Hooks
Code-Splitting
Composition
Portals
memo
Hooks are a new feature that lets you use state and other React features without writing a class. They're released in React v16.7.0.
Hooks are functions that let you "hook into" React state and lifecycle features from function components. Hooks don't work inside classes they let you use React without classes.
State Hook
Renders a counter example. When you click the button, it increments the value:
Using React Hook
import React, { useState } from 'react';
function App() {
// Declare a new state variable, which we'll call "count"
const [count, setCount] = useState(0);
return (
<div>
<p>You clicked {count} times</p>
<button onClick={() => setCount(count + 1)}>
Click me
</button>
</div>
);
}
export default App;useStateis a Hook, we call it inside a function component to add some local state to it.useStatereturns a pair: the current state value and a function that lets you update it, you can call this function from an event handler or somewhere else.useStatehas only one argument is the initial state, in the example above 0 is the initial state.
Using React Class
import React from 'react';
class App extends React.Component {
state = {
count: 0,
}
setCount = () => {
this.setState({
count: this.state.count + 1,
});
}
render() {
return (
<div>
<p>You clicked {this.state.count} times</p>
<button onClick={() => this.setCount()}>
Click me
</button>
</div>
);
}
}
export default App;Declaring multiple state variables.
You can use State Hook more than once in a single component:
function withManyStates() {
// Declare multiple state variables!
const [name, setName] = useState('Brian');
const [age, setAge] = useState(77);
const [todos, setTodos] = useState([{ text: 'Learn Hooks' }]);
}React also provides a few built-in Hooks like useState. You can also create custom Hooks to reuse stateful behavior between different components. We'll look at the built-in Hooks first.
You can learn more about the State Hook on a dedicated page: Using the State Hook.
Effect Hook
You likely performe data fetching, subscriptions, or manually changing the DOM from React components using React lifecycle ( componentDidMount, componentDidUpdate and componentWillUnmount ) in React classes. We call these operations "side effects".
The Effect Hook, useEffect, adds the ability to perform side effects from a function component. It serves the same purpose as React lifecycle ( componentDidMount, componentDidUpdate, and componentWillUnmount ) in React classes, but unified into a single API.
Example, this component sets the document title after React updates the DOM:
Using React Hook
import React, { useState, useEffect } from 'react';
function App() {
const [count, setCount] = useState(0);
// Similar to componentDidMount and componentDidUpdate:
useEffect(() => {
// Update the document title using the browser API
document.title = `You clicked ${count} times`;
});
return (
<div>
<p>You clicked {count} times</p>
<button onClick={() => setCount(count + 1)}>
Click me
</button>
</div>
);
}
export default App;- When you call
useEffect, you're telling React to run your "effect" function after flushing changes to the DOM. - Effects are declared inside the component so they have access to its props and state. By default, React runs the effects after every render including the first render.
Using React Class
import React from 'react';
class App extends React.Component {
state = {
count: 0,
}
componentDidMount() {
document.title = `You clicked ${this.state.count} times`;
}
componentDidUpdate() {
document.title = `You clicked ${this.state.count} times`;
window.addEventListener('resize', this.setCount);
}
setCount = () => {
this.setState({
count: this.state.count + 1,
});
}
render() {
return (
<div>
<p>You clicked {this.state.count} times</p>
<button onClick={() => this.setCount()}>
Click me
</button>
</div>
);
}
}
export default App;Example, React would clean up resize event when the component unmounts, as well as before re-running the effect due to a subsequent render.
Using React Hook
import React, { useState, useEffect } from 'react';
function App() {
const [width, setWidth] = useState(window.innerWidth);
// Similar to componentDidMount and componentDidUpdate:
useEffect(() => {
const handleResize = () => setWidth(window.innerWidth);
window.addEventListener('resize', handleResize);
// Clean up resize event
return () => {
window.removeEventListener('resize', handleResize);
};
});
return (
<div>
<p>Window width: {width}</p>
</div>
);
}
export default App;Using React Class
import React, { Component } from 'react';
class App extends Component {
state = {
width: window.innerWidth,
}
UNSAFE_componentWillMount() {
window.addEventListener('resize', this.handleResize);
}
componentWillUnmount() {
window.removeEventListener('resize', this.handleResize);
}
handleResize = () => {
this.setState({
width: window.innerWidth,
});
}
render() {
return (
<div>
<p>Window width: {this.state.width}</p>
</div>
);
}
}
export default App;Just like with useState, you can use multiple effect in a component:
Using React Hook
import React, { useState, useEffect } from 'react';
function App() {
const [width, setWidth] = useState(window.innerWidth);
useEffect(() => {
document.title = `Window width: ${width}`;
});
// Similar to componentDidMount and componentDidUpdate:
useEffect(() => {
const handleResize = () => setWidth(window.innerWidth);
window.addEventListener('resize', handleResize);
// Clean up resize event
return () => {
window.removeEventListener('resize', handleResize);
};
});
return (
<div>
<p>Window width: {width}</p>
</div>
);
}
export default App;Hooks let you organize side effects in a component by what pieces are related (such as adding and removing a event or subscription), rather than forcing a split based on lifecycle methods.
You can learn more about the Effect Hook on a dedicated page: Using the Effect Hook.
Building Your Own Hooks
Well, sometimes we want to reuse some stateful logic between components. Traditionally, there were two popular solutions to this problem: higher-order components and render props. Custom Hooks let you do this, but without adding more components to your tree.
Earlier on this page, we introduced a App component that calls the useState and useEffect Hooks to get window width and display on browser title. Let's say we also want to reuse this App logic in another component.
First, we'll extract this logic into a custom Hook called useWindowWidth and useDocumentTitle:
function useDocumentTitle(title) {
useEffect(() => {
document.title = title;
});
}
function useWindowWidth() {
const [width, setWidth] = useState(window.innerWidth);
useEffect(() => {
const handleResize = () => setWidth(window.innerWidth);
window.addEventListener('resize', handleResize);
// Clearn up
return () => {
window.removeEventListener('resize', handleResize);
};
});
return width;
}Now we can use it from the component:
import React, { useState, useEffect } from 'react';
function App() {
const width = useWindowWidth();
useDocumentTitle(`Window width: ${width}`);
return (
<div>
<p>Window width: {width}</p>
</div>
);
}
export default App;The state of these components is completely independent. Hooks are a way to reuse stateful logic, not state itself. In fact, each call to a Hook has a completely isolated state and so you can even use the same custom Hook twice in one component.
To build your own hooks you need to follow the useSomething naming convention. If a function's name starts with "use" and it calls other Hooks, we say it is a custom Hook.
You can write custom Hooks that cover a wide range of use cases like form handling, animation, declarative subscriptions, timers, and probably many more we haven't considered.
You can learn more about the Building Your Own Hooks on a dedicated page: Building Your Own Hooks.
Rules of Hooks
Hooks are JavaScript functions, but they impose two additional rules:
- Only call Hooks at the top level. Don't call Hooks inside loops, conditions, or nested functions.
- Only call Hooks from React function components. Don't call Hooks from regular JavaScript functions. (There is just one other valid place to call Hooks is your own custom Hooks.)
The better way you can use a linter plugin to enforce these rules automatically.
You can learn more about the Rules of Hooks on a dedicated page: Rules of Hooks.
The best way to introduce code-splitting into your app is through the dynamic import() syntax.
Before:
import { add } from './math';
console.log(add(16, 26));After:
import("./math").then(math => {
console.log(math.add(16, 26));
});Before:
import OtherComponent from './OtherComponent';
function MyComponent() {
return (
<div>
<OtherComponent />
</div>
);
}After:
import React, {lazy} from 'react';
const OtherComponent = lazy(() => import('./OtherComponent'));
function MyComponent() {
return (
<div>
<OtherComponent />
</div>
);
}This will automatically load the bundle containing the OtherComponent when this component gets rendered.
lazy takes a function that must call a dynamic import(). This must return a Promise which resolves to a module with a default export containing a React component.
If the module containing the OtherComponent is not yet loaded by the time MyComponent renders, we must show some fallback content while we're waiting for it to load - such as a loading indicator. This is done using the Suspense component.
import React, {lazy, Suspense} from 'react';
const OtherComponent = lazy(() => import('./OtherComponent'));
function MyComponent() {
return (
<Suspense fallback={<div>Loading...</div>}>
<OtherComponent />
</Suspense>
);
}Error boundaries are React components that catch JavaScript errors anywhere in their child component tree, log those errors, and display a fallback UI instead of the component tree that crashed.
Error boundaries catch errors during rendering, in lifecycle methods, and in constructors of the whole tree below them.
A class component becomes an error boundary once it defines either (or both) of the lifecycle methods static getDerivedStateFromError() or componentDidCatch(). Use static getDerivedStateFromError() to render a fallback UI after an error has been thrown. Use componentDidCatch() to log error information.
class ErrorBoundary extends React.Component {
constructor(props) {
super(props);
this.state = { hasError: false };
}
static getDerivedStateFromError(error) {
// Update state so the next render will show the fallback UI.
return { hasError: true };
}
componentDidCatch(error, info) {
// You can also log the error to an error reporting service
// logErrorToMyService(error, info);
}
render() {
if (this.state.hasError) {
// You can render any custom fallback UI
return <h1>Something went wrong.</h1>;
}
return this.props.children;
}
}Note that error boundaries only catch errors in the components below them in the tree.
Once you've created your Error Boundary, you can use it anywhere above your lazy components to display an error state when there's a network error.
import MyErrorBoundary from './MyErrorBoundary';
const OtherComponent = React.lazy(() => import('./OtherComponent'));
const AnotherComponent = React.lazy(() => import('./AnotherComponent'));
const MyComponent = () => (
<div>
<MyErrorBoundary>
<Suspense fallback={<div>Loading...</div>}>
<section>
<OtherComponent />
<AnotherComponent />
</section>
</Suspense>
</MyErrorBoundary>
</div>
);Deciding where in your app to introduce code splitting can be a bit tricky.
A good place to start is with routes.
Here's an example of how to setup route-based code splitting into your app using libraries like React Router with lazy.
import { BrowserRouter as Router, Route, Switch } from 'react-router-dom';
import React, { Suspense, lazy } from 'react';
const Home = lazy(() => import('./routes/Home'));
const About = lazy(() => import('./routes/About'));
const App = () => (
<Router>
<Suspense fallback={<div>Loading...</div>}>
<Switch>
<Route exact path="/" component={Home}/>
<Route path="/about" component={About}/>
</Switch>
</Suspense>
</Router>
);
lazycurrently only supports default exports.
// ManyComponents.js
export const MyComponent = /* ... */;
export const MyUnusedComponent = /* ... */;If the module you want to import uses named exports, you can create an intermediate module that reexports it as the default.
// MyComponent.js
export { MyComponent as default } from "./ManyComponents.js";Use like so:
// MyApp.js
import React, { lazy } from 'react';
const MyComponent = lazy(() => import("./MyComponent.js"));Context provides a way to pass data through the component tree without having to pass props down manually at every level.
- When to Use Context
- Before You Use Context
- API
- React.createContext
- Context.Provider
- Class.contextType
- Context.Consumer
- Examples
- Dynamic Context
- Updating Context from a Nested Component
- Consuming Multiple Contexts
- Caveats
Context is designed to share data that can be considered as "global" for a tree of React components, such as the current authenticated user, theme, or preferred language.
Using context, we can avoid passing props through intermediate elements.
Context is primarily used when some data needs to be accessible by many components at different nesting levels.
Context API provides a way to pass data through the component tree without having to pass props down manually to every level. In React, data is often passed from a parent to its child component as a property.
Using the new React Context API depends on three main steps:
- Passing the initial state to
React.createContext. This function then returns an object with aProviderand aConsumer. - Using the
Providercomponent at the top of the tree and making it accept a prop calledvalue. This value can be anything. - Using the
Consumercomponent anywhere below the Provider in the component tree to get a subset of the state.
const MyContext = React.createContext(defaultValue);React.createContext which is passed the initial value (defaultValue). This returns an object with a Provider and a Consumer.
<MyContext.Provider value={/* some value */}>The Provider component is used higher in the tree and accepts a prop called value (which can be anything).
contextTypeis used to:- Perform a side-effect at mount using the value of Context
- Render something based on the value of Context
class MyClass extends React.Component {
static contextType = MyContext;
componentDidMount() {
let value = this.context;
/* perform a side-effect at mount using the value of MyContext */
}
componentDidUpdate() {
let value = this.context;
/* ... */
}
componentWillUnmount() {
let value = this.context;
/* ... */
}
render() {
let value = this.context;
/* render something based on the value of MyContext */
}
}<MyContext.Consumer>
{value => /* render something based on the context value */}
</MyContext.Consumer>The Consumer component is used anywhere below the provider in the tree and accepts a prop called "children" which must be a function that accepts the value and must return a react element (JSX).
theme-context.js
import React from 'react';
export const themes = {
dark: {
foreground: '#ffffff',
background: '#222222',
},
light: {
foreground: '#000000',
background: '#eeeeee',
},
};
export const ThemeContext = React.createContext(
themes.dark, // default value
);themed-button.js
import React from 'react';
import {ThemeContext} from './theme-context';
class ThemedButton extends React.Component {
render() {
let props = this.props;
let theme = this.context;
return (
<button
{...props}
style={{backgroundColor: theme.background}}
/>
);
}
}
ThemedButton.contextType = ThemeContext;
export default ThemedButton;App.js
import React, { Component } from 'react';
import {ThemeContext, themes} from './theme-context';
import ThemedButton from './themed-button';
// An intermediate component that uses the ThemedButton
function Toolbar(props) {
return (
<ThemedButton onClick={props.changeTheme}>
Change Theme
</ThemedButton>
);
}
class App extends Component {
constructor(props) {
super(props);
this.state = {
theme: themes.light,
};
this.toggleTheme = () => {
this.setState(state => ({
theme:
state.theme === themes.dark
? themes.light
: themes.dark,
}));
};
}
render() {
// The ThemedButton button inside the ThemeProvider
// uses the theme from state while the one outside uses
// the default dark theme
return (
<div>
<ThemeContext.Provider value={this.state.theme}>
<Toolbar changeTheme={this.toggleTheme} />
</ThemeContext.Provider>
<div>
<ThemedButton />
</div>
</div>
);
}
}
export default App;It is often necessary to update the context from a component that is nested somewhere deeply in the component tree.
In this case you can pass a function down through the context to allow consumers to update the context:
theme-context.js
import React from 'react';
export const themes = {
dark: {
foreground: '#ffffff',
background: '#222222',
},
light: {
foreground: '#000000',
background: '#eeeeee',
},
};
// Make sure the shape of the default value passed to
// createContext matches the shape that the consumers expect!
export const ThemeContext = React.createContext({
theme: themes.dark,
toggleTheme: () => {},
});theme-toggler-button.js
import React from 'react';
import {ThemeContext} from './theme-context';
function ThemeTogglerButton() {
// The Theme Toggler Button receives not only the theme
// but also a toggleTheme function from the context
return (
<ThemeContext.Consumer>
{({theme, toggleTheme}) => (
<button
onClick={toggleTheme}
style={{backgroundColor: theme.background}}>
Toggle Theme
</button>
)}
</ThemeContext.Consumer>
);
}
export default ThemeTogglerButton;App.js
import React, { Component } from 'react';
import {ThemeContext, themes} from './theme-context';
import ThemeTogglerButton from './theme-toggler-button';
class App extends Component {
constructor(props) {
super(props);
this.toggleTheme = () => {
this.setState(state => ({
theme:
state.theme === themes.dark
? themes.light
: themes.dark,
}));
};
// State also contains the updater function so it will
// be passed down into the context provider
this.state = {
theme: themes.light,
toggleTheme: this.toggleTheme,
};
}
render() {
// The entire state is passed to the provider
return (
<ThemeContext.Provider value={this.state}>
<Content />
</ThemeContext.Provider>
);
}
}
function Content() {
return (
<div>
<ThemeTogglerButton />
</div>
);
}
export default App;The Context API is a neat way to pass state across the app without having to use props.
The Context API was introduced to allow you to pass state (and enable the state to update) across the app, without having to use props for it.
The React team suggests to stick to props if you have just a few levels of children to pass, because it’s still a much less complicated API than the Context API.
In many cases, it enables us to avoid using Redux, simplifying our apps a lot, and also learning how to use React.
How does it work?
You create a context using React.createContext(), which returns a Context object.:
const { Provider, Consumer } = React.createContext()Then you create a wrapper component that returns a Provider component, and you add as children all the components from which you want to access the context:
class Container extends React.Component {
constructor(props) {
super(props)
this.state = {
something: 'hey'
}
}
render() {
return (
<Provider value={{ state: this.state }}>{this.props.children}</Provider>
)
}
}
class HelloWorld extends React.Component {
render() {
return (
<Container>
<Button />
</Container>
)
}
}I used Container as the name of this component because this will be a global provider. You can also create smaller contexts.
Inside a component that’s wrapped in a Provider, you use a Consumer component to make use of the context:
class Button extends React.Component {
render() {
return (
<Consumer>
{context => <button>{context.state.something}</button>}
</Consumer>
)
}
}You can also pass functions into a Provider value, and those functions will be used by the Consumer to update the context state:
<Provider value={{
state: this.state,
updateSomething: () => this.setState({something: 'ho!'})
{this.props.children}
</Provider>
/* ... */
<Consumer>
{(context) => (
<button onClick={context.updateSomething}>{context.state.something}</button>
)}
</Consumer>You can create multiple contexts, to make your state distributed across components, yet expose it and make it reachable by any component you want.
When using multiple files, you create the content in one file, and import it in all the places you use it:
//context.js
import React from 'react'
export default React.createContext()
//component1.js
import Context from './context'
//... use Context.Provider
//component2.js
import Context from './context'
//... use Context.Consumer