Rules of Hooks - React

<Intro> Hooks are defined using JavaScript functions, but they represent a special type of reusable UI logic with restrictions on where they can be called. </Intro> <InlineToc />

Only call Hooks at the top level

Functions whose names start with use are called Hooks in React.

Don’t call Hooks inside loops, conditions, nested functions, or try/catch/finally blocks. Instead, always use Hooks at the top level of your React function, before any early returns. You can only call Hooks while React is rendering a function component:

✅ Call them at the top level in the body of a function component.
✅ Call them at the top level in the body of a custom Hook.

js{2-3,8-9}

function Counter() {
  // ✅ Good: top-level in a function component
  const [count, setCount] = useState(0);
  // ...
}

function useWindowWidth() {
  // ✅ Good: top-level in a custom Hook
  const [width, setWidth] = useState(window.innerWidth);
  // ...
}

It’s not supported to call Hooks (functions starting with use) in any other cases, for example:

🔴 Do not call Hooks inside conditions or loops.
🔴 Do not call Hooks after a conditional return statement.
🔴 Do not call Hooks in event handlers.
🔴 Do not call Hooks in class components.
🔴 Do not call Hooks inside functions passed to useMemo, useReducer, or useEffect.
🔴 Do not call Hooks inside try/catch/finally blocks.

If you break these rules, you might see this error.

js{3-4,11-12,20-21}

function Bad({ cond }) {
  if (cond) {
    // 🔴 Bad: inside a condition (to fix, move it outside!)
    const theme = useContext(ThemeContext);
  }
  // ...
}

function Bad() {
  for (let i = 0; i < 10; i++) {
    // 🔴 Bad: inside a loop (to fix, move it outside!)
    const theme = useContext(ThemeContext);
  }
  // ...
}

function Bad({ cond }) {
  if (cond) {
    return;
  }
  // 🔴 Bad: after a conditional return (to fix, move it before the return!)
  const theme = useContext(ThemeContext);
  // ...
}

function Bad() {
  function handleClick() {
    // 🔴 Bad: inside an event handler (to fix, move it outside!)
    const theme = useContext(ThemeContext);
  }
  // ...
}

function Bad() {
  const style = useMemo(() => {
    // 🔴 Bad: inside useMemo (to fix, move it outside!)
    const theme = useContext(ThemeContext);
    return createStyle(theme);
  });
  // ...
}

class Bad extends React.Component {
  render() {
    // 🔴 Bad: inside a class component (to fix, write a function component instead of a class!)
    useEffect(() => {})
    // ...
  }
}

function Bad() {
  try {
    // 🔴 Bad: inside try/catch/finally block (to fix, move it outside!)
    const [x, setX] = useState(0);
  } catch {
    const [x, setX] = useState(1);
  }
}

You can use the eslint-plugin-react-hooks plugin to catch these mistakes.

<Note>

Custom Hooks may call other Hooks (that's their whole purpose). This works because custom Hooks are also supposed to only be called while a function component is rendering.

</Note>

Only call Hooks from React functions

Don’t call Hooks from regular JavaScript functions. Instead, you can:

✅ Call Hooks from React function components. ✅ Call Hooks from custom Hooks.

By following this rule, you ensure that all stateful logic in a component is clearly visible from its source code.

function FriendList() {
  const [onlineStatus, setOnlineStatus] = useOnlineStatus(); // ✅
}

function setOnlineStatus() { // ❌ Not a component or custom Hook!
  const [onlineStatus, setOnlineStatus] = useOnlineStatus();
}