>
<p>{loading ? 'Loading...' : data}</p>
</Fragment>
)}
</Fragment>
);
};
export default App;
If we run our app and click one of the buttons, our fake fetch loads data as expected.
Hitting the race condition
The trouble comes when we start switching between people in quick succession. Given the fact that our fake fetch has a random delay, we soon find that our fetch results may be returned out of order. Additionally, our selected profile and loaded data can be out of sync. That's a bad look!
What's happening here is relatively intuitive: setData(data)
within the useEffect
hook is only called after the fakeFetch
promise is resolved. Whichever promise resolves last will call setData
last, regardless of which button was actually called last.
Canceling previous fetches
We can fix this race condition by "canceling" the setData
call for any clicks that aren't most recent. We do this by creating a boolean variable scoped within the useEffect
hook and returning a clean-up function from the useEffect
hook that sets this boolean "canceled" variable to true
. When the promise resolves, setData
will only be called if the "canceled" variable is false.
If that description was a bit confusing, the following code sample of the useEffect
hook should help.
useEffect(() => {
let canceled = false;
setLoading(true);
fakeFetch(person).then(data => {
if (!canceled) {
setData(data);
setLoading(false);
}
});
return () => (canceled = true);
}, [person]);
Even if a previous button click's fakeFetch
promise resolves later, its canceled
variable will be set to true
and setData(data)
will not be executed!
Let's take a look at how our new app functions:
PerfectโNo matter how many times we click different buttons, we will always only see data associated with the last button click.
Full code
The full code from this blog post can be found below:
import React, { Fragment, useState, useEffect } from 'react';
const fakeFetch = person => {
return new Promise(res => {
setTimeout(() => res(`${person}'s data`), Math.random() * 5000);
});
};
const App = () => {
const [data, setData] = useState('');
const [loading, setLoading] = useState(false);
const [person, setPerson] = useState(null);
useEffect(() => {
let canceled = false;
setLoading(true);
fakeFetch(person).then(data => {
if (!canceled) {
setData(data);
setLoading(false);
}
});
return () => (canceled = true);
}, [person]);
return (
<Fragment>
<button onClick={() => setPerson('Nick')}>Nick's Profile</button>
<button onClick={() => setPerson('Deb')}>Deb's Profile</button>
<button onClick={() => setPerson('Joe')}>Joe's Profile</button>
{person && (
<Fragment>
<h1>{person}</h1>
<p>{loading ? 'Loading...' : data}</p>
</Fragment>
)}
</Fragment>
);
};
export default App;