React provides powerful abstractions that free you from touching the DOM directly in most cases, but sometimes you simply need to access the underlying API, perhaps to work with a third-party library or existing code.
React is so fast because it never talks to the DOM directly. React maintains a fast in-memory representation of the DOM. `render()` methods return a *description* of the DOM, and React can diff this description with the in-memory representation to compute the fastest way to update the browser.
Additionally, React implements a full synthetic event system such that all event objects are guaranteed to conform to the W3C spec despite browser quirks, and everything bubbles consistently and efficiently across browsers. You can even use some HTML5 events in IE8!
Most of the time you should stay within React's "faked browser" world since it's more performant and easier to reason about. However, sometimes you simply need to access the underlying API, perhaps to work with a third-party library like a jQuery plugin. React provides escape hatches for you to use the underlying DOM API directly.
To interact with the browser, you'll need a reference to a DOM node. React has a `React.findDOMNode(component)` function which you can call to get a reference to the component's DOM node.
> `findDOMNode()` only works on mounted components (that is, components that have been placed in the DOM). If you try to call this on a component that has not been mounted yet (like calling `findDOMNode()` in `render()` on a component that has yet to be created) an exception will be thrown.
In order to get a reference to a React component, you can either use `this` to get the current React component, or you can use refs to refer to a component you own. They work like this:
```javascript
var MyComponent = React.createClass({
handleClick: function() {
// Explicitly focus the text input using the raw DOM API.
React provides lifecycle methods that you can specify to hook into this process. We provide **will** methods, which are called right before something happens, and **did** methods which are called right after something happens.
*`componentWillReceiveProps(object nextProps)` is invoked when a mounted component receives new props. This method should be used to compare `this.props` and `nextProps` to perform state transitions using `this.setState()`.
*`shouldComponentUpdate(object nextProps, object nextState): boolean` is invoked when a component decides whether any changes warrant an update to the DOM. Implement this as an optimization to compare `this.props` with `nextProps` and `this.state` with `nextState` and return false if React should skip updating.
*`componentWillUpdate(object nextProps, object nextState)` is invoked immediately before updating occurs. You cannot call `this.setState()` here.
*`forceUpdate()` can be invoked on any mounted component when you know that some deeper aspect of the component's state has changed without using `this.setState()`.
At Facebook, we support older browsers, including IE8. We've had polyfills in place for a long time to allow us to write forward-thinking JS. This means we don't have a bunch of hacks scattered throughout our codebase and we can still expect our code to "just work". For example, instead of seeing `+new Date()`, we can just write `Date.now()`. Since the open source React is the same as what we use internally, we've carried over this philosophy of using forward thinking JS.
In addition to that philosophy, we've also taken the stance that we, as authors of a JS library, should not be shipping polyfills as a part of our library. If every library did this, there's a good chance you'd be sending down the same polyfill multiple times, which could be a sizable chunk of dead code. If your product needs to support older browsers, chances are you're already using something like [es5-shim](https://github.com/es-shims/es5-shim).
When using HTML5 elements in IE8 including `<section>`, `<article>`, `<nav>`, `<header>`, and `<footer>`, it's also necessary to include [html5shiv](https://github.com/aFarkas/html5shiv) or a similar script.
Although React is pretty good at abstracting browser differences, some browsers are limited or present quirky behaviors that we couldn't find a workaround for.
On IE8 the `onScroll` event doesn't bubble and IE8 doesn't have an API to define handlers to the capturing phase of an event, meaning there is no way for React to listen to these events.
