How to load App class`s componentWillUnmount

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8 Answers

90%

I am sure there are tons of other use cases out there, but these are the cases where I have used componentWillUnMount. I hope it helps you., 1 In componentDidMount, if you are setting isMounted as true then what's the point of having if condition? – Ankit Jindal Dec 12 '20 at 15:30 ,Find centralized, trusted content and collaborate around the technologies you use most.

First one is -

import React, { Component } from 'react';

export default class SideMenu extends Component {

    constructor(props) {
        super(props);
        this.state = {
              };
        this.openMenu = this.openMenu.bind(this);
        this.closeMenu = this.closeMenu.bind(this);
    }

    componentDidMount() {
        document.addEventListener("click", this.closeMenu);
    }

    componentWillUnmount() {
        document.removeEventListener("click", this.closeMenu);
    }

    openMenu() {
    }

    closeMenu() {
    }

    render() {
        return (
            <div>
                    <a
                        href      = "javascript:void(0)"
                        className = "closebtn"
                        onClick   = {this.closeMenu}
                    >
                        ×
                    </a>
                  <div>
                     Some other structure
                  </div>
                </div>
        );
    }
}

Second one is -

import React from 'react';
import { Component } from 'react';
import ReactDom from 'react-dom';
import d3Chart from './d3charts';


export default class Chart extends Component {

    static propTypes = {
            data: React.PropTypes.array,
            domain: React.PropTypes.object
    };

    constructor(props){
        super(props);

    }

    componentDidMount(){
        let el = ReactDom.findDOMNode(this);
        d3Chart.create(el, {
            width: '100%',
            height: '300px'
        }, this.getChartState());
    }

    componentDidUpdate() {
        let el = ReactDom.findDOMNode(this);
        d3Chart.update(el, this.getChartState());
    }

    getChartState() {
        return {
            data: this.props.data,
            domain: this.props.domain
        }
    }

    componentWillUnmount() {
        let el = ReactDom.findDOMNode(this);
        d3Chart.destroy(el);
    }

    render() {
        return (
            <div className="Chart">
            </div>
        );
    }
}
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88%

You should not call setState() in componentWillUnmount() because the component will never be re-rendered. Once a component instance is unmounted, it will never be mounted again.,UNSAFE_componentWillUpdate() will not be invoked if shouldComponentUpdate() returns false.,componentDidUpdate() will not be invoked if shouldComponentUpdate() returns false.

class Welcome extends React.Component {
  render() {
    return <h1>Hello, {this.props.name}</h1>;
  }
}
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72%

The componentWillUnmount() method allows us to execute the React code when the component gets destroyed or unmounted from the DOM (Document Object Model). This method is called during the Unmounting phase of the React Life-cycle i.e before the component gets unmounted.,All the cleanups such as invalidating timers, canceling network requests, or cleaning up any subscriptions that were created in componentDidMount() should be coded in the componentWillUnmount() method block.,Tip: Never call setState() in componentWillUnmount() method.

Syntax:

componentWillUnmount()

Step 1: Create a React application using the following command:

npx create - react - app functiondemo

Step 2: After creating your project folder i.e. functiondemo, move to it using the following command:

cd functiondemo

Step to Run Application: Run the application using the following command from the root directory of the project: 

npm start
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65%

Our App is basically just rendering this Pets component:,And we will use it in our Pet component like this:,So basically we just need to NOT update the state in the callback if the component is not mounted already.

function Pets() {
  const [pets, dispatch] = useReducer(petsReducer, initialState);

  const onChange = ({ target }) => {
    dispatch({ type: "PET_SELECTED", payload: target.value });
  };

  useEffect(() => {
    if (pets.selectedPet) {
      dispatch({ type: "FETCH_PET" });
      getPet(pets.selectedPet).then(data => {
        dispatch({ type: "FETCH_PET_SUCCESS", payload: data });
      });
    } else {
      dispatch({ type: "RESET" });
    }
  }, [pets.selectedPet]);

  return (
    <div>
      <select value={pets.selectedPet} onChange={onChange}>
        <option value="">Select a pet</option>
        <option value="cats">Cats</option>
        <option value="dogs">Dogs</option>
      </select>
      {pets.loading && <div>Loading...</div>}
      {pets.petData && <Pet {...pets.petData} />}
    </div>
  );
}
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75%

componentWillUnmount is a lifecycle method for class-based components that's loaded when the component is being removed from React virtual DOM.,We call clearInterval to remove the timer in the componentWillUnmount hook so that we remove the timer when the component unmounts.,Therefore, we have clearInterval in that function to remove the timer from when the component unmounts.

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40%

The getDerivedStateFromProps() method is called right before rendering the element(s) in the DOM.,Mounting means putting elements into the DOM.,The getDerivedStateFromProps method is called right before the render method:

The constructor method is called, by React, every time you make a component:

class Header extends React.Component {
  constructor(props) {
    super(props);
    this.state = {favoritecolor: "red"};
  }
  render() {
    return (
      <h1>My Favorite Color is {this.state.favoritecolor}</h1>
    );
  }
}

ReactDOM.render(<Header />, document.getElementById('root'));
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22%

Then to cleanup call abort() method from abortController instance inside componentWillUnmount(),Warning: Can’t perform a React state update on an unmounted component. This is a no-op, but it indicates a memory leak in your application. To fix, cancel all subscriptions and asynchronous tasks in a useEffect cleanup function.,Warning: Can’t perform a React state update on an unmounted component. This is a no-op, but it indicates a memory leak in your application. To fix, cancel all subscriptions and asynchronous tasks in the componentWillUnmount method.

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