IPFS
v1.0.1GitHub

Context

Context provides a way to pass data through the component tree without having to pass props down manually at every level.

In a typical React application, data is passed top-down (parent to child) via props, but this can be cumbersome for certain types of props (e.g. locale preference, UI theme) that are required by many components within an application. Context provides a way to share values like these between components without having to explicitly pass a prop through every level of the tree.

When to Use Context

Context is designed to share data that can be considered “global” for a tree of React components, such as the current authenticated user, theme, or preferred language. For example, in the code below we manually thread through a “theme” prop in order to style the Button component:

class App extends React.Component {
  render() {
    return <Toolbar theme="dark" />;
  }
}

function Toolbar(props) {
  // The Toolbar component must take an extra "theme" prop
  // and pass it to the ThemedButton. This can become painful
  // if every single button in the app needs to know the theme
  // because it would have to be passed through all components.
  return (
    <div>
      <ThemedButton theme={props.theme} />
    </div>
  );
}

function ThemedButton(props) {
  return <Button theme={props.theme} />;
}

Using context, we can avoid passing props through intermediate elements:

// Context lets us pass a value deep into the component tree
// without explicitly threading it through every component.
// Create a context for the current theme (with "light" as the default).
const ThemeContext = React.createContext('light');

class App extends React.Component {
  render() {
    // Use a Provider to pass the current theme to the tree below.
    // Any component can read it, no matter how deep it is.
    // In this example, we're passing "dark" as the current value.
    return (
      <ThemeContext.Provider value="dark">
        <Toolbar />
      </ThemeContext.Provider>
    );
  }
}

// A component in the middle doesn't have to
// pass the theme down explicitly anymore.
function Toolbar(props) {
  return (
    <div>
      <ThemedButton />
    </div>
  );
}

function ThemedButton(props) {
  // Use a Consumer to read the current theme context.
  // React will find the closest theme Provider above and use its value.
  // In this example, the current theme is "dark".
  return (
    <ThemeContext.Consumer>
      {theme => <Button {...props} theme={theme} />}
    </ThemeContext.Consumer>
  );
}

Before You Use Context

Context is primarily used when some data needs to be accessible by many components at different nesting levels. Apply it sparingly because it makes component reuse more difficult.

If you only want to avoid passing some props through many levels, component composition is often a simpler solution than context.

For example, consider a Page component that passes a user prop several levels down so that a deeply nested Avatar component can read it:

<Page user={user} />
// ... which renders ...
<PageLayout user={user} />
// ... which renders ...
<NavigationBar user={user} /> 
// ... which renders ...
<Avatar user={user} />

It might feel redundant to pass down the user prop through many levels if in the end only the Avatar component really needs it. It’s also annoying that whenever the Avatar component needs more props from the top, you have to add them at all the intermediate levels too.

To solve this issue without context, change PageLayout and NavigationBar to accept a children prop. Then the Page component could pass <Avatar user={user} /> down as a child, and neither of the components below would need to know about the user prop:

function Page(props) {
  const user = props.user;
  return (
    <PageLayout>
      <NavigationBar>
        <Avatar user={user} />
      </NavigationBar>
    </PageLayout>
  );
}

You’re not limited to a single child for a component. You may pass multiple children, or even have multiple separate “slots” for children, as documented here:

function Page(props) {
  const user = props.user;
  const content = <Feed user={user} />;
  const topBar = (
    <NavigationBar>
      <Avatar user={user} />
    </NavigationBar>
  );
  return (
    <PageLayout
      topBar={topBar}
      content={content}
    />
  );
}

This pattern is sufficient for many cases when you need to decouple a child from its immediate parents. You can take it even further with render props if the child needs to communicate with the parent before rendering.

However, sometimes the same data needs to be accessible by many components in the tree, and at different nesting levels. Context lets you “broadcast” such data, and changes to it, to all components below. Common examples where using context might be simpler than the alternatives include managing the current locale, theme, or a data cache.

API

React.createContext

const {Provider, Consumer} = React.createContext(defaultValue);

Creates a { Provider, Consumer } pair. When React renders a context Consumer, it will read the current context value from the closest matching Provider above it in the tree.

The defaultValue argument is only used by a Consumer when it does not have a matching Provider above it in the tree. This can be helpful for testing components in isolation without wrapping them. Note: passing undefined as a Provider value does not cause Consumers to use defaultValue.

Provider

<Provider value={/* some value */}>

A React component that allows Consumers to subscribe to context changes.

Accepts a value prop to be passed to Consumers that are descendants of this Provider. One Provider can be connected to many Consumers. Providers can be nested to override values deeper within the tree.

Consumer

<Consumer>
  {value => /* render something based on the context value */}
</Consumer>

A React component that subscribes to context changes.

Requires a function as a child. The function receives the current context value and returns a React node. The value argument passed to the function will be equal to the value prop of the closest Provider for this context above in the tree. If there is no Provider for this context above, the value argument will be equal to the defaultValue that was passed to createContext().

Note

For more information about the ‘function as a child’ pattern, see render props.

All Consumers that are descendants of a Provider will re-render whenever the Provider’s value prop changes. The propagation from Provider to its descendant Consumers is not subject to the shouldComponentUpdate method, so the Consumer is updated even when an ancestor component bails out of the update.

Changes are determined by comparing the new and old values using the same algorithm as Object.is.

Note

The way changes are determined can cause some issues when passing objects as value: see Caveats.

Examples

Dynamic Context

A more complex example with dynamic values for the theme:

theme-context.js 

export const themes = {
  light: {
    foreground: '#000000',
    background: '#eeeeee',
  },
  dark: {
    foreground: '#ffffff',
    background: '#222222',
  },
};

export const ThemeContext = React.createContext(
  themes.dark // default value
);

themed-button.js 

import {ThemeContext} from './theme-context';

function ThemedButton(props) {
  return (
    <ThemeContext.Consumer>
      {theme => (
        <button
          {...props}
          style={{backgroundColor: theme.background}}
        />
      )}
    </ThemeContext.Consumer>
  );
}

export default ThemedButton;

app.js 

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 React.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 (
      <Page>
        <ThemeContext.Provider value={this.state.theme}>
          <Toolbar changeTheme={this.toggleTheme} />
        </ThemeContext.Provider>
        <Section>
          <ThemedButton />
        </Section>
      </Page>
    );
  }
}

ReactDOM.render(<App />, document.root);

Updating Context from a Nested Component

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 

// 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 {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 {ThemeContext, themes} from './theme-context';
import ThemeTogglerButton from './theme-toggler-button';

class App extends React.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>
  );
}

ReactDOM.render(<App />, document.root);

Consuming Multiple Contexts

To keep context re-rendering fast, React needs to make each context consumer a separate node in the tree. 

// Theme context, default to light theme
const ThemeContext = React.createContext('light');

// Signed-in user context
const UserContext = React.createContext({
  name: 'Guest',
});

class App extends React.Component {
  render() {
    const {signedInUser, theme} = this.props;

    // App component that provides initial context values
    return (
      <ThemeContext.Provider value={theme}>
        <UserContext.Provider value={signedInUser}>
          <Layout />
        </UserContext.Provider>
      </ThemeContext.Provider>
    );
  }
}

function Layout() {
  return (
    <div>
      <Sidebar />
      <Content />
    </div>
  );
}

// A component may consume multiple contexts
function Content() {
  return (
    <ThemeContext.Consumer>
      {theme => (
        <UserContext.Consumer>
          {user => (
            <ProfilePage user={user} theme={theme} />
          )}
        </UserContext.Consumer>
      )}
    </ThemeContext.Consumer>
  );
}

If two or more context values are often used together, you might want to consider creating your own render prop component that provides both.

Accessing Context in Lifecycle Methods

Accessing values from context in lifecycle methods is a relatively common use case. Instead of adding context to every lifecycle method, you just need to pass it as a prop, and then work with it just like you’d normally work with a prop.

class Button extends React.Component {
  componentDidMount() {
    // ThemeContext value is this.props.theme
  }

  componentDidUpdate(prevProps, prevState) {
    // Previous ThemeContext value is prevProps.theme
    // New ThemeContext value is this.props.theme
  }

  render() {
    const {theme, children} = this.props;
    return (
      <button className={theme || 'light'}>
        {children}
      </button>
    );
  }
}

export default props => (
  <ThemeContext.Consumer>
    {theme => <Button {...props} theme={theme} />}
  </ThemeContext.Consumer>
);

Consuming Context with a HOC

Some types of contexts are consumed by many components (e.g. theme or localization). It can be tedious to explicitly wrap each dependency with a <Context.Consumer> element. A higher-order component can help with this.

For example, a button component might consume a theme context like so:

const ThemeContext = React.createContext('light');

function ThemedButton(props) {
  return (
    <ThemeContext.Consumer>
      {theme => <button className={theme} {...props} />}
    </ThemeContext.Consumer>
  );
}

That’s alright for a few components, but what if we wanted to use the theme context in a lot of places?

We could create a higher-order component called withTheme:

const ThemeContext = React.createContext('light');

// This function takes a component...
export function withTheme(Component) {
  // ...and returns another component...
  return function ThemedComponent(props) {
    // ... and renders the wrapped component with the context theme!
    // Notice that we pass through any additional props as well
    return (
      <ThemeContext.Consumer>
        {theme => <Component {...props} theme={theme} />}
      </ThemeContext.Consumer>
    );
  };
}

Now any component that depends on the theme context can easily subscribe to it using the withTheme function we’ve created:

function Button({theme, ...rest}) {
  return <button className={theme} {...rest} />;
}

const ThemedButton = withTheme(Button);

Forwarding Refs to Context Consumers

One issue with the render prop API is that refs don’t automatically get passed to wrapped elements. To get around this, use React.forwardRef:

fancy-button.js 

class FancyButton extends React.Component {
  focus() {
    // ...
  }

  // ...
}

// Use context to pass the current "theme" to FancyButton.
// Use forwardRef to pass refs to FancyButton as well.
export default React.forwardRef((props, ref) => (
  <ThemeContext.Consumer>
    {theme => (
      <FancyButton {...props} theme={theme} ref={ref} />
    )}
  </ThemeContext.Consumer>
));

app.js 

import FancyButton from './fancy-button';

const ref = React.createRef();

// Our ref will point to the FancyButton component,
// And not the ThemeContext.Consumer that wraps it.
// This means we can call FancyButton methods like ref.current.focus()
<FancyButton ref={ref} onClick={handleClick}>
  Click me!
</FancyButton>;

Caveats

Because context uses reference identity to determine when to re-render, there are some gotchas that could trigger unintentional renders in consumers when a provider’s parent re-renders. For example, the code below will re-render all consumers every time the Provider re-renders because a new object is always created for value:

class App extends React.Component {
  render() {
    return (
      <Provider value={{something: 'something'}}>
        <Toolbar />
      </Provider>
    );
  }
}

To get around this, lift the value into the parent’s state:

class App extends React.Component {
  constructor(props) {
    super(props);
    this.state = {
      value: {something: 'something'},
    };
  }

  render() {
    return (
      <Provider value={this.state.value}>
        <Toolbar />
      </Provider>
    );
  }
}

Legacy API

Note

React previously shipped with an experimental context API. The old API will be supported in all 16.x releases, but applications using it should migrate to the new version. The legacy API will be removed in a future major React version. Read the legacy context docs here.