React Top-Level API

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').

Overview

Components

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.

• React.Component
• React.PureComponent

If you don’t use ES6 classes, you may use the create-react-class module instead. See Using React without ES6 for more information.

Creating React Elements

We recommend using JSX to describe what your UI should look like. Each JSX element is just syntactic sugar for calling React.createElement(). You will not typically invoke the following methods directly if you are using JSX.

• createElement()
• createFactory()

See Using React without JSX for more information.

Transforming Elements

React provides several APIs for manipulating elements:

• cloneElement()
• isValidElement()
• React.Children

Fragments

React also provides a component for rendering a multiple elements without a wrapper.

• React.Fragment

Reference

React.Component

React.Component is the base class for React components when they are defined using ES6 classes:

class Greeting extends React.Component {
  render() {
    return <h1>Hello, {this.props.name}</h1>;
  }
}

See the React.Component API Reference for a list of methods and properties related to the base React.Component class.

React.PureComponent

React.PureComponent is similar to React.Component. The difference between them is that React.Component doesn’t implement 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.

Note

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() when you know deep data structures have changed. Or, consider using immutable objects 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”.

createElement()

React.createElement(
  type,
  [props],
  [...children]
)

Create and return a new React element of the given type. The type argument can be either a tag name string (such as 'div' or 'span'), a React component type (a class or a function), or a React fragment type.

Code written with JSX will be converted to use React.createElement(). You will not typically invoke React.createElement() directly if you are using JSX. See React Without JSX to learn more.

cloneElement()

React.cloneElement(
  element,
  [props],
  [...children]
)

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.

React.cloneElement() is almost equivalent to:

<element.type {...element.props} {...props}>{children}</element.type>

However, it also preserves refs. 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().

createFactory()

React.createFactory(type)

Return a function that produces React elements of a given type. Like React.createElement(), the type argument can be either a tag name string (such as 'div' or 'span'), a React component type (a class or a function), or a React fragment type.

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 to learn more.

isValidElement()

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

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.

React.Children.forEach

React.Children.forEach(children, function[(thisArg)])

Like React.Children.map() but does not return an array.

React.Children.count

React.Children.count(children)

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

React.Children.only(children)

Verifies that children has only one child (a React element) and returns it. Otherwise this method throws an error.

Note:

React.Children.only() does not accept the return value of React.Children.map() because it is an array rather than a React element.

React.Children.toArray

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.

React.Fragment

The React.Fragment component lets you return multiple elements in a render() method without creating an additional DOM element:

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.