why does usestate cause the component to render twice on each update?

- Through Input
​
        const [state, setState] = useState({ fName: "", lName: "" });
        const handleChange = e => {
            const { name, value } = e.target;
            setState(prevState => ({
                ...prevState,
                [name]: value
            }));
        };
​
        <input
            value={state.fName}
            type="text"
            onChange={handleChange}
            name="fName"
        />
        <input
            value={state.lName}
            type="text"
            onChange={handleChange}
            name="lName"
        />
   ***************************
​
 - Through onSubmit or button click
    
        setState(prevState => ({
            ...prevState,
            fName: 'your updated value here'
         }));

ReactDOM.render(
<React.StrictMode>
   <App />
</React.StrictMode>,
rootElement
);

ReactDOM.render(
<React.StrictMode>
   <App />
</React.StrictMode>,
rootElement
);

export default function SearchTable(props) {
  const [filterInput, setFilterInput] = useState("");

  const handleFilterChange = e => {
    const value = e.target.value || "";
    setFilterInput(value);
  };

  console.log("Search Component - Value from filterInput ", filterInput);

  return (
    <form noValidate autoComplete="off">
      <TextField
        id="standard-basic"
        label="Search Component"
        value={filterInput}
        // onChange={() => console.log("Search Component Rendered")}  // Using this this "onChange" event, the component only renders once as it does not use the useState hook
        onChange={handleFilterChange}
        placeholder={"Search"}
      />
    </form>
  );
}

`React intentionally remounts your components in development
to help you find bugs. You can turn off Strict Mode to opt out of the development behavior,
but we recommend keeping it on.` (Docs)
// To disable: Remove <React.StrictMode> or <StrictMode>
      root.render( <React.StrictMode>
         <App />
      </React.StrictMode>);
      // After:
      root.render(
      <App /> );

export default class App extends React.Component {
   render() {
      console.log('this logs twice with React.StrictMode');
      return null;
   }
}

function ExampleComponent () {
  const [debouncedHandlerFn] = useState(() => debounce(handlerFn, 1000))
  return <div onSomeEvent={debouncedHandlerFn} />
}

facebook / react #15074

🔥 StrictMode가 실행되어 Hook을 사용하는 컴포넌트가 2번 렌더링 되는 문제를 해당 코드를 삭제함으로써 해결했습니다.

 setTrades((old) => {
    old[id] = {
       count: old[id].count + 1,
       active: !old[id].active
    };
    return [...old];
 });

  setTrades((old) => {
     const updated = [...old]
     updated[id] = {
        count: old[id].count + 1,
        active: !old[id].active
     }
     return updated
  })

    Operating System:
       Platform: darwin
    Arch: arm64
    Version: Darwin Kernel Version 21.2 .0: Sun Nov 28 20: 28: 41 PST 2021;
    root: xnu - 8019.61 .5~1 / RELEASE_ARM64_T6000
    Binaries:
       Node: 16.14 .0
    npm: 8.5 .5
    Yarn: 1.22 .17
    pnpm: 6.31 .0
    Relevant packages:
       next: 12.1 .4
    react: 18.0 .0
    react - dom: 18.0 .0

// the project is just result of `create-next-app`
// pages/index.js
import { useEffect, useState } from 'react'

export default function Home() {
  console.log('render')

  const [state, setState] = useState(() => {
    console.log('state initialize');
    return 'state';
  })

  useEffect(() => {
    return () => {
      console.log('unmounted')
    }
  }, [])

  return (
    <div>
      Home
    </div>
  )
}

import { useState, useEffect } from 'react';

export default function App() {
  console.log('render');

  const [state, setState] = useState(() => {
    console.log('state initialized');
    return 'state';
  });

  useEffect(() => {
    console.log('run useEffect');
    return () => console.log('unmount/ cleanup Effect');
  }, []);

  return <div>hey</div>;
}

function App() {
  const [counter, setCounter] = useState(0);

  const fetchData = () => {
        console.log('Fetching data...');
        setCounter((oldValue) => oldValue+1);
    }

  useEffect(() => {
      fetchData();
  },[])

  return (
    <div>
        <h1>Times Fetched:</h1>
      <h1>{counter}</h1>
    </div>
  );
}

function App() {
  const [counter, setCounter] = useState(0);
  const dataFetchedRef = useRef(false);

  const fetchData = () => {
        console.log('Fetching data...');
        setCounter((oldValue) => oldValue+1);
    }

  useEffect(() => {
      if (dataFetchedRef.current) return;
      dataFetchedRef.current = true;
      fetchData();
  },[])

  return (
    <div className="App">
        <h1>Times Fetched:</h1>
      <h1>{counter}</h1>
    </div>
  );
}

const root = ReactDOM.createRoot(document.getElementById('root'));
root.render(
<React.StrictMode>
   <App />
</React.StrictMode>

);

npx create - react - app my - app && cd my - app

function App() {
  console.log('I render 😁');

  return (
    <div className="App">
      <header className="App-header">
        <img src={logo} className="App-logo" alt="logo" />
      </header>
    </div>
  );
}

function App() {
  const [clicks, setClicks] = React.useState(0);

  console.log('I render 😁');

  return (
    <div className="App">
      <header className="App-header">
        <img src={logo} className="App-logo" alt="logo" />

        <button onClick={() => setClicks(clicks => clicks + 1)}>
            Clicks: {clicks}
        </button>
      </header>
    </div>
  );
}

ReactDOM.render(
<React.StrictMode>
   <App />
</React.StrictMode>,
document.getElementById('root')
);