Error Boundaries

In the past, JavaScript errors inside components used to corrupt React’s internal state and cause it to emit cryptic errors on next renders. These errors were always caused by an earlier error in the application code, but React did not provide a way to handle them gracefully in components, and could not recover from them.

Introducing Error Boundaries

A JavaScript error in a part of the UI shouldn’t break the whole app. To solve this problem for React users, React 16 introduces a new concept of an “error boundary”.

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.

Note

Error boundaries do not catch errors for:

• Event handlers (learn more)
• Asynchronous code (e.g. setTimeout or requestAnimationFrame callbacks)
• Server side rendering
• Errors thrown in the error boundary itself (rather than its children)

A class component becomes an error boundary if 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, errorInfo) {    // You can also log the error to an error reporting service    logErrorToMyService(error, errorInfo);  }
  render() {
    if (this.state.hasError) {      // You can render any custom fallback UI      return <h1>Something went wrong.</h1>;    }
    return this.props.children; 
  }
}

Then you can use it as a regular component:

<ErrorBoundary>
  <MyWidget />
</ErrorBoundary>

Error boundaries work like a JavaScript catch {} block, but for components. Only class components can be error boundaries. In practice, most of the time you’ll want to declare an error boundary component once and use it throughout your application.

Note that error boundaries only catch errors in the components below them in the tree. An error boundary can’t catch an error within itself. If an error boundary fails trying to render the error message, the error will propagate to the closest error boundary above it. This, too, is similar to how the catch {} block works in JavaScript.

Live Demo

Check out this example of declaring and using an error boundary.

Where to Place Error Boundaries

The granularity of error boundaries is up to you. You may wrap top-level route components to display a “Something went wrong” message to the user, just like how server-side frameworks often handle crashes. You may also wrap individual widgets in an error boundary to protect them from crashing the rest of the application.

New Behavior for Uncaught Errors

This change has an important implication. As of React 16, errors that were not caught by any error boundary will result in unmounting of the whole React component tree.

We debated this decision, but in our experience it is worse to leave corrupted UI in place than to completely remove it. For example, in a product like Messenger leaving the broken UI visible could lead to somebody sending a message to the wrong person. Similarly, it is worse for a payments app to display a wrong amount than to render nothing.

This change means that as you migrate to React 16, you will likely uncover existing crashes in your application that have been unnoticed before. Adding error boundaries lets you provide better user experience when something goes wrong.

For example, Facebook Messenger wraps content of the sidebar, the info panel, the conversation log, and the message input into separate error boundaries. If some component in one of these UI areas crashes, the rest of them remain interactive.

We also encourage you to use JS error reporting services (or build your own) so that you can learn about unhandled exceptions as they happen in production, and fix them.

Component Stack Traces

React 16 prints all errors that occurred during rendering to the console in development, even if the application accidentally swallows them. In addition to the error message and the JavaScript stack, it also provides component stack traces. Now you can see where exactly in the component tree the failure has happened: