componentDidmount react hook code example

Example 1: componentdidmount hooks

For componentDidMount
useEffect(() => {
  // Your code here
}, []);

For componentDidUpdate
useEffect(() => {
  // Your code here
}, [yourDependency]);

For componentWillUnmount
useEffect(() => {
  // componentWillUnmount
  return () => {
     // Your code here
  }
}, [yourDependency]);

Example 2: react hooks componentdidmount

// import useEffect from 'react';

useEffect(() => {
	// your code here
}, []);

Example 3: react useEffect

import React, { useEffect, useState } from 'react';
import ReactDOM from 'react-dom';

function LifecycleDemo() {
  // It takes a function
  useEffect(() => {
    // This gets called after every render, by default
    // (the first one, and every one after that)
    console.log('render!');

    // If you want to implement componentWillUnmount,
    // return a function from here, and React will call
    // it prior to unmounting.
    return () => console.log('unmounting...');
  }, [ // dependencies to watch = leave blank to run once or you will get a stack overflow  ]);

  return "I'm a lifecycle demo";
}

function App() {
  // Set up a piece of state, just so that we have
  // a way to trigger a re-render.
  const [random, setRandom] = useState(Math.random());

  // Set up another piece of state to keep track of
  // whether the LifecycleDemo is shown or hidden
  const [mounted, setMounted] = useState(true);

  // This function will change the random number,
  // and trigger a re-render (in the console,
  // you'll see a "render!" from LifecycleDemo)
  const reRender = () => setRandom(Math.random());

  // This function will unmount and re-mount the
  // LifecycleDemo, so you can see its cleanup function
  // being called.
  const toggle = () => setMounted(!mounted);

  return (
    <>
      <button onClick={reRender}>Re-render</button>
      <button onClick={toggle}>Show/Hide LifecycleDemo</button>
      {mounted && <LifecycleDemo/>}
    </>
  );
}

ReactDOM.render(<App/>, document.querySelector('#root'));

Example 4: useeffect componentdidmount

import React, { useState, useEffect } from 'react';
function Example() {
  const [count, setCount] = useState(0);

  // Similar to componentDidMount and componentDidUpdate:  
  useEffect(() => {    
    // Update the document title using the browser API    
    document.title = `You clicked ${count} times`;  
  });

  );
}

Example 5: useeffect componentdidmount

useEffect(() => {
    messagesRef.on('child added', snapshot => {
    const message = snapshot.val();
    message.key = snapshot.key;

    setMessages(messages.concat(message)); // See Note 1
}, []); // See Note 2

Example 6: componentdidmount in hooks

useEffect(() => {
  // Your code here
}, []);