DEV Community

How to use componentWillUnmount with Functional Components in React

Robert Marshall on May 17, 2020

Most up to date version of this article: https://robertmarshall.dev/blog/componentwillunmount-functional-components-react Functional componen...

Read full post

sridhar.natuva • May 31 '22

I want to display an dialog box on unmount and When user cancels it should stop unmounting the component.

Right now, even if user cancels or says ok, component will anyway unmount on return funtion.

Is there any way to stop unmount on the return function? Or any other approach?

ARNAB MUNSHI • Jun 10 '20

Hi, I need a help

  componentDidMount() {  
    this._subscribe();
  }

  componentWillUnmount() {
    this._unsubscribe();
  }

  async _subscribe() {
    const batteryLevel = await Battery.getBatteryLevelAsync();
    this.setState({ batteryLevel });
    this._subscription = Battery.addBatteryLevelListener(({ batteryLevel }) => {
      this.setState({ batteryLevel });
      console.log('batteryLevel changed!', batteryLevel);
    });
  }

  _unsubscribe() {
    this._subscription && this._subscription.remove();
    this._subscription = null;
  }

How to convert this in functional component ?
I have done something like this ...

  const getLevel = async () => {
    console.log("A: " + (await Battery.getBatteryLevelAsync()));

    let batteryLevel = Math.round((await Battery.getBatteryLevelAsync()) * 100);
    setLevel(batteryLevel);
    Battery.addBatteryLevelListener(({ batteryLevel }) => {
      batteryLevel = Math.round(batteryLevel * 100);
      setLevel(batteryLevel);

      console.log("B: " + batteryLevel);
    });
  };

  useEffect(() => {
    // Anything in here is fired on component mount.
    // componentDidMount
    getLevel();

    // componentWillUnmount
    return () => {
      // Anything in here is fired on component unmount.
    };
  }, []);

I am facing problem in

return function

Robert Marshall • Sep 2 '20

If you don't need anything to fire on component unmount, you can remove the return.

  useEffect(() => {
    getLevel();
  }, []);

gopeeey • Apr 28 '21

Thank you very much for this

amahecool • May 2 '21 • Edited

the returned function fires on component Re-render also. How to you check if its an unmount or just a re-render ?
Basically its just a clean up function which runs every time the useEffect is used (except the first time)

Mohammad Jawad (Kasir) Barati • Jan 1 • Edited

Are you sure? AFAIK it should not be the case, it's only triggered once the component is unmounting or remounting itself. So maybe you can check that out.

Also you might have other useEffect who are defined sooner and as you might know the order of defining useEffect hooks matter

Robert Marshall • May 25 '21

I suppose you could use a useRef to track the render (it will not be cleaned on a rerender). Depends on what you need it to do.

Vladan Profirovic • Apr 26 '21

import React, { useEffect } from 'react';
const ComponentExample => () => {
useEffect( () => {
// Anything in here is fired on component mount.
}, []);
}
hi srry this show me that is fired on component mount but not on will mount, so i try to find what is best solution to replace componentWillMount. Tnx.

Rahul Bhadhoriya • Aug 9 '20

Thanks very much

Hector Bonilha • Aug 2 '22

This is going to simulate componentDidUnmount, not componentWillUnmount

Mohammad Jawad (Kasir) Barati • Jan 1

Hm, interesting. Can you explain it further in detail?

Shivam • Aug 12 '22

Nice this is what I was searching for :).

Alex Vallejo • Mar 15 '21

"Parsing error: Unexpected token, expected ","" is what i get when i try to return that

Robert Marshall • May 25 '21

Looks like you have a typo?

Pablo Bion • Jan 6 '21

Very cool!

Mohammad Jawad (Kasir) Barati • Jan 1

BTW I believe this Stackoverflow Q&A would be a real complimentary content to your post: stackoverflow.com/q/77221997/8784518

Ankit Kumar Gupta • May 26 '21

Hey, do we have some restrictions with using the current state values inside ComponentWillUnmount cleanup function ?
Few of my state values are reseted before my code could run 😔

Robert Marshall • Jul 19 '21

Able to give a bit more context? If you are passing this state out of the component (e.g. to a parent) and then unmounting this component I imagine you would have problems.

Umar Sanil • Jun 9 '21

Thanks