javascript - Uncaught Error : Rendered fewer hooks than expected. 这可能是React Hooks中意外提前返回语句导致的

Question

给定以下组件，当我按下年龄选择器并将值更改为 15 时，我呈现了一个没有驾驶执照字段的表单，我收到错误:

Uncaught Error: Rendered fewer hooks than expected. This may be caused by an accidental early return statement.
    at invariant (react-dom.development.js:55)
    at finishHooks (react-dom.development.js:11581)
    at updateFunctionComponent (react-dom.development.js:14262)
    at beginWork (react-dom.development.js:15103)
    at performUnitOfWork (react-dom.development.js:17817)
    at workLoop (react-dom.development.js:17857)
    at HTMLUnknownElement.callCallback (react-dom.development.js:149)
    at Object.invokeGuardedCallbackDev (react-dom.development.js:199)
    at invokeGuardedCallback (react-dom.development.js:256)
    at replayUnitOfWork (react-dom.development.js:17113)
    at renderRoot (react-dom.development.js:17957)
    at performWorkOnRoot (react-dom.development.js:18808)
    at performWork (react-dom.development.js:18716)
    at flushInteractiveUpdates$1 (react-dom.development.js:18987)
    at batchedUpdates (react-dom.development.js:2210)
    at dispatchEvent (react-dom.development.js:4946)
    at interactiveUpdates$1 (react-dom.development.js:18974)
    at interactiveUpdates (react-dom.development.js:2217)
    at dispatchInteractiveEvent (react-dom.development.js:4923)

示例代码如下:

const {useState} = React;

function App() {
  const [name, setName] = useState('Mary');
  const [age, setAge] = useState(16);

  if (age < 16) {
    return (
      <div>
        Name:{' '}
        <input
          value={name}
          onChange={e => {
            setName(e.target.value);
          }}
        />
        <br />
        Age:{' '}
        <input
          value={age}
          type="number"
          onChange={e => {
            setAge(+e.target.value);
          }}
        />
      </div>
    );
  }

  const [license, setLicense] = useState('A123456');

  return (
    <div>
      Name:{' '}
      <input
        value={name}
        onChange={e => {
          setName(e.target.value);
        }}
      />
      <br />
      Age:{' '}
      <input
        value={age}
        type="number"
        onChange={e => {
          setAge(+e.target.value);
        }}
      />
      <br />
      Driver License:{' '}
      <input
        value={license}
        onChange={e => {
          setLicense(e.target.value);
        }}
      />
    </div>
  );
}

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

<script src="https://unpkg.com/react@16.7.0-alpha.0/umd/react.development.js"></script>
<script src="https://unpkg.com/react-dom@16.7.0-alpha.0/umd/react-dom.development.js"></script>
<div id="app"></div>

Answer 1

问题是在第一个渲染中，调用了 3 个 useState 钩子(Hook) - name、age 和 license 但在年龄更改为 16 以下的值后，不再调用 license 的 useState，导致仅调用前 2 个钩子(Hook)。作为 React docs state :

Don’t call Hooks inside loops, conditions, or nested functions. Instead, always use Hooks at the top level of your React function. By following this rule, you ensure that Hooks are called in the same order each time a component renders. That’s what allows React to correctly preserve the state of Hooks between multiple useState and useEffect calls.

被调用的钩子(Hook)的顺序很重要，如果我们编写导致钩子(Hook)不被调用的代码，React 将无法匹配钩子(Hook)调用及其值。

解决方案是将 license Hook 移动到函数的顶部，以便无论是否需要它都会被调用。

const {useState} = React;

function App() {
  const [name, setName] = useState('Mary');
  const [age, setAge] = useState(16);
  const [license, setLicense] = useState('A123456');

  return (
    <div>
      Name:{' '}
      <input
        value={name}
        onChange={e => {
          setName(e.target.value);
        }}
      />
      <br />
      Age:{' '}
      <input
        value={age}
        type="number"
        onChange={e => {
          setAge(+e.target.value);
        }}
      />
      {age >= 16 && <span>
        <br />
        Driver License:{' '}
        <input
          value={license}
          onChange={e => {
            setLicense(e.target.value);
          }}
        /></span>
       }
    </div>
  );
}

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

<script src="https://unpkg.com/react@16.7.0-alpha.0/umd/react.development.js"></script>
<script src="https://unpkg.com/react-dom@16.7.0-alpha.0/umd/react-dom.development.js"></script>
<div id="app"></div>