score:343

Accepted answer

The pattern that you need to follow depends on your use case.

First: You might have a situation where you need to add event listener during the initial mount and clean them up at unmount and another case where a particular listener needs to be cleaned up and re-added on a prop change.

In such a case, using two different useEffect is better to keep the relevant logic together as well as having performance benefits

useEffect(() => {
   // adding event listeners on mount here
   return () => {
       // cleaning up the listeners here
   }
}, []);

useEffect(() => {
   // adding listeners everytime props.x changes
   return () => {
       // removing the listener when props.x changes
   }
}, [props.x])

Second: You need to trigger an API call or some other side-effect when any of the state or props change from a defined set. In such a case a single useEffect with the relevant dependencies to monitor would be better

useEffect(() => {
    // side effect here on change of any of props.x or stateY
}, [props.x, stateY])

Third: You need separate side-effect for different sets of changes. In such a case, separate out relevant side-effects into different useEffects

useEffect(() => {
   // some side-effect on change of props.x
}, [props.x])

useEffect(() => {
   // another side-effect on change of stateX or stateY 
}, [stateX, stateY])

score:13

It's perfectly fine to have have multiple useEffect.

Here's how one of my setups looks like:

/*
 * Backend tags list have changed add the changes if needed
 */
useEffect(() => {
    setTagsList(setTagsAdded);
}, [setTagsAdded]);

/*
 * Backend files have changed add the changes if needed
 */
useEffect(() => {
    for (let i = 0; i < changedFilesMeta.length; i += 1) {
        // Is the list item value changed
        if (changedFilesMeta[i].id === currentEditableFile.id) {
            unstable_batchedUpdates(() => {
                setTags(changedFilesMeta[i].tags ? changedFilesMeta[i].tags : []);
            });
        }
    }
}, [changedFilesMeta]);

/*
 * Reset when user select new files using the filepicker
 */
useEffect(() => {
    if (setNewFiles.length > 0) {
        unstable_batchedUpdates(() => {
            setCurrentFile(null);
            setDescription('');
            setTitle('');
            setTags([]);
        });
    }
}, [setNewFiles]);

/*
 * User selecet to edit a file, change to that file
 */
useEffect(() => {
    // When user select a file to edit it
    if (currentEditableFile && currentEditableFile !== theCurrentFile) {
        setCurrentFile(currentEditableFile);
        unstable_batchedUpdates(() => {
            setDescription(currentEditableFile.description);
            setTitle(currentEditableFile.title);
            setTags(currentEditableFile.tags);
        });
    }
}, [currentEditableFile]);

score:82

You should use multiple effects to separate concerns as suggested by reactjs.org.


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