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React
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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.

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

React components can also be defined as functions which can be wrapped:

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.

See Using React without JSX for more information.

Transforming Elements

React provides several APIs for manipulating elements:

Fragments

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

Refs

Suspense

Suspense lets components “wait” for something before rendering. Today, Suspense only supports one use case: loading components dynamically with React.lazy. In the future, it will support other use cases like data fetching.

Transitions

Transitions are a new concurrent feature introduced in React 18. They allow you to mark updates as transitions, which tells React that they can be interrupted and avoid going back to Suspense fallbacks for already visible content.

Hooks

Hooks are a new addition in React 16.8. They let you use state and other React features without writing a class. Hooks have a dedicated docs section and a separate API reference:

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”.

React.memo 

const MyComponent = React.memo(function MyComponent(props) {
  /* render using props */
});

React.memo is a higher order component.

If your component renders the same result given the same props, you can wrap it in a call to React.memo for a performance boost in some cases by memoizing the result. This means that React will skip rendering the component, and reuse the last rendered result.

React.memo only checks for prop changes. If your function component wrapped in React.memo has a useState, useReducer or useContext Hook in its implementation, it will still rerender when state or context change.

By default it will only shallowly compare complex objects in the props object. If you want control over the comparison, you can also provide a custom comparison function as the second argument.

function MyComponent(props) {
  /* render using props */
}
function areEqual(prevProps, nextProps) {
  /*
  return true if passing nextProps to render would return
  the same result as passing prevProps to render,
  otherwise return false
  */
}
export default React.memo(MyComponent, areEqual);

This method only exists as a performance optimization. Do not rely on it to “prevent” a render, as this can lead to bugs.

Note

Unlike the shouldComponentUpdate() method on class components, the areEqual function returns true if the props are equal and false if the props are not equal. This is the inverse from shouldComponentUpdate.

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,
  [config],
  [...children]
)

Clone and return a new React element using element as the starting point. config should contain all new props, key, or ref. 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 if no key and ref present in the config.

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. The new ref or key will replace old ones if present.

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 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.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.

React.createRef

React.createRef creates a ref that can be attached to React elements via the ref attribute. 

class MyComponent extends React.Component {
  constructor(props) {
    super(props);

    this.inputRef = React.createRef();  }

  render() {
    return <input type="text" ref={this.inputRef} />;  }

  componentDidMount() {
    this.inputRef.current.focus();  }
}

React.forwardRef

React.forwardRef creates a React component that forwards the ref attribute it receives to another component below in the tree. This technique is not very common but is particularly useful in two scenarios:

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.

const FancyButton = React.forwardRef((props, ref) => (  <button ref={ref} className="FancyButton">    {props.children}
  </button>
));

// You can now get a ref directly to the DOM button:
const ref = React.createRef();
<FancyButton ref={ref}>Click me!</FancyButton>;

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.

For more information, see forwarding refs.

React.lazy

React.lazy() lets you define a component that is loaded dynamically. This helps reduce the bundle size to delay loading components that aren’t used during the initial render.

You can learn how to use it from our code splitting documentation. You might also want to check out this article explaining how to use it in more detail.

// This component is loaded dynamically
const SomeComponent = React.lazy(() => import('./SomeComponent'));

Note that rendering lazy components requires that there’s a <React.Suspense> component higher in the rendering tree. This is how you specify a loading indicator.

React.Suspense

React.Suspense lets you specify the loading indicator in case some components in the tree below it are not yet ready to render. In the future we plan to let Suspense handle more scenarios such as data fetching. You can read about this in our roadmap.

Today, lazy loading components is the only use case supported by <React.Suspense>:

// This component is loaded dynamically
const OtherComponent = React.lazy(() => import('./OtherComponent'));

function MyComponent() {
  return (
    // Displays <Spinner> until OtherComponent loads
    <React.Suspense fallback={<Spinner />}>
      <div>
        <OtherComponent />
      </div>
    </React.Suspense>
  );
}

It is documented in our code splitting guide. Note that lazy components can be deep inside the Suspense tree — it doesn’t have to wrap every one of them. The best practice is to place <Suspense> where you want to see a loading indicator, but to use lazy() wherever you want to do code splitting.

Note

For content that is already shown to the user, switching back to a loading indicator can be disorienting. It is sometimes better to show the “old” UI while the new UI is being prepared. To do this, you can use the new transition APIs startTransition and useTransition to mark updates as transitions and avoid unexpected fallbacks.

React.Suspense in Server Side Rendering

During server side rendering Suspense Boundaries allow you to flush your application in smaller chunks by suspending. When a component suspends we schedule a low priority task to render the closest Suspense boundary’s fallback. If the component unsuspends before we flush the fallback then we send down the actual content and throw away the fallback.

React.Suspense during hydration

Suspense boundaries depend on their parent boundaries being hydrated before they can hydrate, but they can hydrate independently from sibling boundaries. Events on a boundary before it is hydrated will cause the boundary to hydrate at a higher priority than neighboring boundaries. Read more

React.startTransition 

React.startTransition(callback)

React.startTransition lets you mark updates inside the provided callback as transitions. This method is designed to be used when React.useTransition is not available.

Note:

Updates in a transition yield to more urgent updates such as clicks.

Updates in a transition will not show a fallback for re-suspended content, allowing the user to continue interacting while rendering the update.

React.startTransition does not provide an isPending flag. To track the pending status of a transition see React.useTransition.

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