Even Number Counter with React Functional Components

Question

I am trying to figure out re-writing "even number counter" example below, by only using functional components instead of class method.

Here is the original one:

Code

    class ShouldComponentUpdate extends React.Component {
      constructor (props) {
        super(props)
        this.state = {
          value: 0
        }
        this.addValue = this.addValue.bind(this)
      }
    
      addValue () {
        this.setState((state) => ({
          value: state.value + 1
        }))
      }
    
      render () {
        return (
          <div>
            <button onClick={this.addValue}>Add</button>
            <OnlyEvens value={this.state.value} />
          </div>
        )
      }
    }
    
    class OnlyEvens extends React.Component {
      constructor (props) {
        super(props)
      }
    
      shouldComponentUpdate (nextProps, nextState) {
        if (nextProps.value % 2 === 0) return true
      }
    
      render () {
        return (
          <div>
            <h1>{this.props.value}</h1>
          </div>
        )
      }
    }

Well, I try to do something below but I lost in callback function. Can someone review please?

Code

    function ComponentShouldUpdate () {
      const [counter, setCounter] = useState(0)
    
      return (
        <div>
          <button onClick={() => setCounter(counter + 1)}>Add</button>
          <EvenCounter counter={counter} />
        </div>
      )
    }
    
    const EvenCounter = React.memo(({ counter }) => {
      function checkEven (nextProps, nextState) {
        if (nextProps.counter % 2 === 0) {
        //lost in here
        }
      }
      return (
        <div>
          <h2>{counter}</h2>
        </div>
      )
    })

Answer 1

You should separate the counter logic into your own custom hook something like useEvenCounter. Because hooks are composable, it can be reused anywhere else, not just in your particular component

function useEvenCounter(initialValue: number = 0) {
  const [value, setValue] = React.useState(initialValue);
  const increment = React.useCallback(
    () => setValue((v) => (v % 2 === 0 ? v + 2 : v + 1)),
    []
  );

  return [value, increment] as const;
}

Usage

export default function App() {
  const [value, increment] = useEvenCounter();
  const [value2, increment2] = useEvenCounter(1);

  return (
    <div className="App">
      <h1>Hello CodeSandbox</h1>
      <h2>Start editing to see some magic happen!</h2>
      <button onClick={() => increment()}>+</button>
      <div>{value}</div>
      <button onClick={() => increment2()}>+</button>
      <div>{value2}</div>
    </div>
  );
}

Live Demo

You can play around in the live demo below. Hit me up if you don't understand anything