This reference guide documents the SyntheticEvent wrapper that forms part of React’s Event System. See the Handling Events guide to learn more.

Overview

Your event handlers will be passed instances of SyntheticEvent , a cross-browser wrapper around the browser’s native event. It has the same interface as the browser’s native event, including stopPropagation() and preventDefault() , except the events work identically across all browsers.

If you find that you need the underlying browser event for some reason, simply use the nativeEvent attribute to get it. The synthetic events are different from, and do not map directly to, the browser’s native events. For example in onMouseLeave event.nativeEvent will point to a mouseout event. The specific mapping is not part of the public API and may change at any time. Every SyntheticEvent object has the following attributes:

boolean bubbles
boolean cancelable
DOMEventTarget currentTarget
boolean defaultPrevented
number eventPhase
boolean isTrusted
DOMEvent nativeEvent
void preventDefault()
boolean isDefaultPrevented()
void stopPropagation()
boolean isPropagationStopped()
void persist()
DOMEventTarget target
number timeStamp
string type

Note:

As of v17, e.persist() doesn’t do anything because the SyntheticEvent is no longer pooled.

Note:

As of v0.14, returning false from an event handler will no longer stop event propagation. Instead, e.stopPropagation() or e.preventDefault() should be triggered manually, as appropriate.

Supported Events

React normalizes events so that they have consistent properties across different browsers.

The event handlers below are triggered by an event in the bubbling phase. To register an event handler for the capture phase, append Capture to the event name; for example, instead of using onClick , you would use onClickCapture to handle the click event in the capture phase.

• Clipboard Events


• Composition Events


• Keyboard Events


• Focus Events


• Form Events


• Generic Events


• Mouse Events


• Pointer Events


• Selection Events


• Touch Events


• UI Events


• Wheel Events


• Media Events


• Image Events


• Animation Events


• Transition Events


• Other Events

Reference

Clipboard Events

Event names:

onCopy onCut onPaste

Properties:

DOMDataTransfer clipboardData

Composition Events

Event names:

onCompositionEnd onCompositionStart onCompositionUpdate

Properties:

string data

Keyboard Events

Event names:

onKeyDown onKeyPress onKeyUp

Properties:

boolean altKey
number charCode
boolean ctrlKey
boolean getModifierState(key)
string key
number keyCode
string locale
number location
boolean metaKey
boolean repeat
boolean shiftKey
number which

The key property can take any of the values documented in the DOM Level 3 Events spec.

Focus Events

Event names:

onFocus onBlur

These focus events work on all elements in the React DOM, not just form elements.

Properties:

DOMEventTarget relatedTarget

onFocus

The onFocus event is called when the element (or some element inside of it) receives focus. For example, it’s called when the user clicks on a text input.

function Example() {
  return (
    <input
      onFocus={(e) => {
        console.log('Focused on input');
      }}
      placeholder="onFocus is triggered when you click this input."
    />
  )
}

onBlur

The onBlur event handler is called when focus has left the element (or left some element inside of it). For example, it’s called when the user clicks outside of a focused text input.

function Example() {
  return (
    <input
      onBlur={(e) => {
        console.log('Triggered because this input lost focus');
      }}
      placeholder="onBlur is triggered when you click this input and then you click outside of it."
    />
  )
}

Detecting Focus Entering and Leaving

You can use the currentTarget and relatedTarget to differentiate if the focusing or blurring events originated from outside of the parent element. Here is a demo you can copy and paste that shows how to detect focusing a child, focusing the element itself, and focus entering or leaving the whole subtree.

function Example() {
  return (
    <div
      tabIndex={1}
      onFocus={(e) => {
        if (e.currentTarget === e.target) {
          console.log('focused self');
        } else {
          console.log('focused child', e.target);
        }
        if (!e.currentTarget.contains(e.relatedTarget)) {
          // Not triggered when swapping focus between children
          console.log('focus entered self');
        }
      }}
      onBlur={(e) => {
        if (e.currentTarget === e.target) {
          console.log('unfocused self');
        } else {
          console.log('unfocused child', e.target);
        }
        if (!e.currentTarget.contains(e.relatedTarget)) {
          // Not triggered when swapping focus between children
          console.log('focus left self');
        }
      }}
    >
      <input id="1" />
      <input id="2" />
    </div>
  );
}

Form Events

Event names:

onChange onInput onInvalid onReset onSubmit 

For more information about the onChange event, see Forms.

Generic Events

Event names:

onError onLoad

Mouse Events

Event names:

onClick onContextMenu onDoubleClick onDrag onDragEnd onDragEnter onDragExit
onDragLeave onDragOver onDragStart onDrop onMouseDown onMouseEnter onMouseLeave
onMouseMove onMouseOut onMouseOver onMouseUp

The onMouseEnter and onMouseLeave events propagate from the element being left to the one being entered instead of ordinary bubbling and do not have a capture phase.

Properties:

boolean altKey
number button
number buttons
number clientX
number clientY
boolean ctrlKey
boolean getModifierState(key)
boolean metaKey
number pageX
number pageY
DOMEventTarget relatedTarget
number screenX
number screenY
boolean shiftKey

Pointer Events

Event names:

onPointerDown onPointerMove onPointerUp onPointerCancel onGotPointerCapture
onLostPointerCapture onPointerEnter onPointerLeave onPointerOver onPointerOut

The onPointerEnter and onPointerLeave events propagate from the element being left to the one being entered instead of ordinary bubbling and do not have a capture phase.

Properties:

As defined in the W3 spec, pointer events extend Mouse Events with the following properties:

number pointerId
number width
number height
number pressure
number tangentialPressure
number tiltX
number tiltY
number twist
string pointerType
boolean isPrimary

A note on cross-browser support:

Pointer events are not yet supported in every browser (at the time of writing this article, supported browsers include: Chrome, Firefox, Edge, and Internet Explorer). React deliberately does not polyfill support for other browsers because a standard-conform polyfill would significantly increase the bundle size of react-dom .

If your application requires pointer events, we recommend adding a third party pointer event polyfill.

Selection Events

Event names:

onSelect

Touch Events

Event names:

onTouchCancel onTouchEnd onTouchMove onTouchStart

Properties:

boolean altKey
DOMTouchList changedTouches
boolean ctrlKey
boolean getModifierState(key)
boolean metaKey
boolean shiftKey
DOMTouchList targetTouches
DOMTouchList touches

UI Events

Event names:

onScroll

Note

Starting with React 17, the onScroll event does not bubble in React. This matches the browser behavior and prevents the confusion when a nested scrollable element fires events on a distant parent.

Properties:

number detail
DOMAbstractView view

Wheel Events

Event names:

onWheel

Properties:

number deltaMode
number deltaX
number deltaY
number deltaZ

Media Events

Event names:

onAbort onCanPlay onCanPlayThrough onDurationChange onEmptied onEncrypted
onEnded onError onLoadedData onLoadedMetadata onLoadStart onPause onPlay
onPlaying onProgress onRateChange onSeeked onSeeking onStalled onSuspend
onTimeUpdate onVolumeChange onWaiting

Image Events

Event names:

onLoad onError

Animation Events

Event names:

onAnimationStart onAnimationEnd onAnimationIteration

Properties:

string animationName
string pseudoElement
float elapsedTime

Transition Events

Event names:

onTransitionEnd

Properties:

string propertyName
string pseudoElement
float elapsedTime

Other Events

Event names:

onToggle

Importing

import ReactTestUtils from 'react-dom/test-utils'; // ES6
var ReactTestUtils = require('react-dom/test-utils'); // ES5 with npm

Overview

ReactTestUtils makes it easy to test React components in the testing framework of your choice. At Facebook we use Jest for painless JavaScript testing. Learn how to get started with Jest through the Jest website’s React Tutorial.

Note:

We recommend using React Testing Library which is designed to enable and encourage writing tests that use your components as the end users do.

For React versions <= 16, the Enzyme library makes it easy to assert, manipulate, and traverse your React Components’ output.

• act()


• mockComponent()


• isElement()


• isElementOfType()


• isDOMComponent()


• isCompositeComponent()


• isCompositeComponentWithType()


• findAllInRenderedTree()


• scryRenderedDOMComponentsWithClass()


• findRenderedDOMComponentWithClass()


• scryRenderedDOMComponentsWithTag()


• findRenderedDOMComponentWithTag()


• scryRenderedComponentsWithType()


• findRenderedComponentWithType()


• renderIntoDocument()


• Simulate

Reference

act()

To prepare a component for assertions, wrap the code rendering it and performing updates inside an act() call. This makes your test run closer to how React works in the browser.

Note

If you use react-test-renderer , it also provides an act export that behaves the same way.

For example, let’s say we have this Counter component:

class Counter extends React.Component {
  constructor(props) {
    super(props);
    this.state = {count: 0};
    this.handleClick = this.handleClick.bind(this);
  }
  componentDidMount() {
    document.title = `You clicked ${this.state.count} times`;
  }
  componentDidUpdate() {
    document.title = `You clicked ${this.state.count} times`;
  }
  handleClick() {
    this.setState(state => ({
      count: state.count + 1,
    }));
  }
  render() {
    return (
      <div>
        <p>You clicked {this.state.count} times</p>
        <button onClick={this.handleClick}>
          Click me
        </button>
      </div>
    );
  }
}

Here is how we can test it:

import React from 'react';
import ReactDOM from 'react-dom/client';
import { act } from 'react-dom/test-utils';import Counter from './Counter';

let container;

beforeEach(() => {
  container = document.createElement('div');
  document.body.appendChild(container);
});

afterEach(() => {
  document.body.removeChild(container);
  container = null;
});

it('can render and update a counter', () => {
  // Test first render and componentDidMount
  act(() => {    ReactDOM.createRoot(container).render(<Counter />);  });  const button = container.querySelector('button');
  const label = container.querySelector('p');
  expect(label.textContent).toBe('You clicked 0 times');
  expect(document.title).toBe('You clicked 0 times');

  // Test second render and componentDidUpdate
  act(() => {    button.dispatchEvent(new MouseEvent('click', {bubbles: true}));  });  expect(label.textContent).toBe('You clicked 1 times');
  expect(document.title).toBe('You clicked 1 times');
});

• Don’t forget that dispatching DOM events only works when the DOM container is added to the document . You can use a library like React Testing Library to reduce the boilerplate code.


• The recipes document contains more details on how act() behaves, with examples and usage.

mockComponent()

mockComponent(
  componentClass,
  [mockTagName]
)

Pass a mocked component module to this method to augment it with useful methods that allow it to be used as a dummy React component. Instead of rendering as usual, the component will become a simple <div> (or other tag if mockTagName is provided) containing any provided children.

Note:

mockComponent() is a legacy API. We recommend using jest.mock() instead.

isElement()

isElement(element)

Returns true if element is any React element.

isElementOfType()

isElementOfType(
  element,
  componentClass
)

Returns true if element is a React element whose type is of a React componentClass .

isDOMComponent()

isDOMComponent(instance)

Returns true if instance is a DOM component (such as a <div> or <span> ).

isCompositeComponent()

isCompositeComponent(instance)

Returns true if instance is a user-defined component, such as a class or a function.

isCompositeComponentWithType()

isCompositeComponentWithType(
  instance,
  componentClass
)

Returns true if instance is a component whose type is of a React componentClass .

findAllInRenderedTree()

findAllInRenderedTree(
  tree,
  test
)

Traverse all components in tree and accumulate all components where test(component) is true . This is not that useful on its own, but it’s used as a primitive for other test utils.

scryRenderedDOMComponentsWithClass()

scryRenderedDOMComponentsWithClass(
  tree,
  className
)

Finds all DOM elements of components in the rendered tree that are DOM components with the class name matching className .

findRenderedDOMComponentWithClass()

findRenderedDOMComponentWithClass(
  tree,
  className
)

Like scryRenderedDOMComponentsWithClass() but expects there to be one result, and returns that one result, or throws exception if there is any other number of matches besides one.

scryRenderedDOMComponentsWithTag()

scryRenderedDOMComponentsWithTag(
  tree,
  tagName
)

Finds all DOM elements of components in the rendered tree that are DOM components with the tag name matching tagName .

findRenderedDOMComponentWithTag()

findRenderedDOMComponentWithTag(
  tree,
  tagName
)

Like scryRenderedDOMComponentsWithTag() but expects there to be one result, and returns that one result, or throws exception if there is any other number of matches besides one.

scryRenderedComponentsWithType()

scryRenderedComponentsWithType(
  tree,
  componentClass
)

Finds all instances of components with type equal to componentClass .

findRenderedComponentWithType()

findRenderedComponentWithType(
  tree,
  componentClass
)

Same as scryRenderedComponentsWithType() but expects there to be one result and returns that one result, or throws exception if there is any other number of matches besides one.

renderIntoDocument()

renderIntoDocument(element)

Render a React element into a detached DOM node in the document. This function requires a DOM. It is effectively equivalent to:

const domContainer = document.createElement('div');
ReactDOM.createRoot(domContainer).render(element);

Note:

You will need to have window , window.document and window.document.createElement globally available before you import React . Otherwise React will think it can’t access the DOM and methods like setState won’t work.

Other Utilities

Simulate

Simulate.{eventName}(
  element,
  [eventData]
)

Simulate an event dispatch on a DOM node with optional eventData event data.

Simulate has a method for every event that React understands.

Clicking an element

// <button ref={(node) => this.button = node}>...</button>
const node = this.button;
ReactTestUtils.Simulate.click(node);

Changing the value of an input field and then pressing ENTER.

// <input ref={(node) => this.textInput = node} />
const node = this.textInput;
node.value = 'giraffe';
ReactTestUtils.Simulate.change(node);
ReactTestUtils.Simulate.keyDown(node, {key: "Enter", keyCode: 13, which: 13});

Note

You will have to provide any event property that you’re using in your component (e.g. keyCode, which, etc…) as React is not creating any of these for you.

Importing

import TestRenderer from 'react-test-renderer'; // ES6
const TestRenderer = require('react-test-renderer'); // ES5 with npm

Overview

This package provides a React renderer that can be used to render React components to pure JavaScript objects, without depending on the DOM or a native mobile environment.

Essentially, this package makes it easy to grab a snapshot of the platform view hierarchy (similar to a DOM tree) rendered by a React DOM or React Native component without using a browser or jsdom.

Example:

import TestRenderer from 'react-test-renderer';

function Link(props) {
  return <a href={props.page}>{props.children}</a>;
}

const testRenderer = TestRenderer.create(
  <Link page="https://www.facebook.com/">Facebook</Link>
);

console.log(testRenderer.toJSON());
// { type: 'a',
//   props: { href: 'https://www.facebook.com/' },
//   children: [ 'Facebook' ] }

You can use Jest’s snapshot testing feature to automatically save a copy of the JSON tree to a file and check in your tests that it hasn’t changed: Learn more about it.

You can also traverse the output to find specific nodes and make assertions about them.

import TestRenderer from 'react-test-renderer';

function MyComponent() {
  return (
    <div>
      <SubComponent foo="bar" />
      <p className="my">Hello</p>
    </div>
  )
}

function SubComponent() {
  return (
    <p className="sub">Sub</p>
  );
}

const testRenderer = TestRenderer.create(<MyComponent />);
const testInstance = testRenderer.root;

expect(testInstance.findByType(SubComponent).props.foo).toBe('bar');
expect(testInstance.findByProps({className: "sub"}).children).toEqual(['Sub']);

TestRenderer

• TestRenderer.create()


• TestRenderer.act()

TestRenderer instance

• testRenderer.toJSON()


• testRenderer.toTree()


• testRenderer.update()


• testRenderer.unmount()


• testRenderer.getInstance()


• testRenderer.root

TestInstance

• testInstance.find()


• testInstance.findByType()


• testInstance.findByProps()


• testInstance.findAll()


• testInstance.findAllByType()


• testInstance.findAllByProps()


• testInstance.instance


• testInstance.type


• testInstance.props


• testInstance.parent


• testInstance.children

Reference

TestRenderer.create()

TestRenderer.create(element, options);

Create a TestRenderer instance with the passed React element. It doesn’t use the real DOM, but it still fully renders the component tree into memory so you can make assertions about it. Returns a TestRenderer instance.

TestRenderer.act()

TestRenderer.act(callback);

Similar to the act() helper from react-dom/test-utils , TestRenderer.act prepares a component for assertions. Use this version of act() to wrap calls to TestRenderer.create and testRenderer.update .

import {create, act} from 'react-test-renderer';
import App from './app.js'; // The component being tested

// render the component
let root; 
act(() => {
  root = create(<App value={1}/>)
});

// make assertions on root 
expect(root.toJSON()).toMatchSnapshot();

// update with some different props
act(() => {
  root.update(<App value={2}/>);
})

// make assertions on root 
expect(root.toJSON()).toMatchSnapshot();

testRenderer.toJSON()

testRenderer.toJSON()

Return an object representing the rendered tree. This tree only contains the platform-specific nodes like <div> or <View> and their props, but doesn’t contain any user-written components. This is handy for snapshot testing.

testRenderer.toTree()

testRenderer.toTree()

Return an object representing the rendered tree. The representation is more detailed than the one provided by toJSON() , and includes the user-written components. You probably don’t need this method unless you’re writing your own assertion library on top of the test renderer.

testRenderer.update()

testRenderer.update(element)

Re-render the in-memory tree with a new root element. This simulates a React update at the root. If the new element has the same type and key as the previous element, the tree will be updated; otherwise, it will re-mount a new tree.

testRenderer.unmount()

testRenderer.unmount()

Unmount the in-memory tree, triggering the appropriate lifecycle events.

testRenderer.getInstance()

testRenderer.getInstance()

Return the instance corresponding to the root element, if available. This will not work if the root element is a function component because they don’t have instances.

testRenderer.root

testRenderer.root

Returns the root “test instance” object that is useful for making assertions about specific nodes in the tree. You can use it to find other “test instances” deeper below.

testInstance.find()

testInstance.find(test)

Find a single descendant test instance for which test(testInstance) returns true . If test(testInstance) does not return true for exactly one test instance, it will throw an error.

testInstance.findByType()

testInstance.findByType(type)

Find a single descendant test instance with the provided type . If there is not exactly one test instance with the provided type , it will throw an error.

testInstance.findByProps()

testInstance.findByProps(props)

Find a single descendant test instance with the provided props . If there is not exactly one test instance with the provided props , it will throw an error.

testInstance.findAll()

testInstance.findAll(test)

Find all descendant test instances for which test(testInstance) returns true .

testInstance.findAllByType()

testInstance.findAllByType(type)

Find all descendant test instances with the provided type .

testInstance.findAllByProps()

testInstance.findAllByProps(props)

Find all descendant test instances with the provided props .

testInstance.instance

testInstance.instance

The component instance corresponding to this test instance. It is only available for class components, as function components don’t have instances. It matches the this value inside the given component.

testInstance.type

testInstance.type

The component type corresponding to this test instance. For example, a <Button /> component has a type of Button .

testInstance.props

testInstance.props

The props corresponding to this test instance. For example, a <Button size="small" /> component has {size: 'small'} as props.

testInstance.parent

testInstance.parent

The parent test instance of this test instance.

testInstance.children

testInstance.children

The children test instances of this test instance.

Ideas

You can pass createNodeMock function to TestRenderer.create as the option, which allows for custom mock refs.
createNodeMock accepts the current element and should return a mock ref object.
This is useful when you test a component that relies on refs.

import TestRenderer from 'react-test-renderer';

class MyComponent extends React.Component {
  constructor(props) {
    super(props);
    this.input = null;
  }
  componentDidMount() {
    this.input.focus();
  }
  render() {
    return <input type="text" ref={el => this.input = el} />
  }
}

let focused = false;
TestRenderer.create(
  <MyComponent />,
  {
    createNodeMock: (element) => {
      if (element.type === 'input') {
        // mock a focus function
        return {
          focus: () => {
            focused = true;
          }
        };
      }
      return null;
    }
  }
);
expect(focused).toBe(true);

React 18 supports all modern browsers (Edge, Firefox, Chrome, Safari, etc).

If you support older browsers and devices such as Internet Explorer which do not provide modern browser features natively or have non-compliant implementations, consider including a global polyfill in your bundled application.

Here is a list of the modern features React 18 uses:

• Promise


• Symbol


• Object.assign

The correct polyfill for these features depend on your environment. For many users, you can configure your Browserlist settings. For others, you may need to import polyfills like core-js directly.

Single-page Application

A single-page application is an application that loads a single HTML page and all the necessary assets (such as JavaScript and CSS) required for the application to run. Any interactions with the page or subsequent pages do not require a round trip to the server which means the page is not reloaded.

Though you may build a single-page application in React, it is not a requirement. React can also be used for enhancing small parts of existing websites with additional interactivity. Code written in React can coexist peacefully with markup rendered on the server by something like PHP, or with other client-side libraries. In fact, this is exactly how React is being used at Facebook.

ES6, ES2015, ES2016, etc

These acronyms all refer to the most recent versions of the ECMAScript Language Specification standard, which the JavaScript language is an implementation of. The ES6 version (also known as ES2015) includes many additions to the previous versions such as: arrow functions, classes, template literals, let and const statements. You can learn more about specific versions here.

Compilers

A JavaScript compiler takes JavaScript code, transforms it and returns JavaScript code in a different format. The most common use case is to take ES6 syntax and transform it into syntax that older browsers are capable of interpreting. Babel is the compiler most commonly used with React.

Bundlers

Bundlers take JavaScript and CSS code written as separate modules (often hundreds of them), and combine them together into a few files better optimized for the browsers. Some bundlers commonly used in React applications include Webpack and Browserify.

Package Managers

Package managers are tools that allow you to manage dependencies in your project. npm and Yarn are two package managers commonly used in React applications. Both of them are clients for the same npm package registry.

CDN

CDN stands for Content Delivery Network. CDNs deliver cached, static content from a network of servers across the globe.

JSX

JSX is a syntax extension to JavaScript. It is similar to a template language, but it has full power of JavaScript. JSX gets compiled to React.createElement() calls which return plain JavaScript objects called “React elements”. To get a basic introduction to JSX see the docs here and find a more in-depth tutorial on JSX here.

React DOM uses camelCase property naming convention instead of HTML attribute names. For example, tabindex becomes tabIndex in JSX. The attribute class is also written as className since class is a reserved word in JavaScript:

<h1 className="hello">My name is Clementine!</h1>

Elements

React elements are the building blocks of React applications. One might confuse elements with a more widely known concept of “components”. An element describes what you want to see on the screen. React elements are immutable.

const element = <h1>Hello, world</h1>;

Typically, elements are not used directly, but get returned from components.

Components

React components are small, reusable pieces of code that return a React element to be rendered to the page. The simplest version of React component is a plain JavaScript function that returns a React element:

function Welcome(props) {
  return <h1>Hello, {props.name}</h1>;
}

Components can also be ES6 classes:

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

Components can be broken down into distinct pieces of functionality and used within other components. Components can return other components, arrays, strings and numbers. A good rule of thumb is that if a part of your UI is used several times (Button, Panel, Avatar), or is complex enough on its own (App, FeedStory, Comment), it is a good candidate to be a reusable component. Component names should also always start with a capital letter ( <Wrapper/> not <wrapper/> ). See this documentation for more information on rendering components.

props

props are inputs to a React component. They are data passed down from a parent component to a child component.

Remember that props are readonly. They should not be modified in any way:

// Wrong!
props.number = 42;

If you need to modify some value in response to user input or a network response, use state instead.

props.children

props.children is available on every component. It contains the content between the opening and closing tags of a component. For example:

<Welcome>Hello world!</Welcome>

The string Hello world! is available in props.children in the Welcome component:

function Welcome(props) {
  return <p>{props.children}</p>;
}

For components defined as classes, use this.props.children :

class Welcome extends React.Component {
  render() {
    return <p>{this.props.children}</p>;
  }
}

state

A component needs state when some data associated with it changes over time. For example, a Checkbox component might need isChecked in its state, and a NewsFeed component might want to keep track of fetchedPosts in its state.

The most important difference between state and props is that props are passed from a parent component, but state is managed by the component itself. A component cannot change its props , but it can change its state .

For each particular piece of changing data, there should be just one component that “owns” it in its state. Don’t try to synchronize states of two different components. Instead, lift it up to their closest shared ancestor, and pass it down as props to both of them.

Lifecycle Methods

Lifecycle methods are custom functionality that gets executed during the different phases of a component. There are methods available when the component gets created and inserted into the DOM (mounting), when the component updates, and when the component gets unmounted or removed from the DOM.

Controlled vs. Uncontrolled Components

React has two different approaches to dealing with form inputs.

An input form element whose value is controlled by React is called a controlled component . When a user enters data into a controlled component a change event handler is triggered and your code decides whether the input is valid (by re-rendering with the updated value). If you do not re-render then the form element will remain unchanged.

An uncontrolled component works like form elements do outside of React. When a user inputs data into a form field (an input box, dropdown, etc) the updated information is reflected without React needing to do anything. However, this also means that you can’t force the field to have a certain value.

In most cases you should use controlled components.

Keys

A “key” is a special string attribute you need to include when creating arrays of elements. Keys help React identify which items have changed, are added, or are removed. Keys should be given to the elements inside an array to give the elements a stable identity.

Keys only need to be unique among sibling elements in the same array. They don’t need to be unique across the whole application or even a single component.

Don’t pass something like Math.random() to keys. It is important that keys have a “stable identity” across re-renders so that React can determine when items are added, removed, or re-ordered. Ideally, keys should correspond to unique and stable identifiers coming from your data, such as post.id .

Refs

React supports a special attribute that you can attach to any component. The ref attribute can be an object created by React.createRef() function or a callback function, or a string (in legacy API). When the ref attribute is a callback function, the function receives the underlying DOM element or class instance (depending on the type of element) as its argument. This allows you to have direct access to the DOM element or component instance.

Use refs sparingly. If you find yourself often using refs to “make things happen” in your app, consider getting more familiar with top-down data flow.

Events

Handling events with React elements has some syntactic differences:

• React event handlers are named using camelCase, rather than lowercase.


• With JSX you pass a function as the event handler, rather than a string.

Reconciliation

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. This process is called “reconciliation”.