I've come across blog posts that suggest clearing component state on unmount to prevent memory leaks:

• Dhiwise Blog
• Medium Post

If you have components that store large amounts of data in their local state and don't properly clean up or reset that state when unmounted, it can lead to memory leaks. Make sure to handle state cleanup diligently.

Storing large data sets or unnecessary data in the component’s state can lead to memory leaks, especially if the data is not properly cleaned up when it’s no longer needed. Make sure to remove or clear unnecessary data from state or other data structures when appropriate, such as in the cleanup functions mentioned earlier.

However, I was under the impression that we don't actually need to clear state when a component unmounts.

So would we need to do something like this on component unmount (code snippet copied from medium post):

  useEffect(() => {
    const fetchData = async () => {
      const response = await fetch('');
      const newData = await response.json();
      setData(newData);
    };
    fetchData();

    return () => {
      // Cleanup function to clear data
      setData([]);
    };
  }, []);

Side Question: Does the above code snippet cause a memory leak? We are clearing the data state on unmount but after the API call finishes, we are making a setData call again. Would that not cause a memory leak? Would love to get clarity on this. Thanks!

I've come across blog posts that suggest clearing component state on unmount to prevent memory leaks:

• Dhiwise Blog
• Medium Post

If you have components that store large amounts of data in their local state and don't properly clean up or reset that state when unmounted, it can lead to memory leaks. Make sure to handle state cleanup diligently.

Storing large data sets or unnecessary data in the component’s state can lead to memory leaks, especially if the data is not properly cleaned up when it’s no longer needed. Make sure to remove or clear unnecessary data from state or other data structures when appropriate, such as in the cleanup functions mentioned earlier.

However, I was under the impression that we don't actually need to clear state when a component unmounts.

So would we need to do something like this on component unmount (code snippet copied from medium post):

  useEffect(() => {
    const fetchData = async () => {
      const response = await fetch('https://api.example/data');
      const newData = await response.json();
      setData(newData);
    };
    fetchData();

    return () => {
      // Cleanup function to clear data
      setData([]);
    };
  }, []);

Side Question: Does the above code snippet cause a memory leak? We are clearing the data state on unmount but after the API call finishes, we are making a setData call again. Would that not cause a memory leak? Would love to get clarity on this. Thanks!

• reactjs
• memory
• memory-leaks

3 Answers 3

It is always good practice to clean up any memory leaks like timer apis, event handlers or any other external api which can potentially lead to any kind of memory leaks. But In case of local state, I believe there is not need to clean that manually. When the component unmounted, it's local variables are un-referenced Garbage collector will automatically detect un-referenced memory and clean that up (using algorithms like mark-and-sweep). There is no need to manually do that.

It's only best practice for timers or event handlers like socket or websockets etc. You are using over a variable of array data type which can also freed from memory if you don't use cleanup function.

The states will be cleaned up automatically for an unmounted component. The app code does not do anything for it. You may see the same discussed here.

The doc here says not to do the below code. The state setter inside a clean up to be removed. Though this documentation is outdated, the same point is there in the new documentation as well.

return () => {
   // Cleanup function to clear data
   setData([]);
};

Another discussion of the same kind can see here as well