`React` is the entry point to the React library. If you're using one of the prebuilt packages it's available as a global; if you're using CommonJS modules you can `require()` it.
This is the base class for React Components when they're defined using ES6 classes. See [Reusable Components](/react/docs/reusable-components.html#es6-classes) for how to use ES6 classes with React. For what methods are actually provided by the base class, see the [Component API](/react/docs/component-api.html).
Create a component class, given a specification. A component implements a `render` method which returns **one single** child. That child may have an arbitrarily deep child structure. One thing that makes components different than standard prototypal classes is that you don't need to call new on them. They are convenience wrappers that construct backing instances (via new) for you.
Clone and return a new `ReactElement` using `element` as the starting point. The resulting element will have the original element's props with the new props merged in shallowly. New children will replace existing children. Unlike `React.addons.cloneWithProps`, `key` and `ref` from the original element will be preserved. There is no special behavior for merging any props (unlike `cloneWithProps`). See the [v0.13 RC2 blog post](/react/blog/2015/03/03/react-v0.13-rc2.html) for additional details.
`React.DOM` provides convenience wrappers around `React.createElement` for DOM components. These should only be used when not using JSX. For example, `React.DOM.div(null, 'Hello World!')`
### React.PropTypes
`React.PropTypes` includes types that can be used with a component's `propTypes` object to validate props being passed to your components. For more information about `propTypes`, see [Reusable Components](/react/docs/reusable-components.html).
### React.Children
`React.Children` provides utilities for dealing with the `this.props.children` opaque data structure.
#### React.Children.map
```javascript
object React.Children.map(object children, function fn [, object thisArg])
```
Invoke `fn` on every immediate child contained within `children` with `this` set to `thisArg`. If `children` is a nested object or array it will be traversed: `fn` will never be passed the container objects. If children is `null` or `undefined` returns `null` or `undefined` rather than an empty object.
#### React.Children.forEach
```javascript
React.Children.forEach(object children, function fn [, object thisArg])
```
Like `React.Children.map()` but does not return an object.
#### React.Children.count
```javascript
number React.Children.count(object children)
```
Return the total number of components in `children`, equal to the number of times that a callback passed to `map` or `forEach` would be invoked.
#### React.Children.only
```javascript
object React.Children.only(object children)
```
Return the only child in `children`. Throws otherwise.
The `react-dom` package provides DOM-specific methods that can be used at the top level of your app and as an escape hatch to get outside of the React model if you need to. Most of your components should not need to use this module.
Render a ReactElement into the DOM in the supplied `container` and return a [reference](/react/docs/more-about-refs.html) to the component (or returns `null` for [stateless components](/react/docs/reusable-components.html#stateless-functions)).
If the ReactElement was previously rendered into `container`, this will perform an update on it and only mutate the DOM as necessary to reflect the latest React component.
Remove a mounted React component from the DOM and clean up its event handlers and state. If no component was mounted in the container, calling this function does nothing. Returns `true` if a component was unmounted and `false` if there was no component to unmount.
If this component has been mounted into the DOM, this returns the corresponding native browser DOM element. This method is useful for reading values out of the DOM, such as form field values and performing DOM measurements. **In most cases, you can attach a ref to the DOM node and avoid using `findDOMNode` at all.** When `render` returns `null` or `false`, `findDOMNode` returns `null`.
> `findDOMNode()` is an escape hatch used to access the underlying DOM node. In most cases, use of this escape hatch is discouraged because it pierces the component abstraction.
> `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.
Render a ReactElement to its initial HTML. This should only be used on the server. React will return an HTML string. You can use this method to generate HTML on the server and send the markup down on the initial request for faster page loads and to allow search engines to crawl your pages for SEO purposes.
If you call `ReactDOM.render()` on a node that already has this server-rendered markup, React will preserve it and only attach event handlers, allowing you to have a very performant first-load experience.
Similar to `renderToString`, except this doesn't create extra DOM attributes such as `data-react-id`, that React uses internally. This is useful if you want to use React as a simple static page generator, as stripping away the extra attributes can save lots of bytes.
