Answers for "react useEffect"

react useEffect

// Every rerender
useEffect(() => {
	console.log("I run everytime this component rerenders")
});

// onMount
useEffect(() => {
	console.log("I Only run once (When the component gets mounted)")
}, []);

// Condition based 
useEffect(() => {
	console.log("I run everytime my condition is changed")
}, [condition]);

// Condition based with "clean up"
useEffect(() => {
	console.log("I run everytime my condition is changed")
	
	return () => {
    	console.log("Use this return as a 'clean up tool' (this runs before the actual code)")
    }
}, [condition]);

useeffect

function App() {
  const [count, setCount] = useState(0);

  useEffect(() => {
    longResolve().then(() => {
      alert(count);
    });
  }, []);

  return (
    <div>
      <button
        onClick={() => {
          setCount(count + 1);
        }}
      >
        Count: {count}
      </button>
    </div>
  );
}

what is useeffect

useEffect is a hook for encapsulating code that has 'side effects,' and is like a combination of componentDidMount , componentDidUpdate , and componentWillUnmount . Previously, functional components didn't have access to the component life cycle, but with useEffect you can tap into it.23.1.2019

react useEffect

import React, { useEffect } from 'react';

export const App: React.FC = () => {
  
  useEffect(() => {
        
  }, [/*Here can enter some value to call again the content inside useEffect*/])
  
  return (
    <div>Use Effect!</div>
  );
}

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'));

React useEffect

// (1) On Mount and every render
useEffect (() => { 
  dosomething()
});// (2) Only on Mount
useEffect (() => {
  dosomething()
}, []);
// (3) On Mount/every time state of count changes
useEffect (() => {
  dosomething()
}, [count]);
// (4) UseEffect with cleanup clearSomething function
useEffect (() => {
  dosomething();
  return clearSomething(){};
})