Uncaught Error: Rendered fewer hooks than expected. This may be caused by an accidental early return statement in React Hooks

Question

Given the following component, when I press down on the age selector and change the value to 15, such that I render a form without the driver license field, I get the error:

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)

Example code below:

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/[email protected]/umd/react.development.js"></script>
<script src="https://unpkg.com/[email protected]/umd/react-dom.development.js"></script>
<div id="app"></div>

Answer 1

The problem is that in the first render, 3 useState hooks were invoked - name, age and license but after the age is changed to a value below 16, the useState for license is no longer invoked, resulting in only the first 2 hooks being invoked. As the 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.

The order of the hooks being called is important, and if we write code that causes hooks to not be called, React will not be able to match the hook call with its values.

The solution is to move the license hook up to the top of the function so that it gets called regardless whether it is needed or not.

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/[email protected]/umd/react.development.js"></script>
<script src="https://unpkg.com/[email protected]/umd/react-dom.development.js"></script>
<div id="app"></div>