Add docs for getDerivedStateFromError [16.6] (#1223)

* Updated error boundary docs to reference new getDerivedStateFromError

* Updated some wording in response to PR feedback

* These

content/docs/error-boundaries.md

@@ -22,18 +22,21 @@ Error boundaries are React components that **catch JavaScript errors anywhere in
 > * Server side rendering
 > * Errors thrown in the error boundary itself (rather than its children)
 
-A class component becomes an error boundary if it defines a new lifecycle method called `componentDidCatch(error, info)`:
+A class component becomes an error boundary if it defines either (or both) of the lifecycle methods [`static getDerivedStateFromError()`](/docs/react-component.html#static-getderivedstatefromerror) or [`componentDidCatch()`](/docs/react-component.html#componentdidcatch). Use `static getDerivedStateFromError()` to render a fallback UI after an error has been thrown. Use `componentDidCatch()` to log error information.
 
-```js{7-12,15-18}
+```js{7-10,12-15,18-21}
 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) {
-    // Display fallback UI
-    this.setState({ hasError: true });
     // You can also log the error to an error reporting service
     logErrorToMyService(error, info);
   }
@@ -43,6 +46,7 @@ class ErrorBoundary extends React.Component {
       // You can render any custom fallback UI
       return <h1>Something went wrong.</h1>;
     }
+
     return this.props.children; 
   }
 }
@@ -56,32 +60,10 @@ Then you can use it as a regular component:
 </ErrorBoundary>
 ```
 
-The `componentDidCatch()` method works 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.
+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 catch {} block works in JavaScript.
 
-### componentDidCatch Parameters
-
-`error` is an error that has been thrown.
-
-`info` is an object with `componentStack` key. The property has information about component stack during thrown error.
-
-```js
-//...
-componentDidCatch(error, info) {
-
-  /* Example stack information:
-     in ComponentThatThrows (created by App)
-     in ErrorBoundary (created by App)
-     in div (created by App)
-     in App
-  */
-  logComponentStackToMyService(info.componentStack);
-}
-
-//...
-```
-
 ## Live Demo
 
 Check out [this example of declaring and using an error boundary](https://codepen.io/gaearon/pen/wqvxGa?editors=0010) with [React 16](/blog/2017/09/26/react-v16.0.html).

content/docs/reference-react-component.md

@@ -81,8 +81,9 @@ This method is called when a component is being removed from the DOM:
 
 #### Error Handling
 
-This method is called when there is an error during rendering, in a lifecycle method, or in the constructor of any child component.
+These methods are called when there is an error during rendering, in a lifecycle method, or in the constructor of any child component.
 
+- [`static getDerivedStateFromError()`](#getderivedstatefromerror)
 - [`componentDidCatch()`](#componentdidcatch)
 
 ### Other APIs
@@ -290,6 +291,7 @@ This method doesn't have access to the component instance. If you'd like, you ca
 
 Note that this method is fired on *every* render, regardless of the cause. This is in contrast to `UNSAFE_componentWillReceiveProps`, which only fires when the parent causes a re-render and not as a result of a local `setState`.
 
+* * *
 
 ### `getSnapshotBeforeUpdate()`
 
@@ -311,21 +313,110 @@ In the above examples, it is important to read the `scrollHeight` property in `g
 
 * * *
 
+### Error boundaries
+
+[Error boundaries](/docs/error-boundaries.html) 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 if it defines either (or both) of the lifecycle methods `static getDerivedStateFromError()` or `componentDidCatch()`. Updating state from these lifecycles lets you capture an unhandled JavaScript error in the below tree and display a fallback UI.
+
+Only use error boundaries for recovering from unexpected exceptions; **don't try to use them for control flow.**
+
+For more details, see [*Error Handling in React 16*](/blog/2017/07/26/error-handling-in-react-16.html).
+
+> Note
+> 
+> Error boundaries only catch errors in the components **below** them in the tree. An error boundary can’t catch an error within itself.
+
+### `static getDerivedStateFromError()`
+```javascript
+static getDerivedStateFromError(error)
+```
+
+This lifecycle is invoked after an error has been thrown by a descendant component.
+It receives the error that was thrown as a parameter and should return a value to update state.
+
+```js{7-10,13-16}
+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 };
+  }
+
+  render() {
+    if (this.state.hasError) {
+      // You can render any custom fallback UI
+      return <h1>Something went wrong.</h1>;
+    }
+
+    return this.props.children; 
+  }
+}
+```
+
+> Note
+>
+> `getDerivedStateFromError()` is called during the "render" phase, so side-effects are not permitted.
+For those use cases, use `componentDidCatch()` instead.
+
+* * *
+
 ### `componentDidCatch()`
 
 ```javascript
 componentDidCatch(error, info)
 ```
 
-[Error boundaries](/docs/error-boundaries.html) 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.
+This lifecycle is invoked after an error has been thrown by a descendant component.
+It receives two parameters:
 
-A class component becomes an error boundary if it defines this lifecycle method. Calling `setState()` in it lets you capture an unhandled JavaScript error in the below tree and display a fallback UI. Only use error boundaries for recovering from unexpected exceptions; don't try to use them for control flow.
+1. `error` - The error that was thrown.
+2. `info` - An object with a `componentStack` key containing [information about which component threw the error](/docs/error-boundaries.html#component-stack-traces).
 
-For more details, see [*Error Handling in React 16*](/blog/2017/07/26/error-handling-in-react-16.html).
+
+`componentDidCatch()` is called during the "commit" phase, so side-effects are permitted.
+It should be used for things like logging errors:
+
+```js{12-19}
+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) {
+    // Example "componentStack":
+    //   in ComponentThatThrows (created by App)
+    //   in ErrorBoundary (created by App)
+    //   in div (created by App)
+    //   in App
+    logComponentStackToMyService(info.componentStack);
+  }
+
+  render() {
+    if (this.state.hasError) {
+      // You can render any custom fallback UI
+      return <h1>Something went wrong.</h1>;
+    }
+
+    return this.props.children; 
+  }
+}
+```
 
 > Note
 > 
-> Error boundaries only catch errors in the components **below** them in the tree. An error boundary can’t catch an error within itself.
+> In the event of an error, you can render a fallback UI with `componentDidCatch()` by calling `setState`, but this will be deprecated in a future release.
+> Use `static getDerivedStateFromError()` to handle fallback rendering instead.
 
 * * *