Checking if a component is unmounted using react hooks?

Question

I'm checking if a component is unmounted, in order to avoid calling state update functions.

1. This is the first option, and it works

const ref = useRef(false)
  useEffect(() => {
    ref.current = true
    return () => {
      ref.current = false
    }
  }, [])

....
if (ref.current) {
  setAnswers(answers)
  setIsLoading(false)
}
....

2. Second option is using useState, which isMounted is always false, though I changed it to true in component did mount

const [isMounted, setIsMounted] = useState(false)

useEffect(() => {
  setIsMounted(true)
  return () => {
    setIsMounted(false)
  }
}, [])

....
if (isMounted) {
  setAnswers(answers)
  setIsLoading(false)
}
....

Why is the second option not working compared with the first option?

Answer 1

I wrote this custom hook that can check if the component is mounted or not at the current time, useful if you have a long running operation and the component may be unmounted before it finishes and updates the UI state.

import { useCallback, useEffect, useRef } from "react";

export function useIsMounted() {
  const isMountedRef = useRef(true);
  const isMounted = useCallback(() => isMountedRef.current, []);

  useEffect(() => {
    return () => void (isMountedRef.current = false);
  }, []);

  return isMounted;
}

Usage

function MyComponent() {
  const [data, setData] = React.useState()
  const isMounted = useIsMounted()

  React.useEffect(() => {
    fetch().then((data) => {
      // at this point the component may already have been removed from the tree
      // so we need to check first before updating the component state
      if (isMounted()) {
        setData(data)
      }
    })
  }, [...])

  return (...)
}

Live Demo

Answer 2

This is a typescript version of @Nearhuscarl's answer.

import { useCallback, useEffect, useRef } from "react";
/**
 * This hook provides a function that returns whether the component is still mounted.
 * This is useful as a check before calling set state operations which will generates
 * a warning when it is called when the component is unmounted.
 * @returns a function
 */
export function useMounted(): () => boolean {
  const mountedRef = useRef(false);
  useEffect(function useMountedEffect() {
      mountedRef.current = true;
      return function useMountedEffectCleanup() {
        mountedRef.current = false;
      };
    }, []);
  return useCallback(function isMounted() {
      return mountedRef.current;
    }, [mountedRef]);
}

This is the jest test

import { render, waitFor } from '@testing-library/react';
import React, { useEffect } from 'react';
import { delay } from '../delay';
import { useMounted } from "./useMounted";

describe("useMounted", () => {

  it("should work and not rerender", async () => {
    const callback = jest.fn();

    function MyComponent() {
      const isMounted = useMounted();

      useEffect(() => {
        callback(isMounted())
      }, [])

      return (<div data-testid="test">Hello world</div>);
    }

    const { unmount } = render(<MyComponent />)
    expect(callback.mock.calls).toEqual([[true]])
    unmount();
    expect(callback.mock.calls).toEqual([[true]])

  })

  it("should work and not rerender and unmount later", async () => {
    jest.useFakeTimers('modern');
    const callback = jest.fn();

    function MyComponent() {
      const isMounted = useMounted();

      useEffect(() => {
        (async () => {
          await delay(10000);
          callback(isMounted());
        })();
      }, [])

      return (<div data-testid="test">Hello world</div>);
    }

    const { unmount } = render(<MyComponent />)
    await waitFor(() => expect(callback).toBeCalledTimes(0));
    jest.advanceTimersByTime(5000);
    unmount();
    jest.advanceTimersByTime(5000);
    await waitFor(() => expect(callback).toBeCalledTimes(1));
    expect(callback.mock.calls).toEqual([[false]])
  })

})

Sources available in https://github.com/trajano/react-hooks-tests/tree/master/src/useMounted

Answer 3

This cleared up my error message, setting a return in my useEffect cancels out the subscriptions and async tasks.

  import React from 'react'

  const MyComponent = () => {
    
   const [fooState, setFooState] = React.useState(null)

   React.useEffect(()=> {

    //Mounted
     getFetch()
        

    // Unmounted
     return () => {
        setFooState(false)
     }
   })

      return (
        <div>Stuff</div>
      )
    }

export {MyComponent as default}

Answer 4

Please read this answer very carefully until the end.

It seems your component is rendering more than one time and thus the isMounted state will always become false because it doesn't run on every update. It just run once and on unmounted. So, you'll do pass the state in the second option array:

}, [isMounted])

Now, it watches the state and run the effect on every update. But why the first option works?

It's because you're using useRef and it's a synchronous unlike asynchronous useState. Read the docs about useRef again if you're unclear:

This works because useRef() creates a plain JavaScript object. The only difference between useRef() and creating a {current: ...} object yourself is that useRef will give you the same ref object on every render.

---

BTW, you do not need to clean up anything. Cleaning up the process is required for DOM changes, third-party api reflections, etc. But you don't need to habit on cleaning up the states. So, you can just use:

useEffect(() => {
    setIsMounted(true)
}, []) // you may watch isMounted state
     // if you're changing it's value from somewhere else

While you use the useRef hook, you are good to go with cleaning up process because it's related to dom changes.

Answer 5

If you want to use a small library for this, then react-tidy has a custom hook just for doing that called useIsMounted:

import React from 'react'
import {useIsMounted} from 'react-tidy'

function MyComponent() {
  const [data, setData] = React.useState(null)
  const isMounted = useIsMounted()
  React.useEffect(() => {
    fetchData().then((result) => {
      if (isMounted) {
        setData(result)
      }
    })
  }, [])
  // ...
}

Learn more about this hook

Disclaimer I am the writer of this library.