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--- | ||
title: "Built-in React Hooks" | ||
--- | ||
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<Intro> | ||
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*Hooks* let you use different React features from your components. You can either use the built-in Hooks or combine them to build your own. This page lists all built-in Hooks in React. | ||
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</Intro> | ||
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--- | ||
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## State Hooks {/*state-hooks*/} | ||
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*State* lets a component ["remember" information like user input.](/learn/state-a-components-memory) For example, a form component can use state to store the input value, while an image gallery component can use state to store the selected image index. | ||
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To add state to a component, use one of these Hooks: | ||
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* [`useState`](/reference/react/useState) declares a state variable that you can update directly. | ||
* [`useReducer`](/reference/react/useReducer) declares a state variable with the update logic inside a [reducer function.](/learn/extracting-state-logic-into-a-reducer) | ||
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```js | ||
function ImageGallery() { | ||
const [index, setIndex] = useState(0); | ||
// ... | ||
``` | ||
--- | ||
## Context Hooks {/*context-hooks*/} | ||
*Context* lets a component [receive information from distant parents without passing it as props.](/learn/passing-props-to-a-component) For example, your app's top-level component can pass the current UI theme to all components below, no matter how deep. | ||
* [`useContext`](/reference/react/useContext) reads and subscribes to a context. | ||
```js | ||
function Button() { | ||
const theme = useContext(ThemeContext); | ||
// ... | ||
``` | ||
--- | ||
## Ref Hooks {/*ref-hooks*/} | ||
*Refs* let a component [hold some information that isn't used for rendering,](/learn/referencing-values-with-refs) like a DOM node or a timeout ID. Unlike with state, updating a ref does not re-render your component. Refs are an "escape hatch" from the React paradigm. They are useful when you need to work with non-React systems, such as the built-in browser APIs. | ||
* [`useRef`](/reference/react/useRef) declares a ref. You can hold any value in it, but most often it's used to hold a DOM node. | ||
* [`useImperativeHandle`](/reference/react/useImperativeHandle) lets you customize the ref exposed by your component. This is rarely used. | ||
```js | ||
function Form() { | ||
const inputRef = useRef(null); | ||
// ... | ||
``` | ||
--- | ||
## Effect Hooks {/*effect-hooks*/} | ||
*Effects* let a component [connect to and synchronize with external systems.](/learn/synchronizing-with-effects) This includes dealing with network, browser DOM, animations, widgets written using a different UI library, and other non-React code. | ||
* [`useEffect`](/reference/react/useEffect) connects a component to an external system. | ||
```js | ||
function ChatRoom({ roomId }) { | ||
useEffect(() => { | ||
const connection = createConnection(roomId); | ||
connection.connect(); | ||
return () => connection.disconnect(); | ||
}, [roomId]); | ||
// ... | ||
``` | ||
Effects are an "escape hatch" from the React paradigm. Don't use Effects to orchestrate the data flow of your application. If you're not interacting with an external system, [you might not need an Effect.](/learn/you-might-not-need-an-effect) | ||
There are two rarely used variations of `useEffect` with differences in timing: | ||
* [`useLayoutEffect`](/reference/react/useLayoutEffect) fires before the browser repaints the screen. You can measure layout here. | ||
* [`useInsertionEffect`](/reference/react/useInsertionEffect) fires before React makes changes to the DOM. Libraries can insert dynamic CSS here. | ||
--- | ||
## Performance Hooks {/*performance-hooks*/} | ||
A common way to optimize re-rendering performance is to skip unnecessary work. For example, you can tell React to reuse a cached calculation or to skip a re-render if the data has not changed since the previous render. | ||
To skip calculations and unnecessary re-rendering, use one of these Hooks: | ||
- [`useMemo`](/reference/react/useMemo) lets you cache the result of an expensive calculation. | ||
- [`useCallback`](/reference/react/useCallback) lets you cache a function definition before passing it down to an optimized component. | ||
```js | ||
function TodoList({ todos, tab, theme }) { | ||
const visibleTodos = useMemo(() => filterTodos(todos, tab), [todos, tab]); | ||
// ... | ||
} | ||
``` | ||
Sometimes, you can't skip re-rendering because the screen actually needs to update. In that case, you can improve performance by separating blocking updates that must be synchronous (like typing into an input) from non-blocking updates which don't need to block the user interface (like updating a chart). | ||
To prioritize rendering, use one of these Hooks: | ||
- [`useTransition`](/reference/react/useTransition) lets you mark a state transition as non-blocking and allow other updates to interrupt it. | ||
- [`useDeferredValue`](/reference/react/useDeferredValue) lets you defer updating a non-critical part of the UI and let other parts update first. | ||
--- | ||
## Resource Hooks {/*resource-hooks*/} | ||
*Resources* can be accessed by a component without having them as part of their state. For example, a component can read a message from a Promise or read styling information from a context. | ||
To read a value from a resource, use this Hook: | ||
- [`use`](/reference/react/use) lets you read the value of a resource like a [Promise](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise) or [context](/learn/passing-data-deeply-with-context). | ||
```js | ||
function MessageComponent({ messagePromise }) { | ||
const message = use(messagePromise); | ||
const theme = use(ThemeContext); | ||
// ... | ||
} | ||
``` | ||
--- | ||
## Other Hooks {/*other-hooks*/} | ||
These Hooks are mostly useful to library authors and aren't commonly used in the application code. | ||
- [`useDebugValue`](/reference/react/useDebugValue) lets you customize the label React DevTools displays for your custom Hook. | ||
- [`useId`](/reference/react/useId) lets a component associate a unique ID with itself. Typically used with accessibility APIs. | ||
- [`useSyncExternalStore`](/reference/react/useSyncExternalStore) lets a component subscribe to an external store. | ||
--- | ||
## Your own Hooks {/*your-own-hooks*/} | ||
You can also [define your own custom Hooks](/learn/reusing-logic-with-custom-hooks#extracting-your-own-custom-hook-from-a-component) as JavaScript functions. |
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