在JS中，undefined是一个伪值，所以如果selected["a"]是false或'undefined，selected["a"] ? "style selected" : "style"也会以同样的方式求值。
因此，初始状态实际上可以是一个空对象，要在单击某个按钮时关闭所有其他按钮，请创建一个新的空状态，只打开该按钮。

const { useState } = React

const Demo = () => {
  const [selected, setSelected] = useState({});
      
  const select = (e) => {
    setSelected(() => ({
      [e.target.value]: !selected[e.target.value]
    }))
  }
  
  return (
    <div>
      <button onClick={select} className={selected["a"] ? "style selected" : "style"} value="a">a</button>

      <button onClick={select} className={selected["b"] ? "style selected" : "style"} value="b">b</button>

      <button onClick={select} className={selected["c"] ? "style selected" : "style"} value="c">c</button>
    </div>
  )
}

ReactDOM
  .createRoot(root)
  .render(<Demo />)

.selected {
  color: red;
}

<script crossorigin src="https://unpkg.com/react@18/umd/react.development.js"></script>
<script crossorigin src="https://unpkg.com/react-dom@18/umd/react-dom.development.js"></script>

<div id="root"></div>

试试这个

const select = (e) => {
    const prev_selected={...Selected};
    for(let key in prev_selected){
       prev_selected[key]= key===e.target.value;
    }
    setSelected(prev_selected);    
}