diff --git a/content/warnings/invalid-hook-call-warning.md b/content/warnings/invalid-hook-call-warning.md new file mode 100644 index 00000000000..f04ec90138e --- /dev/null +++ b/content/warnings/invalid-hook-call-warning.md @@ -0,0 +1,122 @@ +--- +title: Invalid Hook Call Warning +layout: single +permalink: warnings/invalid-hook-call-warning.html +--- + + You are probably here because you got the following error message: + + > Hooks can only be called inside the body of a function component. + +There are three common reasons you might be seeing it: + +1. You might have **mismatching versions** of React and React DOM. +2. You might be **breaking the [Rules of Hooks](/docs/hooks-rules.html)**. +3. You might have **more than one copy of React** in the same app. + +Let's look at each of these cases. + +## Mismatching Versions of React and React DOM + +You might be using a version of `react-dom` (< 16.8.0) or `react-native` (< 0.60) that doesn't yet support Hooks. You can run `npm ls react-dom` or `npm ls react-native` in your application folder to check which version you're using. If you find more than one of them, this might also create problems (more on that below). + +## Breaking the Rules of Hooks + +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](/docs/hooks-custom.html). + +**Learn more about this in the [Rules of Hooks](/docs/hooks-rules.html).** + +```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); + // ... +} +``` + +To avoid confusion, it’s **not** supported to call Hooks in other cases: + +* 🔴 Do not call Hooks in class components. +* 🔴 Do not call in event handlers. +* 🔴 Do not call Hooks inside functions passed to `useMemo`, `useReducer`, or `useEffect`. + +If you break these rules, you might see this error. + +```js{3-4,11-12,20-21} +function Bad1() { + function handleClick() { + // 🔴 Bad: inside an event handler (to fix, move it outside!) + const theme = useContext(ThemeContext); + } + // ... +} + +function Bad2() { + const style = useMemo(() => { + // 🔴 Bad: inside useMemo (to fix, move it outside!) + const theme = useContext(ThemeContext); + return createStyle(theme); + }); + // ... +} + +class Bad3 extends React.Component { + render() { + // 🔴 Bad: inside a class component + useEffect(() => {}) + // ... + } +} +``` + +You can use the [`eslint-plugin-react-hooks` plugin](https://www.npmjs.com/package/eslint-plugin-react-hooks) to catch some of these mistakes. + +>Note +> +>[Custom Hooks](/docs/hooks-custom.html) *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. + + +## Duplicate React + +In order for Hooks to work, the `react` import from your application code needs to resolve to the same module as the `react` import from inside the `react-dom` package. + +If these `react` imports resolve to two different exports objects, you will see this warning. This may happen if you **accidentally end up with two copies** of the `react` package. + +If you use Node for package management, you can run this check in your project folder: + + npm ls react + +If you see more than one React, you'll need to figure out why this happens and fix your dependency tree. For example, maybe a library you're using incorrectly specifies `react` as a dependency (rather than a peer dependency). Until that library is fixed, [Yarn resolutions](https://yarnpkg.com/lang/en/docs/selective-version-resolutions/) is one possible workaround. + +You can also try to debug this problem by adding some logs and restarting your development server: + +```js +// Add this in node_modules/react-dom/index.js +window.React1 = require('react'); + +// Add this in your component file +require('react-dom'); +window.React2 = require('react'); +console.log(window.React1 === window.React2); +``` + +If it prints `false` then you might have two Reacts and need to figure out why that happened. [This issue](https://github.com/facebook/react/issues/13991) includes some common reasons encountered by the community. + +This problem can also come up when you use `npm link` or an equivalent. In that case, your bundler might "see" two Reacts — one in application folder and one in your library folder. Assuming `myapp` and `mylib` are sibling folders, one possible fix is to run `npm link ../myapp/node_modules/react` from `mylib`. This should make the library use the application's React copy. + +>Note +> +>In general, React supports using multiple independent copies on one page (for example, if an app and a third-party widget both use it). It only breaks if `require('react')` resolves differently between the component and the `react-dom` copy it was rendered with. + +## Other Causes + +If none of this worked, please comment in [this issue](https://github.com/facebook/react/issues/13991) and we'll try to help. Try to create a small reproducing example — you might discover the problem as you're doing it.