Context

With React, it's easy to track the flow of data through your React components. When you look at a component, you can see which props are being passed, which makes your apps easy to reason about.

In some cases, you want to pass data through the component tree without having to pass the props down manually at every level. You can do this directly in React with the powerful "context" API.

Why Not To Use Context #

The vast majority of applications do not need to use context.

If you want your application to be stable, don't use context. It is an experimental API and it is likely to break in future releases of React.

If you aren't familiar with state management libraries like Redux or MobX, don't use context. For many practical applications, these libraries and their React bindings are a good choice for managing state that is relevant to many components. It is far more likely that Redux is the right solution to your problem than that context is the right solution.

If you aren't an experienced React developer, don't use context. There is usually a better way to implement functionality just using props and state.

If you insist on using context despite these warnings, try to isolate your use of context to a small area and avoid using the context API directly when possible so that it's easier to upgrade when the API changes.

How To Use Context #

Suppose you have a structure like:

class Button extends React.Component {
  render() {
    return (
      <button style={{background: this.props.color}}>
        {this.props.children}
      </button>
    );
  }
}

class Message extends React.Component {
  render() {
    return (
      <div>
        {this.props.text} <Button color={this.props.color}>Delete</Button>
      </div>
    );
  }
}

class MessageList extends React.Component {
  render() {
    const color = "purple";
    const children = this.props.messages.map((message) =>
      <Message text={message.text} color={color} />
    );
    return <div>{children}</div>;
  }
}

In this example, we manually thread through a color prop in order to style the Button and Message components appropriately. Using context, we can pass this through the tree automatically:

class Button extends React.Component {
  render() {
    return (
      <button style={{background: this.context.color}}>
        {this.props.children}
      </button>
    );
  }
}

Button.contextTypes = {
  color: React.PropTypes.string
};

class Message extends React.Component {
  render() {
    return (
      <div>
        {this.props.text} <Button>Delete</Button>
      </div>
    );
  }
}

class MessageList extends React.Component {
  getChildContext() {
    return {color: "purple"};
  }

  render() {
    const children = this.props.messages.map((message) =>
      <Message text={message.text} />
    );
    return <div>{children}</div>;
  }
}

MessageList.childContextTypes = {
  color: React.PropTypes.string
};

By adding childContextTypes and getChildContext to MessageList (the context provider), React passes the information down automatically and any component in the subtree (in this case, Button) can access it by defining contextTypes.

If contextTypes is not defined, then context will be an empty object.

Parent-Child Coupling #

Context can also let you build an API where parents and children communicate. For example, one library that works this way is React Router V4:

const BasicExample = () => (
  <Router>
    <div>
      <ul>
        <li><Link to="/">Home</Link></li>
        <li><Link to="/about">About</Link></li>
        <li><Link to="/topics">Topics</Link></li>
      </ul>

      <hr/>

      <Match exactly pattern="/" component={Home} />
      <Match pattern="/about" component={About} />
      <Match pattern="/topics" component={Topics} />
    </div>
  </Router>
)

By passing down some information from the Router component, each Link and Match can communicate back to the containing Router.

Before you build components with an API similar to this, consider if there are cleaner alternatives. For example, you can pass entire React component as props if you'd like to.

Referencing Context in Lifecycle Methods #

If contextTypes is defined within a component, the following lifecycle methods will receive an additional parameter, the context object:

void componentWillReceiveProps(
  object nextProps, object nextContext
)

boolean shouldComponentUpdate(
  object nextProps, object nextState, object nextContext
)

void componentWillUpdate(
  object nextProps, object nextState, object nextContext
)

void componentDidUpdate(
  object prevProps, object prevState, object prevContext
)

Referencing Context in Stateless Functional Components #

Stateless functional components are also able to reference context if contextTypes is defined as a property of the function. The following code shows a Button component written as a stateless functional component.

const Button = ({children}, context) =>
  <button style={{background: context.color}}>
    {children}
  </button>;

Button.contextTypes = {color: React.PropTypes.string};

Updating Context #

Don't do it.

React has an API to update context, but it is fundamentally broken and you should not use it.

The getChildContext function will be called when the state or props changes. In order to update data in the context, trigger a local state update with this.setState. This will trigger a new context and changes will be received by the children.

class MediaQuery extends React.Component {
  constructor(props) {
    super(props);
    this.state = {type:'desktop'};
  }

  getChildContext() {
    return {type: this.state.type};
  }

  componentDidMount() {
    const checkMediaQuery = () => {
      const type = window.matchMedia("(min-width: 1025px)").matches ? 'desktop' : 'mobile';
      if (type !== this.state.type) {
        this.setState({type});
      }
    };

    window.addEventListener('resize', checkMediaQuery);
    checkMediaQuery();
  }

  render() {
    return this.props.children;
  }
}

MediaQuery.childContextTypes = {
  type: React.PropTypes.string
};

The problem is, if a context value provided by component changes, descendants that use that value won't update if an intermediate parent returns false from shouldComponentUpdate. This is totally out of control of the components using context, so there's basically no way to reliably update the context. This blog post has a good explanation of why this is a problem and how you might get around it.