A React hook that tells if the component is mounted.

useIsMounted.mjs

import React from 'react'

export const useIsMounted = () => {
  const ref = React.useRef(false)
  const [, setIsMounted] = React.useState(false)
  React.useEffect(() => {
    ref.current = true
    setIsMounted(true)
    return () => (ref.current = false)
  }, [])
  return () => ref.current
}

@geoguide really trying to understand this myself.

Do you guys have the unit test of the short way as well :)?

Use this one instead: https://github.com/jmlweb/isMounted
It doesn't use state for this, which is good.

yarn add ismounted

Thanks the trick was to return the ref itself not ref.current that way it doesn't need state.

I'm curious why React wouldn't do this in useState itself?

Well the warning doesn't mention where it happens, it just says it happened which means they don't know where this problem occurs to solve it, probably useState is not aware at all on which component it is, to see if it was unmounted not run it, just saying I'm not sure though.

I hope I'm not too late to the party. This is working fine for me so far.

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

export function useIsMounted(): { current: boolean } {
  const isMounted = useRef(false);
  useEffect(() => {
    isMounted.current = true;

    return () => {
      isMounted.current = false;
    };
  }, []);

  const mounted = useCallback(() => {
    return isMounted;
  }, []);

  return mounted();
}

import { useState, useEffect } from 'react';
import { useIsMounted } from '@/helper/useIsMounted';

export default function TestIsMounted () {
const [state, stateSet] = useState(false);

const isMounted = useIsMounted();

useEffect(() => {
  isMounted.current && stateSet(true);
}, [state])

...

return (...)
}

why don’t return isMount.current? When you use the hook, what you want is just true or false. Does return isMount.current break the hook?

Hi Daveteu,

You are correct. I tried returning isMount.current and got it to work. Thank you.

export default function useIsMounted(): boolean {
  const isMounted = useRef(true);
  useEffect(() => {
    return () => {
      isMounted.current = false;
    };
  }, []);

  const mounted = useCallback(() => {
    return isMounted.current;
  }, []);

  return isMounted()
}

const isMounted = useIsMounted();

useEffect(() => {
  isMounted && stateSet(true);
}, [state])

What's the point of the setState part? I think the isMount hook itself is sufficient.

Think about it, if you don't change the state you will not be able to for instance use useEffect on the result of that hook, basically a rerender won't occur, of course you can reach the value if you want to and it is changed but you can't monitor the changes.

Normally you don't use the result from the hook as a dependency of a useEffect. You only use it for check.

E.g.

useEffect(() => {
  if (isMounted) {
    do something.
 }

},[props])

Just be aware that what you want from a useIsMounted hook is not for it to return a boolean, but to return a way to check on demand the mounted status. For example, maybe you awaited a loading promise and want to update some state, but want to first check the component is still mounted before doing so to avoid a React set state on unmounted component error. If you read a simple boolean that was created at the same render that started the loading side effect, it will be stale. The component may be unmounted because that boolean is out of date.

I don't actually have a need for a useIsMounted hook anymore, because I use a useForceUpdate pattern instead, for example: https://github.com/jaydenseric/graphql-react/blob/f7d3a17edb99dc3ffabe8598151df8bf942e2258/useLoadingEntry.mjs#L22

@daveteu You've got a point there, I don't remember why but I needed it in cases like that dependency, it should work without setState for you though.

@tamvo22 @daveteu It's not correct to return isMounted() from hook because it will reflect the mounted state at the time of hook call, not when async function finishes. I.e. the hook needs to return a 'checker function', not result of the function.

But the function needs to be memoized, otherwise when it's added to useEffect dependencies (eslint and it's autofix will suggest doing this), it will trigger effect recalculation on each re-render. So I think this implementation is correct: https://gist.github.com/jaydenseric/a67cfb1b809b1b789daa17dfe6f83daa?permalink_comment_id=3401782#gistcomment-3401782

@timoxley I have since found ways to avoid having to use a useIsMounted hook, but from my rusty memory, the state is updated arbitrarily in the useEffect callback to trigger a re-render? I worked through a lot of gotchas to get it going.

The state update is in fact an instance of the forceUpdate pattern.

@zmeyc Thank you for the clarification! I tested with a delayed API calls and found that you are right. As you pointed out, it only reflected the initial mount status hook call. All this time, I was under the assumption that isMounted would return the useRef value at the time of the API request. When I pass the function isMounted() to call during the API return, it does provide the correct mounted value when my test component mounted and unmounted. Thanks again cheers!

One more option to the initial value being true or false debate:

const useIsMounted = (initialValue = true) => {
  const ref = useRef(initialValue)

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

  return ref
}

Let that value be passed in. If true then it behaves like its mounted initially and only changes to false when it's unmounted. Useful to stop yourself from setting state when an async action is done after a component was unmounted. But if false then it's treated like the original solution where it still flips to false when unmounted but also flips to true when ready in the client.