JAVASCRIPT

useref

import React, { useState, useRef, useEffect } from 'react'
import { render } from 'react-dom'

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

  useEffect(() => {
    intervalRef.current = setInterval(() => setCount(count => count + 1), 1000)

    return () => {
      clearInterval(intervalRef.current)
    }
  }, [])

  return (
    <>
      <div style={{ fontSize: 120 }}>{count}</div>
      <button
        onClick={() => {
          clearInterval(intervalRef.current)
        }}
      >
        Stop
      </button>
    </>
  )
}

render(<App />, document.querySelector('#app'))

useRef

/*
	A common use case is to access a child imperatively: 
*/

function TextInputWithFocusButton() {
  const inputEl = useRef(null);
  const onButtonClick = () => {
    // `current` points to the mounted text input element
    inputEl.current.focus();
  };
  return (
    <>
      <input ref={inputEl} type="text" />
      <button onClick={onButtonClick}>Focus the input</button>
    </>
  );
}

useref

import { useRef } from "react";

function TextInputWithFocusButton() {
  const inputEl = useRef(null);
  
  const onButtonClick = () => {
    // `current` points to the mounted text input element
    inputEl.current.focus();
  };
  
  return (
    <>
      <input ref={inputEl} type="text" />
      <button onClick={onButtonClick}>Focus the input</button>
    </>
  );
}

useRef() in react

function TextInputWithFocusButton() {
  const inputEl = useRef(null);
  const onButtonClick = () => {
    // `current` points to the mounted text input element
    inputEl.current.focus();
  };
  return (
    <>
      <input ref={inputEl} type="text" />
      <button onClick={onButtonClick}>Focus the input</button>
    </>
  );
}

useRef

/**
 * Accessing DOM elements
 *
 * Another useful application of the useRef() hook is to access DOM elements.
 * This is performed in 3 steps:
 * 
 * - Define the reference to access the element const elementRef = useRef();
 * - Assign the reference to ref attribute of the element: <div ref={elementRef}></div>;
 * - After mounting, elementRef.current points to the DOM element.
 */

import { useRef, useEffect } from 'react';
function AccessingElement() {
  const elementRef = useRef();
   useEffect(() => {
    const divElement = elementRef.current;
    console.log(divElement); // logs <div>I'm an element</div>
  }, []);
  return (
    <div ref={elementRef}>
      I'm an element
    </div>
  );
}

useref

function TextInputWithFocusButton() {
 const inputEl = useRef(null);
 const onButtonClick = () => {
    inputEl.current.focus();
  };
return (
    <>
      <input ref={inputEl} type="text" />
      <button onClick={onButtonClick}>Focus</button>
    </>
  );
}

useref example

import { useRef } from "react";

function ExampleUseRef() {
  const inputNode = useRef(null);

  const onClick = () => {
    inputNode.current.focus();
  };

  return (
    <>
      <input ref={inputNode} type="text" />
      <button onClick={onClick}>Focus..</button>
    </>
  );
}

useRef

function TextInputWithFocusButton() {
  const inputEl = useRef(null);
  const onButtonClick = () => {
    // `current` 指向已挂载到 DOM 上的文本输入元素
    inputEl.current.focus();
  };
  return (
    <>
      <input ref={inputEl} type="text" />
      <button onClick={onButtonClick}>Focus the input</button>
    </>
  );
}

what does useref do react

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

what is useref in react

useRef is an kind of alternative to useState hook , with useRef you can get an reference to  an element and than use the DOM/JS properties to modify it .
You can even call useRef as an tool which is used to apply all the DOM properties as you were doing in Javascript before coming to React.
If you use useRef than this  apparoch is called un-controlled components.

useref in react

import { useRef } from 'react';

function LogButtonClicks() {
  const countRef = useRef(0);  
  const handle = () => {
    countRef.current++;    console.log(`Clicked ${countRef.current} times`);
  };

  console.log('I rendered!');

  return <button onClick={handle}>Click me</button>;
}

useref react class component

class CustomTextInput extends React.Component {
  constructor(props) {
    super(props);
    // create a ref to store the textInput DOM element
    this.textInput = React.createRef();
    this.focusTextInput = this.focusTextInput.bind(this);
  }

  focusTextInput() {
    // Explicitly focus the text input using the raw DOM API
    // Note: we're accessing "current" to get the DOM node
    this.textInput.current.focus();
  }

  render() {
    // tell React that we want to associate the <input> ref
    // with the `textInput` that we created in the constructor
    return (
      <div>
        <input
          type="text"
          ref={this.textInput} />
        <input
          type="button"
          value="Focus the text input"
          onClick={this.focusTextInput}
        />
      </div>
    );
  }
}

useRef example

import React, { useRef, useEffect } from 'react';

function CountMyRenders() {
  const countRenderRef = useRef(1);
  
  useEffect(function afterRender() {
    countRenderRef.current++;  });

  return (
    <div>I've rendered {countRenderRef.current} times</div>
  );
}

useref

  const inputEl = useRef(null);

useRef

import { useRef, useState, useEffect } from 'react';
function Stopwatch() {
  const timerIdRef = useRef(0);
  const [count, setCount] = useState(0);
  const startHandler = () => {
    if (timerIdRef.current) { return; }
    timerIdRef.current = setInterval(() => setCount(c => c+1), 1000);
  };
  const stopHandler = () => {
    clearInterval(timerIdRef.current);
    timerIdRef.current = 0;
  };
  useEffect(() => {
    return () => clearInterval(timerIdRef.current);
  }, []);
  return (
    <div>
      <div>Timer: {count}s</div>
      <div>
        <button onClick={startHandler}>Start</button>
        <button onClick={stopHandler}>Stop</button>
      </div>
    </div>
  );
}

useRef

import { useRef, useState, useEffect } from 'react';
function Stopwatch() {
  const timerIdRef = useRef(0);
  const [count, setCount] = useState(0);
  const startHandler = () => {
    if (timerIdRef.current) { return; }
    timerIdRef.current = setInterval(() => setCount(c => c+1), 1000);
  };
  const stopHandler = () => {
    clearInterval(timerIdRef.current);
    timerIdRef.current = 0;
  };
  useEffect(() => {
    return () => clearInterval(timerIdRef.current);
  }, []);
  return (
    <div>
      <div>Timer: {count}s</div>
      <div>
        <button onClick={startHandler}>Start</button>
        <button onClick={stopHandler}>Stop</button>
      </div>
    </div>
  );
}

useref react class component

 constructor(props) {
    super(props);
    this.myRef = React.createRef();
  }

useRef

function FancyInput(props, ref) {
  const inputRef = useRef();
  useImperativeHandle(ref, () => ({
    focus: () => {
      inputRef.current.focus();
    }
  }));
  return <input ref={inputRef} ... />;
}
FancyInput = forwardRef(FancyInput);