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React useEffect cleanup – DEV Community


React’s useEffect hook is an excellent hook to run unintended effects.
You is likely to be questioning what sort of unintended effects we could possibly be speaking about?

Let’s set some examples.

  • Change the doc title primarily based on rendered values
  • Get or set values from native storage
  • Run analytics occasions
  • Present some greeting primarily based on time of day
  • Give attention to a kind subject after load



The essential use of useEffect

Will probably be adequate to make use of the hook as meant, so set an instance normally.

useEffect(() => {
  doc.title = `The web page is loaded`;
});
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As we realized earlier than we are able to set the dependency at which change this wants to fireplace:

useEffect(() => {
  doc.title = `The web page ${title} is loaded`;
}, [title]);
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The above code will solely fireplace as soon as the title variable is modified.

We are able to additionally choose to run solely as soon as on mount, by passing a empty array like this:

useEffect(() => {
  // Solely run as soon as
  doc.title = `The web page ${title} is loaded`;
}, []);
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Cleansing up useEffect

The hook comes with a cleanup perform, which you won’t all the time want, however it may possibly turn out to be useful.

To invoke the cleanup perform you’ll be able to merely add a return perform like so:

useEffect(() => {
  // Your impact

  return () => {
    // Cleanup
  };
}, []);
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The cleanup can stop reminiscence leaks and take away undesirable issues. Some use-cases for this are:

  • Clear up subscriptions
  • Clear up modals
  • Take away occasion listeners
  • Clear timeouts

Let’s create an instance the place we’ve got a perform that provides one thing solely after a selected time.

const [show, setShow] = useState(false);
useEffect(() => {
  let timer = setTimeout(() => setShow(true), 3000);
}, []);
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Nonetheless, this may create a timeout in reminiscence, so it will be finest to scrub this up.

For this let’s add the cleanup perform:

useEffect(() => {
  let timer = setTimeout(() => setShow(true), 3000);
  return () => {
    clearTimeout(timer);
  };
}, []);
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One other instance is, after all, an online socket name that retains polling.

useEffect(() => {
  let ws = new WebSocket('wss://ws.your-websocket/');

  ws.onmessage = (msg) => {
    // Do one thing with the message
  };

  return () => {
    ws.shut();
  };
}, []);
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We open the WebSocket connection, and we are able to use the cleanup perform to shut the connection.

One other factor you need to use it for is monitoring modal shut occasions, for example.

As an example we’ve got a modal in our code. Contained in the modal part, we may add a useEffect that may fireplace on cleanup. This manner, we seize each occasion.

A modal could possibly be invoked by one other part that cleaned up within the meantime, so there is no such thing as a method of telling when the person closes it. (Or they shut the appliance)

We are able to add monitoring to fireplace with a cleanup impact when this occurs.

useEffect(() => {
  return () => {
    trackModalClose();
  };
}, []);
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This manner, we’ve got a rock-solid technique of monitoring when the modal shut must be invoked, and you may even add a test to see if the functions as closed or the modal shut was invoked.



Conclusion

I hope you loved this text on the cleanup perform for the useEffect hook in React.

Let me know you probably have any questions or different use-cases that might be price exploring.



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