Portals? Advanced Guides > Learn React Today

Portals provide a first-class way to render children into a DOM node that exists outside the DOM hierarchy of the parent component.

ReactDOM.createPortal(child, container)

The first argument (child) is any renderable React child, such as an element, string, or fragment. The second argument (container) is a DOM element.

Usage

Normally, when you return an element from a component’s render method, it’s mounted into the DOM as a child of the nearest parent node:

render() {
  // React mounts a new div and renders the children into it
  return (
    <div>
      {this.props.children}
    </div>
  );
}

However, sometimes it’s useful to insert a child into a different location in the DOM:

render() {
  // React does *not* create a new div. It renders the children into `domNode`.
  // `domNode` is any valid DOM node, regardless of its location in the DOM.
  return ReactDOM.createPortal(
    this.props.children,
    domNode
  );
}

A typical use case for portals is when a parent component has an overflow: hidden or z-index style, but you need the child to visually “break out” of its container. For example, dialogs, hovercards, and tooltips.

Note:

When working with portals, remember that managing keyboard focus becomes very important.

For modal dialogs, ensure that everyone can interact with them by following the WAI-ARIA Modal Authoring Practices.

Try it on CodePen

Event Bubbling Through Portals

Even though a portal can be anywhere in the DOM tree, it behaves like a normal React child in every other way. Features like context work exactly the same regardless of whether the child is a portal, as the portal still exists in the React tree regardless of position in the DOM tree.

This includes event bubbling. An event fired from inside a portal will propagate to ancestors in the containing React tree, even if those elements are not ancestors in the DOM tree. Assuming the following HTML structure:

<html>
  <body>
    <div id="app-root"></div>
    <div id="modal-root"></div>
  </body>
</html>

Parent component in #app-root would be able to catch an uncaught, bubbling event from the sibling node #modal-root.

// These two containers are siblings in the DOM
const appRoot = document.getElementById('app-root');
const modalRoot = document.getElementById('modal-root');

class Modal extends React.Component {
  constructor(props) {
    super(props);
    this.el = document.createElement('div');
  }

  componentDidMount() {
    // The portal element is inserted in the DOM tree after
    // the Modal's children are mounted, meaning that children
    // will be mounted on a detached DOM node. If a child
    // component requires to be attached to the DOM tree
    // immediately when mounted, for example to measure a
    // DOM node, or uses 'autoFocus' in a descendant, add
    // state to Modal and only render the children when Modal
    // is inserted in the DOM tree.
    modalRoot.appendChild(this.el);
  }

  componentWillUnmount() {
    modalRoot.removeChild(this.el);
  }

  render() {
    return ReactDOM.createPortal(
      this.props.children,
      this.el,
    );
  }
}

class Parent extends React.Component {
  constructor(props) {
    super(props);
    this.state = {clicks: 0};
    this.handleClick = this.handleClick.bind(this);
  }

  handleClick() {
    // This will fire when the button in Child is clicked,
    // updating Parent's state, even though button
    // is not direct descendant in the DOM.
    this.setState(state => ({
      clicks: state.clicks + 1
    }));
  }

  render() {
    return (
      <div onClick={this.handleClick}>
        <p>Number of clicks: {this.state.clicks}</p>
        <p>
          Open up the browser DevTools
          to observe that the button
          is not a child of the div
          with the onClick handler.
        </p>
        <Modal>
          <Child />
        </Modal>
      </div>
    );
  }
}

function Child() {
  // The click event on this button will bubble up to parent,
  // because there is no 'onClick' attribute defined
  return (
    <div className="modal">
      <button>Click</button>
    </div>
  );
}

ReactDOM.render(<Parent />, appRoot);

Try it on CodePen

Catching an event bubbling up from a portal in a parent component allows the development of more flexible abstractions that are not inherently reliant on portals. For example, if you render a <Modal /> component, the parent can capture its events regardless of whether it’s implemented using portals.

How to Optimizing Performance? Advanced Guides > Learn React Today

Internally, React uses several clever techniques to minimize the number of costly DOM operations required to update the UI. For many applications, using React will lead to a fast user interface without doing much work to specifically optimize for performance. Nevertheless, there are several ways you can speed up your React application.

Use the Production Build

If you’re benchmarking or experiencing performance problems in your React apps, make sure you’re testing with the minified production build.

By default, React includes many helpful warnings. These warnings are very useful in development. However, they make React larger and slower so you should make sure to use the production version when you deploy the app.

If you aren’t sure whether your build process is set up correctly, you can check it by installing React Developer Tools for Chrome. If you visit a site with React in production mode, the icon will have a dark background:

React DevTools on a website with production version of React

If you visit a site with React in development mode, the icon will have a red background:

React DevTools on a website with development version of React

It is expected that you use the development mode when working on your app, and the production mode when deploying your app to the users.

You can find instructions for building your app for production below.

Create React App

If your project is built with Create React App, run:

npm run build

This will create a production build of your app in the build/ folder of your project.

Remember that this is only necessary before deploying to production. For normal development, use npm start.

Single-File Builds

We offer production-ready versions of React and React DOM as single files:

<script src="https://unpkg.com/react@16/umd/react.production.min.js"></script>
<script src="https://unpkg.com/react-dom@16/umd/react-dom.production.min.js"></script>

Remember that only React files ending with .production.min.js are suitable for production.

Brunch

For the most efficient Brunch production build, install the terser-brunch plugin:

# If you use npm
npm install --save-dev terser-brunch

# If you use Yarn
yarn add --dev terser-brunch

Then, to create a production build, add the -p flag to the build command:

brunch build -p

Remember that you only need to do this for production builds. You shouldn’t pass the -p flag or apply this plugin in development, because it will hide useful React warnings and make the builds much slower.

Browserify

For the most efficient Browserify production build, install a few plugins:

# If you use npm
npm install --save-dev envify terser uglifyify 

# If you use Yarn
yarn add --dev envify terser uglifyify 

To create a production build, make sure that you add these transforms (the order matters):

  • The envify transform ensures the right build environment is set. Make it global (-g).
  • The uglifyify transform removes development imports. Make it global too (-g).
  • Finally, the resulting bundle is piped to terser for mangling (read why).

For example:

browserify ./index.js \
  -g [ envify --NODE_ENV production ] \
  -g uglifyify \
  | terser --compress --mangle > ./bundle.js

Remember that you only need to do this for production builds. You shouldn’t apply these plugins in development because they will hide useful React warnings, and make the builds much slower.

Rollup

For the most efficient Rollup production build, install a few plugins:

# If you use npm
npm install --save-dev rollup-plugin-commonjs rollup-plugin-replace rollup-plugin-terser

# If you use Yarn
yarn add --dev rollup-plugin-commonjs rollup-plugin-replace rollup-plugin-terser

To create a production build, make sure that you add these plugins (the order matters):

  • The replace plugin ensures the right build environment is set.
  • The commonjs plugin provides support for CommonJS in Rollup.
  • The terser plugin compresses and mangles the final bundle.
plugins: [
  // ...
  require('rollup-plugin-replace')({
    'process.env.NODE_ENV': JSON.stringify('production')
  }),
  require('rollup-plugin-commonjs')(),
  require('rollup-plugin-terser')(),
  // ...
]

For a complete setup example see this gist.

Remember that you only need to do this for production builds. You shouldn’t apply the terser plugin or the replace plugin with 'production' value in development because they will hide useful React warnings, and make the builds much slower.

webpack

Note:

If you’re using Create React App, please follow the instructions above.
This section is only relevant if you configure webpack directly.

Webpack v4+ will minify your code by default in production mode.

const TerserPlugin = require('terser-webpack-plugin');

module.exports = {
  mode: 'production',
  optimization: {
    minimizer: [new TerserPlugin({ /* additional options here */ })],
  },
};

You can learn more about this in webpack documentation.

Remember that you only need to do this for production builds. You shouldn’t apply TerserPlugin in development because it will hide useful React warnings, and make the builds much slower.

Profiling Components with the Chrome Performance Tab

In the development mode, you can visualize how components mount, update, and unmount, using the performance tools in supported browsers. For example:

To do this in Chrome:

  1. Temporarily disable all Chrome extensions, especially React DevTools. They can significantly skew the results!
  2. Make sure you’re running the application in the development mode.
  3. Open the Chrome DevTools Performance tab and press Record.
  4. Perform the actions you want to profile. Don’t record more than 20 seconds or Chrome might hang.
  5. Stop recording.
  6. React events will be grouped under the User Timing label.

For a more detailed walkthrough, check out this article by Ben Schwarz.

Note that the numbers are relative so components will render faster in production. Still, this should help you realize when unrelated UI gets updated by mistake, and how deep and how often your UI updates occur.

Currently Chrome, Edge, and IE are the only browsers supporting this feature, but we use the standard User Timing API so we expect more browsers to add support for it.

Profiling Components with the DevTools Profiler

react-dom 16.5+ and react-native 0.57+ provide enhanced profiling capabilities in DEV mode with the React DevTools Profiler. An overview of the Profiler can be found in the blog post “Introducing the React Profiler”. A video walkthrough of the profiler is also available on YouTube.

If you haven’t yet installed the React DevTools, you can find them here:

Note

A production profiling bundle of react-dom is also available as react-dom/profiling. Read more about how to use this bundle at fb.me/react-profiling

Virtualize Long Lists

If your application renders long lists of data (hundreds or thousands of rows), we recommended using a technique known as “windowing”. This technique only renders a small subset of your rows at any given time, and can dramatically reduce the time it takes to re-render the components as well as the number of DOM nodes created.

react-window and react-virtualized are popular windowing libraries. They provide several reusable components for displaying lists, grids, and tabular data. You can also create your own windowing component, like Twitter did, if you want something more tailored to your application’s specific use case.

Avoid Reconciliation

React builds and maintains an internal representation of the rendered UI. It includes the React elements you return from your components. This representation lets React avoid creating DOM nodes and accessing existing ones beyond necessity, as that can be slower than operations on JavaScript objects. Sometimes it is referred to as a “virtual DOM”, but it works the same way on React Native.

When a component’s props or state change, React decides whether an actual DOM update is necessary by comparing the newly returned element with the previously rendered one. When they are not equal, React will update the DOM.

Even though React only updates the changed DOM nodes, re-rendering still takes some time. In many cases it’s not a problem, but if the slowdown is noticeable, you can speed all of this up by overriding the lifecycle function shouldComponentUpdate, which is triggered before the re-rendering process starts. The default implementation of this function returns true, leaving React to perform the update:

shouldComponentUpdate(nextProps, nextState) {
  return true;
}

If you know that in some situations your component doesn’t need to update, you can return false from shouldComponentUpdate instead, to skip the whole rendering process, including calling render() on this component and below.

In most cases, instead of writing shouldComponentUpdate() by hand, you can inherit from React.PureComponent. It is equivalent to implementing shouldComponentUpdate() with a shallow comparison of current and previous props and state.

shouldComponentUpdate In Action

Here’s a subtree of components. For each one, SCU indicates what shouldComponentUpdate returned, and vDOMEq indicates whether the rendered React elements were equivalent. Finally, the circle’s color indicates whether the component had to be reconciled or not.

should component update

Since shouldComponentUpdate returned false for the subtree rooted at C2, React did not attempt to render C2, and thus didn’t even have to invoke shouldComponentUpdate on C4 and C5.

For C1 and C3, shouldComponentUpdate returned true, so React had to go down to the leaves and check them. For C6 shouldComponentUpdate returned true, and since the rendered elements weren’t equivalent React had to update the DOM.

The last interesting case is C8. React had to render this component, but since the React elements it returned were equal to the previously rendered ones, it didn’t have to update the DOM.

Note that React only had to do DOM mutations for C6, which was inevitable. For C8, it bailed out by comparing the rendered React elements, and for C2’s subtree and C7, it didn’t even have to compare the elements as we bailed out on shouldComponentUpdate, and render was not called.

Examples

If the only way your component ever changes is when the props.color or the state.count variable changes, you could have shouldComponentUpdate check that:

class CounterButton extends React.Component {
  constructor(props) {
    super(props);
    this.state = {count: 1};
  }

  shouldComponentUpdate(nextProps, nextState) {
    if (this.props.color !== nextProps.color) {
      return true;
    }
    if (this.state.count !== nextState.count) {
      return true;
    }
    return false;
  }

  render() {
    return (
      <button
        color={this.props.color}
        onClick={() => this.setState(state => ({count: state.count + 1}))}>
        Count: {this.state.count}
      </button>
    );
  }
}

In this code, shouldComponentUpdate is just checking if there is any change in props.color or state.count. If those values don’t change, the component doesn’t update. If your component got more complex, you could use a similar pattern of doing a “shallow comparison” between all the fields of props and state to determine if the component should update. This pattern is common enough that React provides a helper to use this logic – just inherit from React.PureComponent. So this code is a simpler way to achieve the same thing:

class CounterButton extends React.PureComponent {
  constructor(props) {
    super(props);
    this.state = {count: 1};
  }

  render() {
    return (
      <button
        color={this.props.color}
        onClick={() => this.setState(state => ({count: state.count + 1}))}>
        Count: {this.state.count}
      </button>
    );
  }
}

Most of the time, you can use React.PureComponent instead of writing your own shouldComponentUpdate. It only does a shallow comparison, so you can’t use it if the props or state may have been mutated in a way that a shallow comparison would miss.

This can be a problem with more complex data structures. For example, let’s say you want a ListOfWords component to render a comma-separated list of words, with a parent WordAdder component that lets you click a button to add a word to the list. This code does not work correctly:

class ListOfWords extends React.PureComponent {
  render() {
    return <div>{this.props.words.join(',')}</div>;
  }
}

class WordAdder extends React.Component {
  constructor(props) {
    super(props);
    this.state = {
      words: ['marklar']
    };
    this.handleClick = this.handleClick.bind(this);
  }

  handleClick() {
    // This section is bad style and causes a bug
    const words = this.state.words;
    words.push('marklar');
    this.setState({words: words});
  }

  render() {
    return (
      <div>
        <button onClick={this.handleClick} />
        <ListOfWords words={this.state.words} />
      </div>
    );
  }
}

The problem is that PureComponent will do a simple comparison between the old and new values of this.props.words. Since this code mutates the words array in the handleClick method of WordAdder, the old and new values of this.props.words will compare as equal, even though the actual words in the array have changed. The ListOfWords will thus not update even though it has new words that should be rendered.

The Power Of Not Mutating Data

The simplest way to avoid this problem is to avoid mutating values that you are using as props or state. For example, the handleClick method above could be rewritten using concat as:

handleClick() {
  this.setState(state => ({
    words: state.words.concat(['marklar'])
  }));
}

ES6 supports a spread syntax for arrays which can make this easier. If you’re using Create React App, this syntax is available by default.

handleClick() {
  this.setState(state => ({
    words: [...state.words, 'marklar'],
  }));
};

You can also rewrite code that mutates objects to avoid mutation, in a similar way. For example, let’s say we have an object named colormap and we want to write a function that changes colormap.right to be 'blue'. We could write:

function updateColorMap(colormap) {
  colormap.right = 'blue';
}

To write this without mutating the original object, we can use Object.assign method:

function updateColorMap(colormap) {
  return Object.assign({}, colormap, {right: 'blue'});
}

updateColorMap now returns a new object, rather than mutating the old one. Object.assign is in ES6 and requires a polyfill.

There is a JavaScript proposal to add object spread properties to make it easier to update objects without mutation as well:

function updateColorMap(colormap) {
  return {...colormap, right: 'blue'};
}

If you’re using Create React App, both Object.assign and the object spread syntax are available by default.

When you deal with deeply nested objects, updating them in an immutable way can feel convoluted. If you run into this problem, check out Immer or immutability-helper. These libraries let you write highly readable code without losing the benefits of immutability.

JSX In Depth? Advanced Guides > Learn React Today

Fundamentally, JSX just provides syntactic sugar for the React.createElement(component, props, ...children) function. The JSX code:

<MyButton color="blue" shadowSize={2}>
  Click Me
</MyButton>

compiles into:

React.createElement(
  MyButton,
  {color: 'blue', shadowSize: 2},
  'Click Me'
)

You can also use the self-closing form of the tag if there are no children. So:

<div className="sidebar" />

compiles into:

React.createElement(
  'div',
  {className: 'sidebar'},
  null
)

If you want to test out how some specific JSX is converted into JavaScript, you can try out the online Babel compiler.

Specifying The React Element Type

The first part of a JSX tag determines the type of the React element.

Capitalized types indicate that the JSX tag is referring to a React component. These tags get compiled into a direct reference to the named variable, so if you use the JSX <Foo /> expression, Foo must be in scope.

React Must Be in Scope

Since JSX compiles into calls to React.createElement, the React library must also always be in scope from your JSX code.

For example, both of the imports are necessary in this code, even though React and CustomButton are not directly referenced from JavaScript:

import React from 'react';
import CustomButton from './CustomButton';

function WarningButton() {
  // return React.createElement(CustomButton, {color: 'red'}, null);
  return <CustomButton color="red" />;
}

If you don’t use a JavaScript bundler and loaded React from a <script> tag, it is already in scope as the React global.

Using Dot Notation for JSX Type

You can also refer to a React component using dot-notation from within JSX. This is convenient if you have a single module that exports many React components. For example, if MyComponents.DatePicker is a component, you can use it directly from JSX with:

import React from 'react';

const MyComponents = {
  DatePicker: function DatePicker(props) {
    return <div>Imagine a {props.color} datepicker here.</div>;
  }
}

function BlueDatePicker() {
  return <MyComponents.DatePicker color="blue" />;
}

User-Defined Components Must Be Capitalized

When an element type starts with a lowercase letter, it refers to a built-in component like <div> or <span> and results in a string 'div' or 'span' passed to React.createElement. Types that start with a capital letter like <Foo /> compile to React.createElement(Foo) and correspond to a component defined or imported in your JavaScript file.

We recommend naming components with a capital letter. If you do have a component that starts with a lowercase letter, assign it to a capitalized variable before using it in JSX.

For example, this code will not run as expected:

import React from 'react';

// Wrong! This is a component and should have been capitalized:
function hello(props) {
  // Correct! This use of <div> is legitimate because div is a valid HTML tag:
  return <div>Hello {props.toWhat}</div>;
}

function HelloWorld() {
  // Wrong! React thinks <hello /> is an HTML tag because it's not capitalized:
  return <hello toWhat="World" />;
}

To fix this, we will rename hello to Hello and use <Hello /> when referring to it:

import React from 'react';

// Correct! This is a component and should be capitalized:
function Hello(props) {
  // Correct! This use of <div> is legitimate because div is a valid HTML tag:
  return <div>Hello {props.toWhat}</div>;
}

function HelloWorld() {
  // Correct! React knows <Hello /> is a component because it's capitalized.
  return <Hello toWhat="World" />;
}

Choosing the Type at Runtime

You cannot use a general expression as the React element type. If you do want to use a general expression to indicate the type of the element, just assign it to a capitalized variable first. This often comes up when you want to render a different component based on a prop:

import React from 'react';
import { PhotoStory, VideoStory } from './stories';

const components = {
  photo: PhotoStory,
  video: VideoStory
};

function Story(props) {
  // Wrong! JSX type can't be an expression.
  return <components[props.storyType] story={props.story} />;
}

To fix this, we will assign the type to a capitalized variable first:

import React from 'react';
import { PhotoStory, VideoStory } from './stories';

const components = {
  photo: PhotoStory,
  video: VideoStory
};

function Story(props) {
  // Correct! JSX type can be a capitalized variable.
  const SpecificStory = components[props.storyType];
  return <SpecificStory story={props.story} />;
}

Props in JSX

There are several different ways to specify props in JSX.

JavaScript Expressions as Props

You can pass any JavaScript expression as a prop, by surrounding it with {}. For example, in this JSX:

<MyComponent foo={1 + 2 + 3 + 4} />

For MyComponent, the value of props.foo will be 10 because the expression 1 + 2 + 3 + 4 gets evaluated.

if statements and for loops are not expressions in JavaScript, so they can’t be used in JSX directly. Instead, you can put these in the surrounding code. For example:

function NumberDescriber(props) {
  let description;
  if (props.number % 2 == 0) {
    description = <strong>even</strong>;
  } else {
    description = <i>odd</i>;
  }
  return <div>{props.number} is an {description} number</div>;
}

You can learn more about conditional rendering and loops in the corresponding sections.

String Literals

You can pass a string literal as a prop. These two JSX expressions are equivalent:

<MyComponent message="hello world" />

<MyComponent message={'hello world'} />

When you pass a string literal, its value is HTML-unescaped. So these two JSX expressions are equivalent:

<MyComponent message="<3" />

<MyComponent message={'<3'} />

This behavior is usually not relevant. It’s only mentioned here for completeness.

Props Default to “True”

If you pass no value for a prop, it defaults to true. These two JSX expressions are equivalent:

<MyTextBox autocomplete />

<MyTextBox autocomplete={true} />

In general, we don’t recommend using this because it can be confused with the ES6 object shorthand {foo} which is short for {foo: foo} rather than {foo: true}. This behavior is just there so that it matches the behavior of HTML.

Spread Attributes

If you already have props as an object, and you want to pass it in JSX, you can use ... as a “spread” operator to pass the whole props object. These two components are equivalent:

function App1() {
  return <Greeting firstName="Ben" lastName="Hector" />;
}

function App2() {
  const props = {firstName: 'Ben', lastName: 'Hector'};
  return <Greeting {...props} />;
}

You can also pick specific props that your component will consume while passing all other props using the spread operator.

const Button = props => {
  const { kind, ...other } = props;
  const className = kind === "primary" ? "PrimaryButton" : "SecondaryButton";
  return <button className={className} {...other} />;
};

const App = () => {
  return (
    <div>
      <Button kind="primary" onClick={() => console.log("clicked!")}>
        Hello World!
      </Button>
    </div>
  );
};

In the example above, the kind prop is safely consumed and is not passed on to the <button> element in the DOM. All other props are passed via the ...other object making this component really flexible. You can see that it passes an onClick and children props.

Spread attributes can be useful but they also make it easy to pass unnecessary props to components that don’t care about them or to pass invalid HTML attributes to the DOM. We recommend using this syntax sparingly.

Children in JSX

In JSX expressions that contain both an opening tag and a closing tag, the content between those tags is passed as a special prop: props.children. There are several different ways to pass children:

String Literals

You can put a string between the opening and closing tags and props.children will just be that string. This is useful for many of the built-in HTML elements. For example:

<MyComponent>Hello world!</MyComponent>

This is valid JSX, and props.children in MyComponent will simply be the string "Hello world!". HTML is unescaped, so you can generally write JSX just like you would write HTML in this way:

<div>This is valid HTML & JSX at the same time.</div>

JSX removes whitespace at the beginning and ending of a line. It also removes blank lines. New lines adjacent to tags are removed; new lines that occur in the middle of string literals are condensed into a single space. So these all render to the same thing:

<div>Hello World</div>

<div>
  Hello World
</div>

<div>
  Hello
  World
</div>

<div>

  Hello World
</div>

JSX Children

You can provide more JSX elements as the children. This is useful for displaying nested components:

<MyContainer>
  <MyFirstComponent />
  <MySecondComponent />
</MyContainer>

You can mix together different types of children, so you can use string literals together with JSX children. This is another way in which JSX is like HTML, so that this is both valid JSX and valid HTML:

<div>
  Here is a list:
  <ul>
    <li>Item 1</li>
    <li>Item 2</li>
  </ul>
</div>

A React component can also return an array of elements:

render() {
  // No need to wrap list items in an extra element!
  return [
    // Don't forget the keys :)
    <li key="A">First item</li>,
    <li key="B">Second item</li>,
    <li key="C">Third item</li>,
  ];
}

JavaScript Expressions as Children

You can pass any JavaScript expression as children, by enclosing it within {}. For example, these expressions are equivalent:

<MyComponent>foo</MyComponent>

<MyComponent>{'foo'}</MyComponent>

This is often useful for rendering a list of JSX expressions of arbitrary length. For example, this renders an HTML list:

function Item(props) {
  return <li>{props.message}</li>;
}

function TodoList() {
  const todos = ['finish doc', 'submit pr', 'nag dan to review'];
  return (
    <ul>
      {todos.map((message) => <Item key={message} message={message} />)}
    </ul>
  );
}

JavaScript expressions can be mixed with other types of children. This is often useful in lieu of string templates:

function Hello(props) {
  return <div>Hello {props.addressee}!</div>;
}

Functions as Children

Normally, JavaScript expressions inserted in JSX will evaluate to a string, a React element, or a list of those things. However, props.children works just like any other prop in that it can pass any sort of data, not just the sorts that React knows how to render. For example, if you have a custom component, you could have it take a callback as props.children:

// Calls the children callback numTimes to produce a repeated component
function Repeat(props) {
  let items = [];
  for (let i = 0; i < props.numTimes; i++) {
    items.push(props.children(i));
  }
  return <div>{items}</div>;
}

function ListOfTenThings() {
  return (
    <Repeat numTimes={10}>
      {(index) => <div key={index}>This is item {index} in the list</div>}
    </Repeat>
  );
}

Children passed to a custom component can be anything, as long as that component transforms them into something React can understand before rendering. This usage is not common, but it works if you want to stretch what JSX is capable of.

Booleans, Null, and Undefined Are Ignored

falsenullundefined, and true are valid children. They simply don’t render. These JSX expressions will all render to the same thing:

<div />

<div></div>

<div>{false}</div>

<div>{null}</div>

<div>{undefined}</div>

<div>{true}</div>

This can be useful to conditionally render React elements. This JSX renders the <Header /> component only if showHeader is true:

<div>
  {showHeader && <Header />}
  <Content />
</div>

One caveat is that some “falsy” values, such as the 0 number, are still rendered by React. For example, this code will not behave as you might expect because 0 will be printed when props.messages is an empty array:

<div>
  {props.messages.length &&
    <MessageList messages={props.messages} />
  }
</div>

To fix this, make sure that the expression before && is always boolean:

<div>
  {props.messages.length > 0 &&
    <MessageList messages={props.messages} />
  }
</div>

Conversely, if you want a value like falsetruenull, or undefined to appear in the output, you have to convert it to a string first:

<div>
  My JavaScript variable is {String(myVariable)}.
</div>

Higher-Order Components | HOC | Advanced Guides > Learn React Today

A higher-order component (HOC) is an advanced technique in React for reusing component logic. HOCs are not part of the React API, per se. They are a pattern that emerges from React’s compositional nature.

Concretely, a higher-order component is a function that takes a component and returns a new component.

const EnhancedComponent = higherOrderComponent(WrappedComponent);

Whereas a component transforms props into UI, a higher-order component transforms a component into another component.

HOCs are common in third-party React libraries, such as Redux’s connect and Relay’s createFragmentContainer.

In this document, we’ll discuss why higher-order components are useful, and how to write your own.

Use HOCs For Cross-Cutting Concerns

Note

We previously recommended mixins as a way to handle cross-cutting concerns. We’ve since realized that mixins create more trouble than they are worth. Read more about why we’ve moved away from mixins and how you can transition your existing components.

Components are the primary unit of code reuse in React. However, you’ll find that some patterns aren’t a straightforward fit for traditional components.

For example, say you have a CommentList component that subscribes to an external data source to render a list of comments:

class CommentList extends React.Component {
  constructor(props) {
    super(props);
    this.handleChange = this.handleChange.bind(this);
    this.state = {
      // "DataSource" is some global data source
      comments: DataSource.getComments()
    };
  }

  componentDidMount() {
    // Subscribe to changes
    DataSource.addChangeListener(this.handleChange);
  }

  componentWillUnmount() {
    // Clean up listener
    DataSource.removeChangeListener(this.handleChange);
  }

  handleChange() {
    // Update component state whenever the data source changes
    this.setState({
      comments: DataSource.getComments()
    });
  }

  render() {
    return (
      <div>
        {this.state.comments.map((comment) => (
          <Comment comment={comment} key={comment.id} />
        ))}
      </div>
    );
  }
}

Later, you write a component for subscribing to a single blog post, which follows a similar pattern:

class BlogPost extends React.Component {
  constructor(props) {
    super(props);
    this.handleChange = this.handleChange.bind(this);
    this.state = {
      blogPost: DataSource.getBlogPost(props.id)
    };
  }

  componentDidMount() {
    DataSource.addChangeListener(this.handleChange);
  }

  componentWillUnmount() {
    DataSource.removeChangeListener(this.handleChange);
  }

  handleChange() {
    this.setState({
      blogPost: DataSource.getBlogPost(this.props.id)
    });
  }

  render() {
    return <TextBlock text={this.state.blogPost} />;
  }
}

CommentList and BlogPost aren’t identical — they call different methods on DataSource, and they render different output. But much of their implementation is the same:

  • On mount, add a change listener to DataSource.
  • Inside the listener, call setState whenever the data source changes.
  • On unmount, remove the change listener.

You can imagine that in a large app, this same pattern of subscribing to DataSource and calling setState will occur over and over again. We want an abstraction that allows us to define this logic in a single place and share it across many components. This is where higher-order components excel.

We can write a function that creates components, like CommentList and BlogPost, that subscribe to DataSource. The function will accept as one of its arguments a child component that receives the subscribed data as a prop. Let’s call the function withSubscription:

const CommentListWithSubscription = withSubscription(
  CommentList,
  (DataSource) => DataSource.getComments()
);

const BlogPostWithSubscription = withSubscription(
  BlogPost,
  (DataSource, props) => DataSource.getBlogPost(props.id)
);

The first parameter is the wrapped component. The second parameter retrieves the data we’re interested in, given a DataSource and the current props.

When CommentListWithSubscription and BlogPostWithSubscription are rendered, CommentList and BlogPost will be passed a data prop with the most current data retrieved from DataSource:

// This function takes a component...
function withSubscription(WrappedComponent, selectData) {
  // ...and returns another component...
  return class extends React.Component {
    constructor(props) {
      super(props);
      this.handleChange = this.handleChange.bind(this);
      this.state = {
        data: selectData(DataSource, props)
      };
    }

    componentDidMount() {
      // ... that takes care of the subscription...
      DataSource.addChangeListener(this.handleChange);
    }

    componentWillUnmount() {
      DataSource.removeChangeListener(this.handleChange);
    }

    handleChange() {
      this.setState({
        data: selectData(DataSource, this.props)
      });
    }

    render() {
      // ... and renders the wrapped component with the fresh data!
      // Notice that we pass through any additional props
      return <WrappedComponent data={this.state.data} {...this.props} />;
    }
  };
}

Note that a HOC doesn’t modify the input component, nor does it use inheritance to copy its behavior. Rather, a HOC composes the original component by wrapping it in a container component. A HOC is a pure function with zero side-effects.

And that’s it! The wrapped component receives all the props of the container, along with a new prop, data, which it uses to render its output. The HOC isn’t concerned with how or why the data is used, and the wrapped component isn’t concerned with where the data came from.

Because withSubscription is a normal function, you can add as many or as few arguments as you like. For example, you may want to make the name of the data prop configurable, to further isolate the HOC from the wrapped component. Or you could accept an argument that configures shouldComponentUpdate, or one that configures the data source. These are all possible because the HOC has full control over how the component is defined.

Like components, the contract between withSubscription and the wrapped component is entirely props-based. This makes it easy to swap one HOC for a different one, as long as they provide the same props to the wrapped component. This may be useful if you change data-fetching libraries, for example.

Don’t Mutate the Original Component. Use Composition.

Resist the temptation to modify a component’s prototype (or otherwise mutate it) inside a HOC.

function logProps(InputComponent) {
  InputComponent.prototype.componentWillReceiveProps = function(nextProps) {
    console.log('Current props: ', this.props);
    console.log('Next props: ', nextProps);
  };
  // The fact that we're returning the original input is a hint that it has
  // been mutated.
  return InputComponent;
}

// EnhancedComponent will log whenever props are received
const EnhancedComponent = logProps(InputComponent);

There are a few problems with this. One is that the input component cannot be reused separately from the enhanced component. More crucially, if you apply another HOC to EnhancedComponent that also mutates componentWillReceiveProps, the first HOC’s functionality will be overridden! This HOC also won’t work with function components, which do not have lifecycle methods.

Mutating HOCs are a leaky abstraction—the consumer must know how they are implemented in order to avoid conflicts with other HOCs.

Instead of mutation, HOCs should use composition, by wrapping the input component in a container component:

function logProps(WrappedComponent) {
  return class extends React.Component {
    componentWillReceiveProps(nextProps) {
      console.log('Current props: ', this.props);
      console.log('Next props: ', nextProps);
    }
    render() {
      // Wraps the input component in a container, without mutating it. Good!
      return <WrappedComponent {...this.props} />;
    }
  }
}

This HOC has the same functionality as the mutating version while avoiding the potential for clashes. It works equally well with class and function components. And because it’s a pure function, it’s composable with other HOCs, or even with itself.

You may have noticed similarities between HOCs and a pattern called container components. Container components are part of a strategy of separating responsibility between high-level and low-level concerns. Containers manage things like subscriptions and state, and pass props to components that handle things like rendering UI. HOCs use containers as part of their implementation. You can think of HOCs as parameterized container component definitions.

Convention: Pass Unrelated Props Through to the Wrapped Component

HOCs add features to a component. They shouldn’t drastically alter its contract. It’s expected that the component returned from a HOC has a similar interface to the wrapped component.

HOCs should pass through props that are unrelated to its specific concern. Most HOCs contain a render method that looks something like this:

render() {
  // Filter out extra props that are specific to this HOC and shouldn't be
  // passed through
  const { extraProp, ...passThroughProps } = this.props;

  // Inject props into the wrapped component. These are usually state values or
  // instance methods.
  const injectedProp = someStateOrInstanceMethod;

  // Pass props to wrapped component
  return (
    <WrappedComponent
      injectedProp={injectedProp}
      {...passThroughProps}
    />
  );
}

This convention helps ensure that HOCs are as flexible and reusable as possible.

Convention: Maximizing Composability

Not all HOCs look the same. Sometimes they accept only a single argument, the wrapped component:

const NavbarWithRouter = withRouter(Navbar);

Usually, HOCs accept additional arguments. In this example from Relay, a config object is used to specify a component’s data dependencies:

const CommentWithRelay = Relay.createContainer(Comment, config);

The most common signature for HOCs looks like this:

// React Redux's `connect`
const ConnectedComment = connect(commentSelector, commentActions)(CommentList);

What?! If you break it apart, it’s easier to see what’s going on.

// connect is a function that returns another function
const enhance = connect(commentListSelector, commentListActions);
// The returned function is a HOC, which returns a component that is connected
// to the Redux store
const ConnectedComment = enhance(CommentList);

In other words, connect is a higher-order function that returns a higher-order component!

This form may seem confusing or unnecessary, but it has a useful property. Single-argument HOCs like the one returned by the connect function have the signature Component => Component. Functions whose output type is the same as its input type are really easy to compose together.

// Instead of doing this...
const EnhancedComponent = withRouter(connect(commentSelector)(WrappedComponent))

// ... you can use a function composition utility
// compose(f, g, h) is the same as (...args) => f(g(h(...args)))
const enhance = compose(
  // These are both single-argument HOCs
  withRouter,
  connect(commentSelector)
)
const EnhancedComponent = enhance(WrappedComponent)

(This same property also allows connect and other enhancer-style HOCs to be used as decorators, an experimental JavaScript proposal.)

The compose utility function is provided by many third-party libraries including lodash (as lodash.flowRight), Redux, and Ramda.

Convention: Wrap the Display Name for Easy Debugging

The container components created by HOCs show up in the React Developer Tools like any other component. To ease debugging, choose a display name that communicates that it’s the result of a HOC.

The most common technique is to wrap the display name of the wrapped component. So if your higher-order component is named withSubscription, and the wrapped component’s display name is CommentList, use the display name WithSubscription(CommentList):

function withSubscription(WrappedComponent) {
  class WithSubscription extends React.Component {/* ... */}
  WithSubscription.displayName = `WithSubscription(${getDisplayName(WrappedComponent)})`;
  return WithSubscription;
}

function getDisplayName(WrappedComponent) {
  return WrappedComponent.displayName || WrappedComponent.name || 'Component';
}

Caveats

Higher-order components come with a few caveats that aren’t immediately obvious if you’re new to React.

Don’t Use HOCs Inside the render Method

React’s diffing algorithm (called reconciliation) uses component identity to determine whether it should update the existing subtree or throw it away and mount a new one. If the component returned from render is identical (===) to the component from the previous render, React recursively updates the subtree by diffing it with the new one. If they’re not equal, the previous subtree is unmounted completely.

Normally, you shouldn’t need to think about this. But it matters for HOCs because it means you can’t apply a HOC to a component within the render method of a component:

render() {
  // A new version of EnhancedComponent is created on every render
  // EnhancedComponent1 !== EnhancedComponent2
  const EnhancedComponent = enhance(MyComponent);
  // That causes the entire subtree to unmount/remount each time!
  return <EnhancedComponent />;
}

The problem here isn’t just about performance — remounting a component causes the state of that component and all of its children to be lost.

Instead, apply HOCs outside the component definition so that the resulting component is created only once. Then, its identity will be consistent across renders. This is usually what you want, anyway.

In those rare cases where you need to apply a HOC dynamically, you can also do it inside a component’s lifecycle methods or its constructor.

Static Methods Must Be Copied Over

Sometimes it’s useful to define a static method on a React component. For example, Relay containers expose a static method getFragment to facilitate the composition of GraphQL fragments.

When you apply a HOC to a component, though, the original component is wrapped with a container component. That means the new component does not have any of the static methods of the original component.

// Define a static method
WrappedComponent.staticMethod = function() {/*...*/}
// Now apply a HOC
const EnhancedComponent = enhance(WrappedComponent);

// The enhanced component has no static method
typeof EnhancedComponent.staticMethod === 'undefined' // true

To solve this, you could copy the methods onto the container before returning it:

function enhance(WrappedComponent) {
  class Enhance extends React.Component {/*...*/}
  // Must know exactly which method(s) to copy :(
  Enhance.staticMethod = WrappedComponent.staticMethod;
  return Enhance;
}

However, this requires you to know exactly which methods need to be copied. You can use hoist-non-react-statics to automatically copy all non-React static methods:

import hoistNonReactStatic from 'hoist-non-react-statics';
function enhance(WrappedComponent) {
  class Enhance extends React.Component {/*...*/}
  hoistNonReactStatic(Enhance, WrappedComponent);
  return Enhance;
}

Another possible solution is to export the static method separately from the component itself.

// Instead of...
MyComponent.someFunction = someFunction;
export default MyComponent;

// ...export the method separately...
export { someFunction };

// ...and in the consuming module, import both
import MyComponent, { someFunction } from './MyComponent.js';

Refs Aren’t Passed Through

While the convention for higher-order components is to pass through all props to the wrapped component, this does not work for refs. That’s because ref is not really a prop — like key, it’s handled specially by React. If you add a ref to an element whose component is the result of a HOC, the ref refers to an instance of the outermost container component, not the wrapped component.

Fragments? Advanced Guides > Learn React Today

A common pattern in React is for a component to return multiple elements. Fragments let you group a list of children without adding extra nodes to the DOM.

render() {
  return (
    <React.Fragment>
      <ChildA />
      <ChildB />
      <ChildC />
    </React.Fragment>
  );
}

There is also a new short syntax for declaring them.

Motivation

A common pattern is for a component to return a list of children. Take this example React snippet:

class Table extends React.Component {
  render() {
    return (
      <table>
        <tr>
          <Columns />
        </tr>
      </table>
    );
  }
}

<Columns /> would need to return multiple <td> elements in order for the rendered HTML to be valid. If a parent div was used inside the render() of <Columns />, then the resulting HTML will be invalid.

class Columns extends React.Component {
  render() {
    return (
      <div>
        <td>Hello</td>
        <td>World</td>
      </div>
    );
  }
}

results in a <Table /> output of:

<table>
  <tr>
    <div>
      <td>Hello</td>
      <td>World</td>
    </div>
  </tr>
</table>

Fragments solve this problem.

Usage

class Columns extends React.Component {
  render() {
    return (
      <React.Fragment>
        <td>Hello</td>
        <td>World</td>
      </React.Fragment>
    );
  }
}

which results in a correct <Table /> output of:

<table>
  <tr>
    <td>Hello</td>
    <td>World</td>
  </tr>
</table>

Short Syntax

There is a new, shorter syntax you can use for declaring fragments. It looks like empty tags:

class Columns extends React.Component {
  render() {
    return (
      <>
        <td>Hello</td>
        <td>World</td>
      </>
    );
  }
}

You can use <></> the same way you’d use any other element except that it doesn’t support keys or attributes.

Keyed Fragments

Fragments declared with the explicit <React.Fragment> syntax may have keys. A use case for this is mapping a collection to an array of fragments — for example, to create a description list:

function Glossary(props) {
  return (
    <dl>
      {props.items.map(item => (
        // Without the `key`, React will fire a key warning
        <React.Fragment key={item.id}>
          <dt>{item.term}</dt>
          <dd>{item.description}</dd>
        </React.Fragment>
      ))}
    </dl>
  );
}

key is the only attribute that can be passed to Fragment. In the future, we may add support for additional attributes, such as event handlers.

Live Demo

You can try out the new JSX fragment syntax with this CodePen.