ReactJS lifecycle method inside a function Component

Edit: With the introduction of Hooks it is possible to implement a lifecycle kind of behavior as well as the state in the functional Components. Currently

Hooks are a new feature proposal that lets you use state and other React features without writing a class. They are released in React as a part of v16.8.0

useEffect hook can be used to replicate lifecycle behavior, and useState can be used to store state in a function component.

Basic syntax:

useEffect(callbackFunction, [dependentProps]) => cleanupFunction

You can implement your use case in hooks like

const grid = (props) => {
    console.log(props);
    let {skuRules} = props;

    useEffect(() => {
        if(!props.fetched) {
            props.fetchRules();
        }
        console.log('mount it!');
    }, []); // passing an empty array as second argument triggers the callback in useEffect only after the initial render thus replicating `componentDidMount` lifecycle behaviour

    return(
        <Content title="Promotions" breadcrumbs={breadcrumbs} fetched={skuRules.fetched}>
            <Box title="Sku Promotion">
                <ActionButtons buttons={actionButtons} />
                <SkuRuleGrid 
                    data={skuRules.payload}
                    fetch={props.fetchSkuRules}
                />
            </Box>      
        </Content>  
    )
}

useEffect can also return a function that will be run when the component is unmounted. This can be used to unsubscribe to listeners, replicating the behavior of componentWillUnmount:

Eg: componentWillUnmount

useEffect(() => {
    window.addEventListener('unhandledRejection', handler);
    return () => {
       window.removeEventListener('unhandledRejection', handler);
    }
}, [])

To make useEffect conditional on specific events, you may provide it with an array of values to check for changes:

Eg: componentDidUpdate

componentDidUpdate(prevProps, prevState) {
     const { counter } = this.props;
     if (this.props.counter !== prevState.counter) {
      // some action here
     }
}

Hooks Equivalent

useEffect(() => {
     // action here
}, [props.counter]); // checks for changes in the values in this array

If you include this array, make sure to include all values from the component scope that change over time (props, state), or you may end up referencing values from previous renders.

There are some subtleties to using useEffect; check out the API Here.

Before v16.7.0

The property of function components is that they don't have access to Reacts lifecycle functions or the this keyword. You need to extend the React.Component class if you want to use the lifecycle function.

class Grid extends React.Component  {
    constructor(props) {
       super(props)
    }

    componentDidMount () {
        if(!this.props.fetched) {
            this.props.fetchRules();
        }
        console.log('mount it!');
    }
    render() {
    return(
        <Content title="Promotions" breadcrumbs={breadcrumbs} fetched={skuRules.fetched}>
            <Box title="Sku Promotion">
                <ActionButtons buttons={actionButtons} />
                <SkuRuleGrid 
                    data={skuRules.payload}
                    fetch={props.fetchSkuRules}
                />
            </Box>      
        </Content>  
    )
  }
}

Function components are useful when you only want to render your Component without the need of extra logic.

You can use react-pure-lifecycle to add lifecycle functions to functional components.

Example:

import React, { Component } from 'react';
import lifecycle from 'react-pure-lifecycle';

const methods = {
  componentDidMount(props) {
    console.log('I mounted! Here are my props: ', props);
  }
};

const Channels = props => (
<h1>Hello</h1>
)

export default lifecycle(methods)(Channels);

You can make your own "lifecycle methods" using hooks for maximum nostalgia.

Utility functions:

import { useEffect, useRef } from "react";

export const useComponentDidMount = handler => {
  return useEffect(() => handler(), []);
};

export const useComponentDidUpdate = (handler, deps) => {
  const isInitialMount = useRef(true);

  useEffect(() => {
    if (isInitialMount.current) {
      isInitialMount.current = false;

      return;
    }

    return handler();
  }, deps);
};

export const useComponentWillUnmount = handler => {
  return useEffect(() => handler, []);
};

Usage:

import {
  useComponentDidMount,
  useComponentDidUpdate,
  useComponentWillUnmount
} from "./utils";

export const MyComponent = ({ myProp }) => {
  useComponentDidMount(() => {
    console.log("Component did mount!");
  });

  useComponentDidUpdate(() => {
    console.log("Component did update!");
  });

  useComponentDidUpdate(() => {
    console.log("myProp did update!");
  }, [myProp]);

  useComponentWillUnmount(() => {
    console.log("Component will unmount!");
  });

  return <div>Hello world</div>;
};  

Solution One: You can use new react HOOKS API. Currently in React v16.8.0

Hooks let you use more of React’s features without classes. Hooks provide a more direct API to the React concepts you already know: props, state, context, refs, and lifecycle. Hooks solves all the problems addressed with Recompose.

A Note from the Author of recompose (acdlite, Oct 25 2018):

Hi! I created Recompose about three years ago. About a year after that, I joined the React team. Today, we announced a proposal for Hooks. Hooks solves all the problems I attempted to address with Recompose three years ago, and more on top of that. I will be discontinuing active maintenance of this package (excluding perhaps bugfixes or patches for compatibility with future React releases), and recommending that people use Hooks instead. Your existing code with Recompose will still work, just don't expect any new features.

Solution Two:

If you are using react version that does not support hooks, no worries, use recompose(A React utility belt for function components and higher-order components.) instead. You can use recompose for attaching lifecycle hooks, state, handlers etc to a function component.

Here’s a render-less component that attaches lifecycle methods via the lifecycle HOC (from recompose).

// taken from https://gist.github.com/tsnieman/056af4bb9e87748c514d#file-auth-js-L33

function RenderlessComponent() {
  return null; 
}

export default lifecycle({

  componentDidMount() {
    const { checkIfAuthed } = this.props;
    // Do they have an active session? ("Remember me")
    checkIfAuthed();
  },

  componentWillReceiveProps(nextProps) {
    const {
      loadUser,
    } = this.props;

    // Various 'indicators'..
    const becameAuthed = (!(this.props.auth) && nextProps.auth);
    const isCurrentUser = (this.props.currentUser !== null);

    if (becameAuthed) {
      loadUser(nextProps.auth.uid);
    }

    const shouldSetCurrentUser = (!isCurrentUser && nextProps.auth);
    if (shouldSetCurrentUser) {
      const currentUser = nextProps.users[nextProps.auth.uid];
      if (currentUser) {
        this.props.setCurrentUser({
          'id': nextProps.auth.uid,
          ...currentUser,
        });
      }
    }
  }
})(RenderlessComponent);