`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](/docs/react-without-es6.html) for more information.
We recommend [using JSX](/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.
`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):
`React.PureComponent` is similar to [`React.Component`](#reactcomponent). The difference between them is that [`React.Component`](#reactcomponent) doesn't implement [`shouldComponentUpdate()`](/docs/react-component.html#shouldcomponentupdate), but `React.PureComponent` implements it 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()`](/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](/docs/rendering-elements.html) of the given type. The type argument can be either a tag name string (such as `'div'` or `'span'`), a [React component](/docs/components-and-props.html) type (a class or a function), or a [React fragment](#reactfragment) type.
Code written with [JSX](/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](/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'`), a [React component](/docs/components-and-props.html) type (a class or a function), or a [React fragment](#reactfragment) type.
You will not typically invoke `React.createFactory()` directly if you are using JSX. See [React Without JSX](/docs/react-without-jsx.html) to learn more.
Invokes a function on every immediate child contained within `children` with `this` set to `thisArg`. If `children` is an array it will be traversed and the function will be called for each child in the array. If children is `null` or `undefined`, this method will return `null` or `undefined` rather than an array.
> Note
>
> If `children` is a `Fragment` it will be treated as a single child and not traversed.
>`React.Children.only()` does not accept the return value of [`React.Children.map()`](#reactchildrenmap) because it is an array rather than a React element.
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.
The `React.Fragment` component lets you return multiple elements in a `render()` method without creating an additional DOM element:
```javascript
render() {
return (
<React.Fragment>
Some text.
<h2>A heading</h2>
</React.Fragment>
);
}
```
You can also use it with the shorthand `<></>` syntax. For more information, see [React v16.2.0: Improved Support for Fragments](/blog/2017/11/28/react-v16.2.0-fragment-support.html).
`React.forwardRef` creates a React component that forwards the [ref](/docs/refs-and-the-dom.html) attribute it receives to another component below in the tree. This technique is not very common but is particularly useful in two scenarios:
* [Forwarding refs to DOM components](/docs/forwarding-refs.html#forwarding-refs-to-dom-components)
* [Forwarding refs in higher-order-components](/docs/forwarding-refs.html#forwarding-refs-in-higher-order-components)
`React.forwardRef` accepts a rendering function as an argument. React will call this function with `props` and `ref` as two arguments. This function should return a React node.
In the above example, React passes a `ref` given to `<FancyButton ref={ref}>` element as a second argument to the rendering function inside the `React.forwardRef` call. This rendering function passes the `ref` to the `<button ref={ref}>` element.
As a result, after React attaches the ref, `ref.current` will point directly to the `<button>` DOM element instance.
