1/*
2	A common use case is to access a child imperatively: 
3*/
4
5function TextInputWithFocusButton() {
6  const inputEl = useRef(null);
7  const onButtonClick = () => {
8    // `current` points to the mounted text input element
9    inputEl.current.focus();
10  };
11  return (
12    <>
13      <input ref={inputEl} type="text" />
14      <button onClick={onButtonClick}>Focus the input</button>
15    </>
16  );
17}

1import React, { useRef } from 'react';
2
3function TextInputWithFocusButton() {
4  const inputEl = useRef(null);
5  const onButtonClick = () => {
6    // `current` points to the mounted text input element
7    inputEl.current.focus();
8  };
9  return (
10    <>
11      <input ref={inputEl} type="text" />
12      <button onClick={onButtonClick}>Focus the input</button>
13    </>
14  );
15}

1import React, { useEffect, useState } from 'react';
2import ReactDOM from 'react-dom';
3
4function LifecycleDemo() {
5  // It takes a function
6  useEffect(() => {
7    // This gets called after every render, by default
8    // (the first one, and every one after that)
9    console.log('render!');
10
11    // If you want to implement componentWillUnmount,
12    // return a function from here, and React will call
13    // it prior to unmounting.
14    return () => console.log('unmounting...');
15  }, [ // dependencies to watch = leave blank to run once or you will get a stack overflow  ]);
16
17  return "I'm a lifecycle demo";
18}
19
20function App() {
21  // Set up a piece of state, just so that we have
22  // a way to trigger a re-render.
23  const [random, setRandom] = useState(Math.random());
24
25  // Set up another piece of state to keep track of
26  // whether the LifecycleDemo is shown or hidden
27  const [mounted, setMounted] = useState(true);
28
29  // This function will change the random number,
30  // and trigger a re-render (in the console,
31  // you'll see a "render!" from LifecycleDemo)
32  const reRender = () => setRandom(Math.random());
33
34  // This function will unmount and re-mount the
35  // LifecycleDemo, so you can see its cleanup function
36  // being called.
37  const toggle = () => setMounted(!mounted);
38
39  return (
40    <>
41      <button onClick={reRender}>Re-render</button>
42      <button onClick={toggle}>Show/Hide LifecycleDemo</button>
43      {mounted && <LifecycleDemo/>}
44    </>
45  );
46}
47
48ReactDOM.render(<App/>, document.querySelector('#root'));

1function TextInputWithFocusButton() {
2  const inputEl = useRef(null);
3  const onButtonClick = () => {
4    // `current` points to the mounted text input element
5    inputEl.current.focus();
6  };
7  return (
8    <>
9      <input ref={inputEl} type="text" />
10      <button onClick={onButtonClick}>Focus the input</button>
11    </>
12  );
13}

1const refContainer = useRef(initialValue);
2//useRef returns a mutable ref object whose .current property is initialized to the passed argument (initialValue). 
3//The returned object will persist for the full lifetime of the component.