useRef() vs. const vs. useState()

by cadenzah

HTML

<script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.7.0-alpha.2/umd/react.development.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.7.0-alpha.2/umd/react-dom.production.min.js"></script>
<div id="app"></div>

CSS

body {
  background: #20262E;
  padding: 20px;
  font-family: Helvetica;
}

#app {
  background: #fff;
  border-radius: 4px;
  padding: 20px;
  transition: all 0.2s;
}

li {
  margin: 8px 0;
}

h2 {
  font-weight: bold;
  margin-bottom: 15px;
}

.done {
  color: rgba(0, 0, 0, 0.3);
  text-decoration: line-through;
}

input {
  margin-right: 5px;
}

Babel + JSX

const { useState, useEffect, useMemo, useCallback, useRef } = React;
// defined a variable outside function component
let countCache = 0;

function Counter() {
  const [count, setCount] = useState(0);

  const countRef = useRef(count);

  useEffect(() => {
    // Update count on "after" every render
    countCache = count;
    countRef.current = count;
  });
  
  return (
    <div>
      <button onClick={() => setCount(p => p + 1)}>click me</button>
      <h3>state {count}</h3>
      <h3>variable {countCache}</h3>
      <h3>reference {countRef.current}</h3>
    </div>
  );
}

function App() {
	const [count, setCount] = useState(0);
  const latestCount = useRef(count);
  latestCount.current = count;
  
  useEffect(() => {
    setTimeout(() => console.log(latestCount.current), 5000);
  }, []);
  
	return (
  	<div>
      {count}
      <button onClick={() => setCount(count+1)}>+</button>
      <button onClick={() => setCount(count-1)}>-</button>
      <hr />
      <Counter />
      <Counter />
  	</div>
  )
}

ReactDOM.render(<App />, document.querySelector("#app"));