Added post about upgrading your code to avoid isMounted()

_posts/2015-12-16-ismounted-antipattern.md

@@ -0,0 +1,77 @@
+---
+title: "isMounted is an Antipattern"
+author: jimfb
+---
+
+As we move closer to officially deprecating isMounted, it's worth understanding why the function is an antipattern, and how to write code without the isMounted function.
+
+The primary use case for `isMounted()` is to avoid calling `setState()` after a component has unmounted, because calling `setState()` after a component has unmounted will emit a warning. The “setState warning” exists to help you catch bugs, because calling `setState()` on an unmounted component is an indication that your app/component has somehow failed to clean up properly. Specifically, calling `setState()` in an unmounted component means that your app is still holding a reference to the component after the component has been unmounted - which often indicates a memory leak!
+
+To avoid the error message, people often add lines like this:
+
+```js
+if(this.isMounted()) { // This is bad.
+  this.setState({...});
+}
+```
+
+Checking `isMounted` before calling `setState()` does eliminate the warning, but it also defeats the purpose of the warning, since now you will never get the warning (even when you should!)
+
+Other uses of `isMounted()` are similarly erroneous; using `isMounted()` is a code smell because the only reason you would check is because you think you might be holding a reference after the component has unmounted.
+
+An easy migration strategy for anyone upgrading their code to avoid `isMounted()` is to track the mounted status yourself.  Just set a `_isMounted` property to true in `componentDidMount` and set it to false in `componentWillUnmount`, and use this variable to check your component's status.
+
+An optimal solution would be to find places where `setState()` might be called after a component has unmounted, and fix them. Such situations most commonly occur due to callbacks, when a component is waiting for some data and gets unmounted before the data arrives. Ideally, any callbacks should be canceled in `componentWillUnmount`, prior to unmounting.
+
+For instance, if you are using a Flux store in your component, you must unsubscribe in `componentWillUnmount`:
+
+```javascript{9}
+class MyComponent extends React.Component {
+  componentDidMount() {
+    mydatastore.subscribe(this);
+  }
+  render() {
+    ...
+  }
+  componentWillUnmount() {
+    mydatastore.unsubscribe(this);
+  }
+}
+```
+
+If you use ES6 promises, you may need to wrap your promise in order to make it cancelable.
+
+```js
+const cancelablePromise = makeCancelable(
+  new Promise(r => component.setState({...}}))
+);
+
+cancelablePromise
+  .promise
+  .then(() => console.log('resolved'))
+  .catch((reason) => console.log('isCanceled', reason.isCanceled));
+
+cancelablePromise.cancel(); // Cancel the promise
+```
+
+Where `makeCancelable` is [defined by @istarkov](https://github.com/facebook/react/issues/5465#issuecomment-157888325) as:
+
+```js
+const makeCancelable = (promise) => {
+  let hasCanceled_ = false;
+
+  return {
+    promise: new Promise(
+      (resolve, reject) => promise
+        .then(r => hasCanceled_
+          ? reject({isCanceled: true})
+          : resolve(r)
+        )
+    ),
+    cancel() {
+      hasCanceled_ = true;
+    },
+  };
+};
+```
+As an added bonus for getting your code cleaned up early, getting rid of `isMounted()` makes it one step easier for you to upgrade to ES6 classes, where using `isMounted()` is already prohibited.  Happy coding!

docs/ref-02-component-api.md

@@ -106,7 +106,7 @@ boolean isMounted()
 
 > Note:
 >
-> This method is not available on ES6 `class` components that extend `React.Component`. It may be removed entirely in a future version of React.
+> This method is not available on ES6 `class` components that extend `React.Component`. It will likely be removed entirely in a future version of React, so you might as well [start migrating away from isMounted() now](/react/blog/2015/12/16/ismounted-antipattern.html).
 
 
 ### setProps
@@ -143,4 +143,4 @@ Like `setProps()` but deletes any pre-existing props instead of merging the two
 
 > Note:
 >
-> This method is deprecated and will be removed soon. This method is not available on ES6 `class` components that extend `React.Component`. Instead of calling `replaceProps`, try invoking ReactDOM.render() again with the new props. For additional notes, see our [blog post about using the Top Level API](/react/blog/2015/10/01/react-render-and-top-level-api.html)
+> This method is deprecated and will be removed soon. This method is not available on ES6 `class` components that extend `React.Component`. Instead of calling `replaceProps`, try invoking ReactDOM.render() again with the new props. For additional notes, see our [blog post about using the Top Level API](/react/blog/2015/10/01/react-render-and-top-level-api.html)