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CSS

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JavaScript

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HTML 

              
                #app
!

CSS 

              
                * {
  box-sizing: border-box
}

pre, code {
  font-family: monospace
  padding: .5em
  background: #eee
  border-radius: .2em
}

code {
  color: #e76ea1 - 20
  padding: .25em .5em
}

p {
  margin: 0 0 .5em
}

#app {
  padding: 1em
}

.flex {
  display: flex
  //flex-wrap: wrap
}

.flex-item {
  flex-grow: 1
  padding: 0 1em 1em
}
!

JS 

              
                const useState    = React.useState;
const useMemo     = React.useMemo;
const useCallback = React.useCallback;

function Counter({ count, square, onClick, onReset }) {
  return (
    <div>
      <p>Count: {count}</p>
      <p>Square: {square}</p>
      <button onClick={onClick}>+1</button> 
      <button onClick={onReset}>reset</button>
    </div>
  );
}

// 🐞
// ----------------------------------------------------
function CounterWithBug({ initCount }) {
  const [count, setCount] = useState(initCount);
  const [square, setSquare] = useState(initCount * initCount);
  
  const updateSquare = () => {
    setSquare( count * count );
  }
  
  const onCountup = () => {
    setCount( count + 1 );
    updateSquare();
  }
  
  const onReset = () => {
    setCount( initCount );
    setSquare( initCount * initCount );
  }
  
  return (
    <>
      <span role="img">🐞</span>
      <Counter
        count={count}
        square={square}
        onClick={onCountup}
        onReset={onReset}
       />
      <pre>{
 `const onCountup = () => {
  setCount( count + 1 );
  setSquare( count * count );
}`
        }</pre>
    </>
  );
}

// 👍
// ----------------------------------------------------
function CounterWithVar({ initCount }) {
  const [count, setCount] = useState(initCount);
  const [square, setSquare] = useState(initCount * initCount);
  
  const updateSquare = ( val ) => {
    setSquare( val * val );
  }
  
  const onCountup = () => {
    const newCount = count + 1;
    setCount( newCount );
    updateSquare( newCount );
  }
  
  const onReset = () => {
    setCount( initCount );
    setSquare( initCount * initCount );
  }
  
  return (
    <>
      <span role="img">👍</span>
      <Counter
        count={count}
        square={square}
        onClick={onCountup}
        onReset={onReset}
       />
      <pre>{
 `const onCountup = () => {
  const newCount = count + 1
  setCount( newCount );
  setSquare( newCount * newCount );
}`
        }</pre>
    </>
  );
}

// 👍 use Setter function
// ----------------------------------------------------
function CounterWithSetter({ initCount }) {
  const [count, setCount] = useState(initCount);
  const [square, setSquare] = useState(initCount * initCount);
  
  const updateSquare = ( val ) => {
    setSquare( val * val );
  }
  
  const onCountup = () => {
    setCount( (preCount) => {
      const newCount = preCount + 1;
      updateSquare( newCount );
      return newCount;
    } );
    
  }
  
  const onReset = () => {
    setCount( initCount );
    setSquare( initCount * initCount );
  }
  
  return (
    <>
      <span role="img">👍</span>
      <Counter
        count={count}
        square={square}
        onClick={onCountup}
        onReset={onReset}
       />
      <pre>{
 `const onCountup = () => {
  setCount(preCount => {
    const newCount = preCount + 1;
    setSquare( newCount * newCount );
    return newCount;
  });
}`
        }</pre>
    </>
  );
}

// ⚠️ use Callback Anti pattern
// ----------------------------------------------------
function CounterWithUseCallbackAntiPattern({ initCount }) {
  const [count, setCount] = useState(initCount);
  const square = useMemo(() => {
    return count * count;
  }, [count]);
  
  // generate function everytime when count changes.
  const onCountup = useCallback(() => {
    setCount( count + 1 );
  }, [count]);
  
  const onReset = useCallback(() => {
    setCount( initCount );
  }, [initCount]);
  
  return (
    <>
      <span role="img">⚠️ useCallback</span>
      <Counter
        count={count}
        square={square}
        onClick={onCountup}
        onReset={onReset}
       />
      <pre>{
 `const onCountup = useCallback(() => {
    setCount( count + 1 );
  }, [count]);`
        }</pre>
      <p>generate function everytime when <code>count</code> changes.</p>
    </>
  );
}

// 🐞use Callback
// ----------------------------------------------------
function CounterWithUseCallbackHasBug({ initCount }) {
  const [count, setCount] = useState(initCount);
  const square = useMemo(() => {
    return count * count;
  }, [count]);
  
  const onCountup = useCallback(() => {
    // 🐞count bind first time value.
    setCount( count + 1 );
  }, []);
  
  const onReset = useCallback(() => {
    setCount( initCount );
  }, [initCount]);
  
  return (
    <>
      <span role="img">🐞useCallback</span>
      <Counter
        count={count}
        square={square}
        onClick={onCountup}
        onReset={onReset}
       />
      <pre>{
 `const onCountup = useCallback(() => {
    setCount( count + 1 );
  }, []);`
        }</pre>
      <p>function create once, but <code>count</code> var was binded first value.</p>
    </>
  );
}

// 👍use Callback
// ----------------------------------------------------
function CounterWithUseCallback({ initCount }) {
  const [count, setCount] = useState(initCount);
  const square = useMemo(() => {
    return count * count;
  }, [count]);
  
  const onCountup = useCallback(() => {
    setCount( preCount => preCount + 1 );
  }, []);
  
  const onReset = useCallback(() => {
    setCount( initCount );
  }, [initCount]);
  
  return (
    <>
      <span role="img">👍useCallback</span>
      <Counter
        count={count}
        square={square}
        onClick={onCountup}
        onReset={onReset}
       />
      <pre>{
 `const onCountup = useCallback(() => {
    setCount( preCount => preCount + 1 );
  }, []);`
        }</pre>
    </>
  );
}

// 👍 use Memo
// ----------------------------------------------------
function CounterWithUseMemo({ initCount }) {
  const [count, setCount] = useState(initCount);
  const square = useMemo(() => {
    return count * count;
  }, [count]);
  
  const onCountup = () => {
    setCount( preCount => preCount + 1 );
  }
  
  const onReset = () => {
    setCount( initCount );
  }
  
  return (
    <>
      <span role="img">👍 useMemo</span>
      <Counter
        count={count}
        square={square}
        onClick={onCountup}
        onReset={onReset}
       />
      <pre>{
 `const square = useMemo(() => {
    return count * count;
  }, [count]);

const onCountup = () => {
  setCount( preCount => preCount + 1 );
}
`
        }</pre>
    </>
  );
}


// ----------------------------------------------------------------------
const initCount = 2;
function App() {
  return (
    <>
      <div className="flex">
        <div className="flex-item">
          <CounterWithBug initCount={initCount} />
        </div>
        <div className="flex-item">
          <CounterWithVar initCount={initCount} />
        </div>
        <div className="flex-item">
          <CounterWithSetter initCount={initCount} />
        </div>
      </div>
      <p>useMemo / useCallback</p>
      <div className="flex">
        <div className="flex-item">
          <CounterWithUseMemo initCount={initCount} />
        </div>
        <div className="flex-item">
          <CounterWithUseCallbackAntiPattern initCount={initCount} />
        </div>
        <div className="flex-item">
          <CounterWithUseCallbackHasBug initCount={initCount} />
        </div>
        <div className="flex-item">
          <CounterWithUseCallback initCount={initCount} />
        </div>
      </div>
    </>
  );
}

ReactDOM.render(<App />, document.getElementById('app'));
!
999px

Console