`React` is the entry point to the React library. If you load React from a `<script>` tag, these top-level APIs are available on the `React` global. If you use ES6 with npm, you can write `import React from 'react'`. If you use ES5 with npm, you can write `var React = require('react')`.
React components let you split the UI into independent, reusable pieces, and think about each piece in isolation. React components can be defined by subclassing `React.Component` or `React.PureComponent`.
If you don't use ES6 classes, you may use the `create-react-class` module instead. See [Using React without ES6](/react/docs/react-without-es6.html) for more information.
We recommend [using JSX](/react/docs/introducing-jsx.html) to describe what your UI should look like. Each JSX element is just syntactic sugar for calling [`React.createElement()`](#createelement). You will not typically invoke the following methods directly if you are using JSX.
- [`createElement()`](#createelement)
- [`createFactory()`](#createfactory)
See [Using React without JSX](/react/docs/react-without-jsx.html) for more information.
### Transforming Elements
`React` also provides some other APIs:
- [`cloneElement()`](#cloneelement)
- [`isValidElement()`](#isvalidelement)
- [`React.Children`](#react.children)
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## Reference
### `React.Component`
`React.Component` is the base class for React components when they are defined using [ES6 classes](https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Classes).
```javascript
class Greeting extends React.Component {
render() {
return <h1>Hello, {this.props.name}</h1>;
}
}
```
See the [React.Component API Reference](/react/docs/react-component.html) for a list of methods and properties related to the base `React.Component` class.
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### `React.PureComponent`
`React.PureComponent` is exactly like [`React.Component`](#react.component) but implements [`shouldComponentUpdate()`](/react/docs/react-component.html#shouldcomponentupdate) with a shallow prop and state comparison.
If your React component's `render()` function renders the same result given the same props and state, you can use `React.PureComponent` for a performance boost in some cases.
> `React.PureComponent`'s `shouldComponentUpdate()` only shallowly compares the objects. If these contain complex data structures, it may produce false-negatives for deeper differences. Only extend `PureComponent` when you expect to have simple props and state, or use [`forceUpdate()`](/react/docs/react-component.html#forceupdate) when you know deep data structures have changed. Or, consider using [immutable objects](https://facebook.github.io/immutable-js/) to facilitate fast comparisons of nested data.
> Furthermore, `React.PureComponent`'s `shouldComponentUpdate()` skips prop updates for the whole component subtree. Make sure all the children components are also "pure".
Create and return a new [React element](/react/docs/rendering-elements.html) of the given type. The type argument can be either a tag name string (such as `'div'` or `'span'`), or a [React component](/react/docs/components-and-props.html) type (a class or a function).
Convenience wrappers around `React.createElement()` for DOM components are provided by `React.DOM`. For example, `React.DOM.a(...)` is a convenience wrapper for `React.createElement('a', ...)`. They are considered legacy, and we encourage you to either use JSX or use `React.createElement()` directly instead.
Code written with [JSX](/react/docs/introducing-jsx.html) will be converted to use `React.createElement()`. You will not typically invoke `React.createElement()` directly if you are using JSX. See [React Without JSX](/react/docs/react-without-jsx.html) to learn more.
Clone and return a new React element 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. `key` and `ref` from the original element will be preserved.
However, it also preserves `ref`s. This means that if you get a child with a `ref` on it, you won't accidentally steal it from your ancestor. You will get the same `ref` attached to your new element.
This API was introduced as a replacement of the deprecated `React.addons.cloneWithProps()`.
Return a function that produces React elements of a given type. Like [`React.createElement()`](#createElement), the type argument can be either a tag name string (such as `'div'` or `'span'`), or a [React component](/react/docs/components-and-props.html) type (a class or a function).
This helper is considered legacy, and we encourage you to either use JSX or use `React.createElement()` directly instead.
You will not typically invoke `React.createFactory()` directly if you are using JSX. See [React Without JSX](/react/docs/react-without-jsx.html) to learn more.
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### `isValidElement()`
```javascript
React.isValidElement(object)
```
Verifies the object is a React element. Returns `true` or `false`.
* * *
### `React.Children`
`React.Children` provides utilities for dealing with the `this.props.children` opaque data structure.
#### `React.Children.map`
```javascript
React.Children.map(children, function[(thisArg)])
```
Invokes a function on every immediate child contained within `children` with `this` set to `thisArg`. If `children` is a keyed fragment or array it will be traversed: the function will never be passed the container objects. If children is `null` or `undefined`, returns `null` or `undefined` rather than an array.
Returns 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
React.Children.only(children)
```
Returns the only child in `children`. Throws otherwise.
#### `React.Children.toArray`
```javascript
React.Children.toArray(children)
```
Returns the `children` opaque data structure as a flat array with keys assigned to each child. Useful if you want to manipulate collections of children in your render methods, especially if you want to reorder or slice `this.props.children` before passing it down.
> Note:
>
> `React.Children.toArray()` changes keys to preserve the semantics of nested arrays when flattening lists of children. That is, `toArray` prefixes each key in the returned array so that each element's key is scoped to the input array containing it.
