- React is an open-source frontend JavaScript library which is used for building user interfaces especially for single page applications. It is used for handling view layer for web and mobile apps. React was created by Jordan Walke, a software engineer working for Facebook. React was first deployed on Facebook's News Feed in 2011 and on Instagram in 2012.
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The major features of React are:
- It uses VirtualDOM instead RealDOM considering that RealDOM manipulations are expensive.
- Supports server-side rendering.
- Follows Unidirectional data flow or data binding.
- Uses reusable/composable UI components to develop the view.
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JSX is a XML-like syntax extension to ECMAScript (the acronym stands for JavaScript XML). Basically it just provides syntactic sugar for the
React.createElement()function, giving us expressiveness of JavaScript along with HTML like template syntax.In the example below text inside
<h1>tag return as JavaScript function to the render function.class App extends React.Component { render() { return( <div> <h1>{'Welcome to React world!'}</h1> </div> ) } }
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An Element is a plain object describing what you want to appear on the screen in terms of the DOM nodes or other components. Elements can contain other Elements in their props. Creating a React element is cheap. Once an element is created, it is never mutated.
The object representation of React Element would be as follows:
const element = React.createElement( 'div', {id: 'login-btn'}, 'Login' )
The above
React.createElement()function returns an object:{ type: 'div', props: { children: 'Login', id: 'login-btn' } }And finally it renders to the DOM using
ReactDOM.render():<div id='login-btn'>Login</div>
Whereas a component can be declared in several different ways. It can be a class with a
render()method. Alternatively, in simple cases, it can be defined as a function. In either case, it takes props as an input, and returns a JSX tree as the output:const Button = ({ onLogin }) => <div id={'login-btn'} onClick={onLogin}>Login</div>
Then JSX gets transpiled to a
React.createElement()function tree:const Button = ({ onLogin }) => React.createElement( 'div', { id: 'login-btn', onClick: onLogin }, 'Login' )
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There are two possible ways to create a component.
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Function Components: This is the simplest way to create a component. Those are pure JavaScript functions that accept props object as first parameter and return React elements:
function Greeting({ message }) { return <h1>{`Hello, ${message}`}</h1> }
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Class Components: You can also use ES6 class to define a component. The above function component can be written as:
class Greeting extends React.Component { render() { return <h1>{`Hello, ${this.props.message}`}</h1> } }
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- If the component needs state or lifecycle methods then use class component otherwise use function component. However, from React 16.8 with the addition of Hooks, you could use state , lifecycle methods and other features that were only available in class component right in your function component.
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React.PureComponentis exactly the same asReact.Componentexcept that it handles theshouldComponentUpdate()method for you. When props or state changes, PureComponent will do a shallow comparison on both props and state. Component on the other hand won't compare current props and state to next out of the box. Thus, the component will re-render by default whenevershouldComponentUpdateis called.
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State of a component is an object that holds some information that may change over the lifetime of the component. We should always try to make our state as simple as possible and minimize the number of stateful components. Let's create an user component with message state,
class User extends React.Component { constructor(props) { super(props) this.state = { message: 'Welcome to React world' } } render() { return ( <div> <h1>{this.state.message}</h1> </div> ) } }
State is similar to props, but it is private and fully controlled by the component. i.e, It is not accessible to any component other than the one that owns and sets it.
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Props are inputs to components. They are single values or objects containing a set of values that are passed to components on creation using a naming convention similar to HTML-tag attributes. They are data passed down from a parent component to a child component.
The primary purpose of props in React is to provide following component functionality:
- Pass custom data to your component.
- Trigger state changes.
- Use via
this.props.reactPropinside component'srender()method.
For example, let us create an element with
reactPropproperty:<Element reactProp={'1'} />
This
reactProp(or whatever you came up with) name then becomes a property attached to React's native props object which originally already exists on all components created using React library.props.reactProp
- Both props and state are plain JavaScript objects. While both of them hold information that influences the output of render, they are different in their functionality with respect to component. Props get passed to the component similar to function parameters whereas state is managed within the component similar to variables declared within a function.
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If you try to update state directly then it won't re-render the component.
//Wrong this.state.message = 'Hello world'
Instead use
setState()method. It schedules an update to a component's state object. When state changes, the component responds by re-rendering.//Correct this.setState({ message: 'Hello World' })
Note: You can directly assign to the state object either in constructor or using latest javascript's class field declaration syntax.
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The callback function is invoked when setState finished and the component gets rendered. Since
setState()is asynchronous the callback function is used for any post action.Note: It is recommended to use lifecycle method rather than this callback function.
setState({ name: 'John' }, () => console.log('The name has updated and component re-rendered'))
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- In HTML, the event name should be in lowercase:
<button onclick='activateLasers()'>
Whereas in React it follows camelCase convention:
<button onClick={activateLasers}>
- In HTML, you can return
falseto prevent default behavior:
<a href='#' onclick='console.log("The link was clicked."); return false;' />
Whereas in React you must call
preventDefault()explicitly:function handleClick(event) { event.preventDefault() console.log('The link was clicked.') }
- In HTML, you need to invoke the function by appending
()Whereas in react you should not append()with the function name. (refer "activateLasers" function in the first point for example)
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There are 3 possible ways to achieve this:
- Binding in Constructor: In JavaScript classes, the methods are not bound by default. The same thing applies for React event handlers defined as class methods. Normally we bind them in constructor.
class Component extends React.Componenet { constructor(props) { super(props) this.handleClick = this.handleClick.bind(this) } handleClick() { // ... } }
- Public class fields syntax: If you don't like to use bind approach then public class fields syntax can be used to correctly bind callbacks.
handleClick = () => { console.log('this is:', this) }
<button onClick={this.handleClick}> {'Click me'} </button>
- Arrow functions in callbacks: You can use arrow functions directly in the callbacks.
<button onClick={(event) => this.handleClick(event)}> {'Click me'} </button>
Note: If the callback is passed as prop to child components, those components might do an extra re-rendering. In those cases, it is preferred to go with
.bind()or public class fields syntax approach considering performance.
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You can use an arrow function to wrap around an event handler and pass parameters:
<button onClick={() => this.handleClick(id)} />
This is an equivalent to calling
.bind:<button onClick={this.handleClick.bind(this, id)} />
Apart from these two approaches, you can also pass arguments to a function which is defined as array function
<button onClick={this.handleClick(id)} /> handleClick = (id) => () => { console.log("Hello, your ticket number is", id) };
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SyntheticEventis a cross-browser wrapper around the browser's native event. It's API is same as the browser's native event, includingstopPropagation()andpreventDefault(), except the events work identically across all browsers.
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You can use either if statements or ternary expressions which are available from JS to conditionally render expressions. Apart from these approaches, you can also embed any expressions in JSX by wrapping them in curly braces and then followed by JS logical operator
&&.<h1>Hello!</h1> { messages.length > 0 && !isLogin? <h2> You have {messages.length} unread messages. </h2> : <h2> You don't have unread messages. </h2> }
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A
keyis a special string attribute you should include when creating arrays of elements. Keys help React identify which items have changed, are added, or are removed.Most often we use IDs from our data as keys:
const todoItems = todos.map((todo) => <li key={todo.id}> {todo.text} </li> )
When you don't have stable IDs for rendered items, you may use the item index as a key as a last resort:
const todoItems = todos.map((todo, index) => <li key={index}> {todo.text} </li> )
Note:
- Using indexes for keys is not recommended if the order of items may change. This can negatively impact performance and may cause issues with component state.
- If you extract list item as separate component then apply keys on list component instead of
litag. - There will be a warning message in the console if the
keyprop is not present on list items.
- The ref is used to return a reference to the element. They should be avoided in most cases, however, they can be useful when you need a direct access to the DOM element or an instance of a component.
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There are two approaches
- This is a recently added approach. Refs are created using
React.createRef()method and attached to React elements via therefattribute. In order to use refs throughout the component, just assign the ref to the instance property within constructor.
class MyComponent extends React.Component { constructor(props) { super(props) this.myRef = React.createRef() } render() { return <div ref={this.myRef} /> } }
- You can also use ref callbacks approach regardless of React version. For example, the search bar component's input element accessed as follows,
class SearchBar extends Component { constructor(props) { super(props); this.txtSearch = null; this.state = { term: '' }; this.setInputSearchRef = e => { this.txtSearch = e; } } onInputChange(event) { this.setState({ term: this.txtSearch.value }); } render() { return ( <input value={this.state.term} onChange={this.onInputChange.bind(this)} ref={this.setInputSearchRef} /> ); } }
You can also use refs in function components using closures. Note: You can also use inline ref callbacks even though it is not a recommended approach
- This is a recently added approach. Refs are created using
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Ref forwarding is a feature that lets some components take a ref they receive, and pass it further down to a child.
const ButtonElement = React.forwardRef((props, ref) => ( <button ref={ref} className="CustomButton"> {props.children} </button> )); // Create ref to the DOM button: const ref = React.createRef(); <ButtonElement ref={ref}>{'Forward Ref'}</ButtonElement>
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It is preferred to use callback refs over
findDOMNode()API. BecausefindDOMNode()prevents certain improvements in React in the future.The legacy approach of using
findDOMNode:class MyComponent extends Component { componentDidMount() { findDOMNode(this).scrollIntoView() } render() { return <div /> } }
The recommended approach is:
class MyComponent extends Component { constructor(props){ super(props); this.node = createRef(); } componentDidMount() { this.node.current.scrollIntoView(); } render() { return <div ref={this.node} /> } }
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If you worked with React before, you might be familiar with an older API where the
refattribute is a string, likeref={'textInput'}, and the DOM node is accessed asthis.refs.textInput. We advise against it because string refs have below issues, and are considered legacy. String refs were removed in React v16.- They force React to keep track of currently executing component. This is problematic because it makes react module stateful, and thus causes weird errors when react module is duplicated in the bundle.
- They are not composable — if a library puts a ref on the passed child, the user can't put another ref on it. Callback refs are perfectly composable.
- They don't work with static analysis like Flow. Flow can't guess the magic that framework does to make the string ref appear on
this.refs, as well as its type (which could be different). Callback refs are friendlier to static analysis. - It doesn't work as most people would expect with the "render callback" pattern (e.g. )
class MyComponent extends Component { renderRow = (index) => { // This won't work. Ref will get attached to DataTable rather than MyComponent: return <input ref={'input-' + index} />; // This would work though! Callback refs are awesome. return <input ref={input => this['input-' + index] = input} />; } render() { return <DataTable data={this.props.data} renderRow={this.renderRow} /> } }
- The Virtual DOM (VDOM) is an in-memory representation of Real DOM. The representation of a UI is kept in memory and synced with the "real" DOM. It's a step that happens between the render function being called and the displaying of elements on the screen. This entire process is called reconciliation.
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The Virtual DOM works in three simple steps.
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Whenever any underlying data changes, the entire UI is re-rendered in Virtual DOM representation.
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Then the difference between the previous DOM representation and the new one is calculated.
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Once the calculations are done, the real DOM will be updated with only the things that have actually changed.
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- The Shadow DOM is a browser technology designed primarily for scoping variables and CSS in web components. The Virtual DOM is a concept implemented by libraries in JavaScript on top of browser APIs.
- Fiber is the new reconciliation engine or reimplementation of core algorithm in React v16. The goal of React Fiber is to increase its suitability for areas like animation, layout, gestures, ability to pause, abort, or reuse work and assign priority to different types of updates; and new concurrency primitives.
- The goal of React Fiber is to increase its suitability for areas like animation, layout, and gestures. Its headline feature is incremental rendering: the ability to split rendering work into chunks and spread it out over multiple frames.
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A component that controls the input elements within the forms on subsequent user input is called Controlled Component, i.e, every state mutation will have an associated handler function.
For example, to write all the names in uppercase letters, we use handleChange as below,
handleChange(event) { this.setState({value: event.target.value.toUpperCase()}) }
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The Uncontrolled Components are the ones that store their own state internally, and you query the DOM using a ref to find its current value when you need it. This is a bit more like traditional HTML.
In the below UserProfile component, the
nameinput is accessed using ref.class UserProfile extends React.Component { constructor(props) { super(props) this.handleSubmit = this.handleSubmit.bind(this) this.input = React.createRef() } handleSubmit(event) { alert('A name was submitted: ' + this.input.current.value) event.preventDefault() } render() { return ( <form onSubmit={this.handleSubmit}> <label> {'Name:'} <input type="text" ref={this.input} /> </label> <input type="submit" value="Submit" /> </form> ); } }
In most cases, it's recommend to use controlled components to implement forms.
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JSX elements will be transpiled to
React.createElement()functions to create React elements which are going to be used for the object representation of UI. WhereascloneElementis used to clone an element and pass it new props.
- When several components need to share the same changing data then it is recommended to lift the shared state up to their closest common ancestor. That means if two child components share the same data from its parent, then move the state to parent instead of maintaining local state in both of the child components.
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The component lifecycle has three distinct lifecycle phases:
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Mounting: The component is ready to mount in the browser DOM. This phase covers initialization from
constructor(),getDerivedStateFromProps(),render(), andcomponentDidMount()lifecycle methods. -
Updating: In this phase, the component get updated in two ways, sending the new props and updating the state either from
setState()orforceUpdate(). This phase coversgetDerivedStateFromProps(),shouldComponentUpdate(),render(),getSnapshotBeforeUpdate()andcomponentDidUpdate()lifecycle methods. -
Unmounting: In this last phase, the component is not needed and get unmounted from the browser DOM. This phase includes
componentWillUnmount()lifecycle method.
It's worth mentioning that React internally has a concept of phases when applying changes to the DOM. They are separated as follows
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Render The component will render without any side-effects. This applies for Pure components and in this phase, React can pause, abort, or restart the render.
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Pre-commit Before the component actually applies the changes to the DOM, there is a moment that allows React to read from the DOM through the
getSnapshotBeforeUpdate(). -
Commit React works with the DOM and executes the final lifecycles respectively
componentDidMount()for mounting,componentDidUpdate()for updating, andcomponentWillUnmount()for unmounting.
React 16.3+ Phases (or an interactive version)
Before React 16.3
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React 16.3+
- getDerivedStateFromProps: Invoked right before calling
render()and is invoked on every render. This exists for rare use cases where you need derived state. Worth reading if you need derived state. - componentDidMount: Executed after first rendering and here all AJAX requests, DOM or state updates, and set up event listeners should occur.
- shouldComponentUpdate: Determines if the component will be updated or not. By default it returns
true. If you are sure that the component doesn't need to render after state or props are updated, you can return false value. It is a great place to improve performance as it allows you to prevent a re-render if component receives new prop. - getSnapshotBeforeUpdate: Executed right before rendered output is committed to the DOM. Any value returned by this will be passed into
componentDidUpdate(). This is useful to capture information from the DOM i.e. scroll position. - componentDidUpdate: Mostly it is used to update the DOM in response to prop or state changes. This will not fire if
shouldComponentUpdate()returnsfalse. - componentWillUnmount It will be used to cancel any outgoing network requests, or remove all event listeners associated with the component.
Before 16.3
- componentWillMount: Executed before rendering and is used for App level configuration in your root component.
- componentDidMount: Executed after first rendering and here all AJAX requests, DOM or state updates, and set up event listeners should occur.
- componentWillReceiveProps: Executed when particular prop updates to trigger state transitions.
- shouldComponentUpdate: Determines if the component will be updated or not. By default it returns
true. If you are sure that the component doesn't need to render after state or props are updated, you can return false value. It is a great place to improve performance as it allows you to prevent a re-render if component receives new prop. - componentWillUpdate: Executed before re-rendering the component when there are props & state changes confirmed by
shouldComponentUpdate()which returns true. - componentDidUpdate: Mostly it is used to update the DOM in response to prop or state changes.
- componentWillUnmount: It will be used to cancel any outgoing network requests, or remove all event listeners associated with the component.
- getDerivedStateFromProps: Invoked right before calling
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A higher-order component (HOC) is a function that takes a component and returns a new component. Basically, it's a pattern that is derived from React's compositional nature.
We call them pure components because they can accept any dynamically provided child component but they won't modify or copy any behavior from their input components.
const EnhancedComponent = higherOrderComponent(WrappedComponent)
HOC can be used for many use cases:
- Code reuse, logic and bootstrap abstraction.
- Render hijacking.
- State abstraction and manipulation.
- Props manipulation.
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You can add/edit props passed to the component using props proxy pattern like this:
function HOC(WrappedComponent) { return class Test extends Component { render() { const newProps = { title: 'New Header', footer: false, showFeatureX: false, showFeatureY: true } return <WrappedComponent {...this.props} {...newProps} /> } } }
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Context provides a way to pass data through the component tree without having to pass props down manually at every level. For example, authenticated user, locale preference, UI theme need to be accessed in the application by many components.
const {Provider, Consumer} = React.createContext(defaultValue)
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Children is a prop (
this.prop.children) that allow you to pass components as data to other components, just like any other prop you use. Component tree put between component's opening and closing tag will be passed to that component aschildrenprop.There are a number of methods available in the React API to work with this prop. These include
React.Children.map,React.Children.forEach,React.Children.count,React.Children.only,React.Children.toArray. A simple usage of children prop looks as below,const MyDiv = React.createClass({ render: function() { return <div>{this.props.children}</div> } }) ReactDOM.render( <MyDiv> <span>{'Hello'}</span> <span>{'World'}</span> </MyDiv>, node )
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The comments in React/JSX are similar to JavaScript Multiline comments but are wrapped in curly braces.
Single-line comments:
<div> {/* Single-line comments(In vanilla JavaScript, the single-line comments are represented by double slash(//)) */} {`Welcome ${user}, let's play React`} </div>
Multi-line comments:
<div> {/* Multi-line comments for more than one line */} {`Welcome ${user}, let's play React`} </div>
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A child class constructor cannot make use of
thisreference untilsuper()method has been called. The same applies for ES6 sub-classes as well. The main reason of passing props parameter tosuper()call is to accessthis.propsin your child constructors.Passing props:
class MyComponent extends React.Component { constructor(props) { super(props) console.log(this.props) // prints { name: 'John', age: 42 } } }
Not passing props:
class MyComponent extends React.Component { constructor(props) { super() console.log(this.props) // prints undefined // but props parameter is still available console.log(props) // prints { name: 'John', age: 42 } } render() { // no difference outside constructor console.log(this.props) // prints { name: 'John', age: 42 } } }
The above code snippets reveals that
this.propsis different only within the constructor. It would be the same outside the constructor.
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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.
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If you are using ES6 or the Babel transpiler to transform your JSX code then you can accomplish this with computed property names.
handleInputChange(event) { this.setState({ [event.target.id]: event.target.value }) }
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You need to make sure that function is not being called while passing the function as a parameter.
render() { // Wrong: handleClick is called instead of passed as a reference! return <button onClick={this.handleClick()}>{'Click Me'}</button> }
Instead, pass the function itself without parenthesis:
render() { // Correct: handleClick is passed as a reference! return <button onClick={this.handleClick}>{'Click Me'}</button> }
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No, currently
React.lazyfunction supports default exports only. If you would like to import modules which are named exports, you can create an intermediate module that reexports it as the default. It also ensures that tree shaking keeps working and don’t pull unused components. Let's take a component file which exports multiple named components,and reexport// MoreComponents.js export const SomeComponent = /* ... */; export const UnusedComponent = /* ... */;
MoreComponents.jscomponents in an intermediate fileIntermediateComponent.jsNow you can import the module using lazy function as below,// IntermediateComponent.js export { SomeComponent as default } from "./MoreComponents.js";
import React, { lazy } from 'react'; const SomeComponent = lazy(() => import("./IntermediateComponent.js"));
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classis a keyword in JavaScript, and JSX is an extension of JavaScript. That's the principal reason why React usesclassNameinstead ofclass. Pass a string as theclassNameprop.render() { return <span className={'menu navigation-menu'}>{'Menu'}</span> }
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It's common pattern in React which is used 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 shorter syntax, but it's not supported in many tools:
render() { return ( <> <ChildA /> <ChildB /> <ChildC /> </> ) }
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- Fragments are a bit faster and use less memory by not creating an extra DOM node. This only has a real benefit on very large and deep trees.
- Some CSS mechanisms like Flexbox and CSS Grid have a special parent-child relationships, and adding divs in the middle makes it hard to keep the desired layout.
- The DOM Inspector is less cluttered.
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Portal is a recommended 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 is any render-able React child, such as an element, string, or fragment. The second argument is a DOM element.
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If the behaviour is independent of its state then it can be a stateless component. You can use either a function or a class for creating stateless components. But unless you need to use a lifecycle hook in your components, you should go for function components. There are a lot of benefits if you decide to use function components here; they are easy to write, understand, and test, a little faster, and you can avoid the
thiskeyword altogether.
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If the behaviour of a component is dependent on the state of the component then it can be termed as stateful component. These stateful components are always class components and have a state that gets initialized in the
constructor.class App extends Component { constructor(props) { super(props) this.state = { count: 0 } } render() { // ... } }
React 16.8 Update: Hooks let you use state and other React features without writing classes.
The Equivalent Functional Component
import React, {useState} from 'react'; const App = (props) => { const [count, setCount] = useState(0); return ( // JSX ) }
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When the application is running in development mode, React will automatically check all props that we set on components to make sure they have correct type. If the type is incorrect, React will generate warning messages in the console. It's disabled in production mode due to performance impact. The mandatory props are defined with
isRequired.The set of predefined prop types:
PropTypes.numberPropTypes.stringPropTypes.arrayPropTypes.objectPropTypes.funcPropTypes.nodePropTypes.elementPropTypes.boolPropTypes.symbolPropTypes.any
We can define
propTypesforUsercomponent as below:import React from 'react' import PropTypes from 'prop-types' class User extends React.Component { static propTypes = { name: PropTypes.string.isRequired, age: PropTypes.number.isRequired } render() { return ( <> <h1>{`Welcome, ${this.props.name}`}</h1> <h2>{`Age, ${this.props.age}`}</h2> </> ) } }
Note: In React v15.5 PropTypes were moved from
React.PropTypestoprop-typeslibrary.
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- Increases the application's performance with Virtual DOM.
- JSX makes code easy to read and write.
- It renders both on client and server side (SSR).
- Easy to integrate with frameworks (Angular, Backbone) since it is only a view library.
- Easy to write unit and integration tests with tools such as Jest.
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- React is just a view library, not a full framework.
- There is a learning curve for beginners who are new to web development.
- Integrating React into a traditional MVC framework requires some additional configuration.
- The code complexity increases with inline templating and JSX.
- Too many smaller components leading to over engineering or boilerplate.
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Error boundaries are components that catch JavaScript errors anywhere in their child component tree, log those errors, and display a fallback UI instead of the component tree that crashed.
A class component becomes an error boundary if it defines a new lifecycle method called
componentDidCatch(error, info)orstatic getDerivedStateFromError():class ErrorBoundary extends React.Component { constructor(props) { super(props) this.state = { hasError: false } } componentDidCatch(error, info) { // You can also log the error to an error reporting service logErrorToMyService(error, info) } static getDerivedStateFromError(error) { // Update state so the next render will show the fallback UI. return { hasError: true }; } render() { if (this.state.hasError) { // You can render any custom fallback UI return <h1>{'Something went wrong.'}</h1> } return this.props.children } }
After that use it as a regular component:
<ErrorBoundary> <MyWidget /> </ErrorBoundary>
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React v15 provided very basic support for error boundaries using
unstable_handleErrormethod. It has been renamed tocomponentDidCatchin React v16.
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Normally we use PropTypes library (
React.PropTypesmoved to aprop-typespackage since React v15.5) for type checking in the React applications. For large code bases, it is recommended to use static type checkers such as Flow or TypeScript, that perform type checking at compile time and provide auto-completion features.
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The
react-dompackage provides DOM-specific methods that can be used at the top level of your app. Most of the components are not required to use this module. Some of the methods of this package are:render()hydrate()unmountComponentAtNode()findDOMNode()createPortal()
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This method is used to render a React element into the DOM in the supplied container and return a reference to the component. If the React element was previously rendered into container, it will perform an update on it and only mutate the DOM as necessary to reflect the latest changes.
ReactDOM.render(element, container[, callback])If the optional callback is provided, it will be executed after the component is rendered or updated.
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The
ReactDOMServerobject enables you to render components to static markup (typically used on node server). This object is mainly used for server-side rendering (SSR). The following methods can be used in both the server and browser environments:renderToString()renderToStaticMarkup()
For example, you generally run a Node-based web server like Express, Hapi, or Koa, and you call
renderToStringto render your root component to a string, which you then send as response.// using Express import { renderToString } from 'react-dom/server' import MyPage from './MyPage' app.get('/', (req, res) => { res.write('<!DOCTYPE html><html><head><title>My Page</title></head><body>') res.write('<div id="content">') res.write(renderToString(<MyPage/>)) res.write('</div></body></html>') res.end() })
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The
dangerouslySetInnerHTMLattribute is React's replacement for usinginnerHTMLin the browser DOM. Just likeinnerHTML, it is risky to use this attribute considering cross-site scripting (XSS) attacks. You just need to pass a__htmlobject as key and HTML text as value.In this example MyComponent uses
dangerouslySetInnerHTMLattribute for setting HTML markup:function createMarkup() { return { __html: 'First · Second' } } function MyComponent() { return <div dangerouslySetInnerHTML={createMarkup()} /> }
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The
styleattribute accepts a JavaScript object with camelCased properties rather than a CSS string. This is consistent with the DOM style JavaScript property, is more efficient, and prevents XSS security holes.const divStyle = { color: 'blue', backgroundImage: 'url(' + imgUrl + ')' }; function HelloWorldComponent() { return <div style={divStyle}>Hello World!</div> }
Style keys are camelCased in order to be consistent with accessing the properties on DOM nodes in JavaScript (e.g.
node.style.backgroundImage).
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Handling events in 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.
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When you use
setState(), then apart from assigning to the object state React also re-renders the component and all its children. You would get error like this: Can only update a mounted or mounting component. So we need to usethis.stateto initialize variables inside constructor.
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Keys should be stable, predictable, and unique so that React can keep track of elements.
In the below code snippet each element's key will be based on ordering, rather than tied to the data that is being represented. This limits the optimizations that React can do.
{todos.map((todo, index) => <Todo {...todo} key={index} /> )}
If you use element data for unique key, assuming todo.id is unique to this list and stable, React would be able to reorder elements without needing to reevaluate them as much.
{todos.map((todo) => <Todo {...todo} key={todo.id} /> )}
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It is recommended to avoid async initialization in
componentWillMount()lifecycle method.componentWillMount()is invoked immediately before mounting occurs. It is called beforerender(), therefore setting state in this method will not trigger a re-render. Avoid introducing any side-effects or subscriptions in this method. We need to make sure async calls for component initialization happened incomponentDidMount()instead ofcomponentWillMount().componentDidMount() { axios.get(`api/todos`) .then((result) => { this.setState({ messages: [...result.data] }) }) }
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If the props on the component are changed without the component being refreshed, the new prop value will never be displayed because the constructor function will never update the current state of the component. The initialization of state from props only runs when the component is first created.
The below component won't display the updated input value:
class MyComponent extends React.Component { constructor(props) { super(props) this.state = { records: [], inputValue: this.props.inputValue }; } render() { return <div>{this.state.inputValue}</div> } }
Using props inside render method will update the value:
class MyComponent extends React.Component { constructor(props) { super(props) this.state = { record: [] } } render() { return <div>{this.props.inputValue}</div> } }
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In some cases you want to render different components depending on some state. JSX does not render
falseorundefined, so you can use conditional short-circuiting to render a given part of your component only if a certain condition is true.const MyComponent = ({ name, address }) => ( <div> <h2>{name}</h2> {address && <p>{address}</p> } </div> )
If you need an
if-elsecondition then use ternary operator.const MyComponent = ({ name, address }) => ( <div> <h2>{name}</h2> {address ? <p>{address}</p> : <p>{'Address is not available'}</p> } </div> )
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When we spread props we run into the risk of adding unknown HTML attributes, which is a bad practice. Instead we can use prop destructuring with
...restoperator, so it will add only required props. For example,const ComponentA = () => <ComponentB isDisplay={true} className={'componentStyle'} /> const ComponentB = ({ isDisplay, ...domProps }) => <div {...domProps}>{'ComponentB'}</div>
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You can decorate your class components, which is the same as passing the component into a function. Decorators are flexible and readable way of modifying component functionality.
@setTitle('Profile') class Profile extends React.Component { //.... } /* title is a string that will be set as a document title WrappedComponent is what our decorator will receive when put directly above a component class as seen in the example above */ const setTitle = (title) => (WrappedComponent) => { return class extends React.Component { componentDidMount() { document.title = title } render() { return <WrappedComponent {...this.props} /> } } }
Note: Decorators are a feature that didn't make it into ES7, but are currently a stage 2 proposal.
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There are memoize libraries available which can be used on function components. For example
moizelibrary can memoize the component in another component.import moize from 'moize' import Component from './components/Component' // this module exports a non-memoized component const MemoizedFoo = moize.react(Component) const Consumer = () => { <div> {'I will memoize the following entry:'} <MemoizedFoo/> </div> }
Update: Since React v16.6.0, we have a
React.memo. It provides a higher order component which memoizes component unless the props change. To use it, simply wrap the component using React.memo before you use it.const MemoComponent = React.memo(function MemoComponent(props) { /* render using props */ }); OR export default React.memo(MyFunctionComponent);
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React is already equipped to handle rendering on Node servers. A special version of the DOM renderer is available, which follows the same pattern as on the client side.
import ReactDOMServer from 'react-dom/server' import App from './App' ReactDOMServer.renderToString(<App />)
This method will output the regular HTML as a string, which can be then placed inside a page body as part of the server response. On the client side, React detects the pre-rendered content and seamlessly picks up where it left off.
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You should use Webpack's
DefinePluginmethod to setNODE_ENVtoproduction, by which it strip out things like propType validation and extra warnings. Apart from this, if you minify the code, for example, Uglify's dead-code elimination to strip out development only code and comments, it will drastically reduce the size of your bundle.
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The
create-react-appCLI tool allows you to quickly create & run React applications with no configuration step.Let's create Todo App using CRA:
# Installation $ npm install -g create-react-app # Create new project $ create-react-app todo-app $ cd todo-app # Build, test and run $ npm run build $ npm run test $ npm start
It includes everything we need to build a React app:
- React, JSX, ES6, and Flow syntax support.
- Language extras beyond ES6 like the object spread operator.
- Autoprefixed CSS, so you don’t need -webkit- or other prefixes.
- A fast interactive unit test runner with built-in support for coverage reporting.
- A live development server that warns about common mistakes.
- A build script to bundle JS, CSS, and images for production, with hashes and sourcemaps.
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The lifecycle methods are called in the following order when an instance of a component is being created and inserted into the DOM.
constructor()static getDerivedStateFromProps()render()componentDidMount()
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The following lifecycle methods going to be unsafe coding practices and will be more problematic with async rendering.
componentWillMount()componentWillReceiveProps()componentWillUpdate()
Starting with React v16.3 these methods are aliased with
UNSAFE_prefix, and the unprefixed version will be removed in React v17.
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The new static
getDerivedStateFromProps()lifecycle method is invoked after a component is instantiated as well as before it is re-rendered. It can return an object to update state, ornullto indicate that the new props do not require any state updates.class MyComponent extends React.Component { static getDerivedStateFromProps(props, state) { // ... } }
This lifecycle method along with
componentDidUpdate()covers all the use cases ofcomponentWillReceiveProps().
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The new
getSnapshotBeforeUpdate()lifecycle method is called right before DOM updates. The return value from this method will be passed as the third parameter tocomponentDidUpdate().class MyComponent extends React.Component { getSnapshotBeforeUpdate(prevProps, prevState) { // ... } }
This lifecycle method along with
componentDidUpdate()covers all the use cases ofcomponentWillUpdate().
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Both render props and higher-order components render only a single child but in most of the cases Hooks are a simpler way to serve this by reducing nesting in your tree.
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It is recommended to name the component by reference instead of using
displayName.Using
displayNamefor naming component:export default React.createClass({ displayName: 'TodoApp', // ... })
The recommended approach:
export default class TodoApp extends React.Component { // ... }
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Recommended ordering of methods from mounting to render stage:
staticmethodsconstructor()getChildContext()componentWillMount()componentDidMount()componentWillReceiveProps()shouldComponentUpdate()componentWillUpdate()componentDidUpdate()componentWillUnmount()- click handlers or event handlers like
onClickSubmit()oronChangeDescription() - getter methods for render like
getSelectReason()orgetFooterContent() - optional render methods like
renderNavigation()orrenderProfilePicture() render()
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A switching component is a component that renders one of many components. We need to use object to map prop values to components.
For example, a switching component to display different pages based on
pageprop:import HomePage from './HomePage' import AboutPage from './AboutPage' import ServicesPage from './ServicesPage' import ContactPage from './ContactPage' const PAGES = { home: HomePage, about: AboutPage, services: ServicesPage, contact: ContactPage } const Page = (props) => { const Handler = PAGES[props.page] || ContactPage return <Handler {...props} /> } // The keys of the PAGES object can be used in the prop types to catch dev-time errors. Page.propTypes = { page: PropTypes.oneOf(Object.keys(PAGES)).isRequired }
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The reason behind for this is that
setState()is an asynchronous operation. React batches state changes for performance reasons, so the state may not change immediately aftersetState()is called. That means you should not rely on the current state when callingsetState()since you can't be sure what that state will be. The solution is to pass a function tosetState(), with the previous state as an argument. By doing this you can avoid issues with the user getting the old state value on access due to the asynchronous nature ofsetState().Let's say the initial count value is zero. After three consecutive increment operations, the value is going to be incremented only by one.
// assuming this.state.count === 0 this.setState({ count: this.state.count + 1 }) this.setState({ count: this.state.count + 1 }) this.setState({ count: this.state.count + 1 }) // this.state.count === 1, not 3
If we pass a function to
setState(), the count gets incremented correctly.this.setState((prevState, props) => ({ count: prevState.count + props.increment })) // this.state.count === 3 as expected
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React.StrictModeis a useful component for highlighting potential problems in an application. Just like<Fragment>,<StrictMode>does not render any extra DOM elements. It activates additional checks and warnings for its descendants. These checks apply for development mode only.import React from 'react' function ExampleApplication() { return ( <div> <Header /> <React.StrictMode> <div> <ComponentOne /> <ComponentTwo /> </div> </React.StrictMode> <Footer /> </div> ) }
In the example above, the strict mode checks apply to
<ComponentOne>and<ComponentTwo>components only.
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Mixins are a way to totally separate components to have a common functionality. Mixins should not be used and can be replaced with higher-order components or decorators.
One of the most commonly used mixins is
PureRenderMixin. You might be using it in some components to prevent unnecessary re-renders when the props and state are shallowly equal to the previous props and state:const PureRenderMixin = require('react-addons-pure-render-mixin') const Button = React.createClass({ mixins: [PureRenderMixin], // ... })
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The primary use case for
isMounted()is to avoid callingsetState()after a component has been unmounted, because it will emit a warning.if (this.isMounted()) { this.setState({...}) }
Checking
isMounted()before callingsetState()does eliminate the warning, but it also defeats the purpose of the warning. UsingisMounted()is a code smell because the only reason you would check is because you think you might be holding a reference after the component has unmounted.An optimal solution would be to find places where
setState()might be called after a component has unmounted, and fix them. Such situations most commonly occur due to callbacks, when a component is waiting for some data and gets unmounted before the data arrives. Ideally, any callbacks should be canceled incomponentWillUnmount(), prior to unmounting.
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Pointer Events provide a unified way of handling all input events. In the old days we had a mouse and respective event listeners to handle them but nowadays we have many devices which don't correlate to having a mouse, like phones with touch surface or pens. We need to remember that these events will only work in browsers that support the Pointer Events specification.
The following event types are now available in React DOM:
onPointerDownonPointerMoveonPointerUponPointerCancelonGotPointerCaptureonLostPointerCapturonPointerEnteronPointerLeaveonPointerOveronPointerOut
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If you are rendering your component using JSX, the name of that component has to begin with a capital letter otherwise React will throw an error as unrecognized tag. This convention is because only HTML elements and SVG tags can begin with a lowercase letter.
class SomeComponent extends Component { // Code goes here }
You can define component class which name starts with lowercase letter, but when it's imported it should have capital letter. Here lowercase is fine:
class myComponent extends Component { render() { return <div /> } } export default myComponent
While when imported in another file it should start with capital letter:
import MyComponent from './MyComponent'
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Yes. In the past, React used to ignore unknown DOM attributes. If you wrote JSX with an attribute that React doesn't recognize, React would just skip it. For example, this:
<div mycustomattribute={'something'} />
Would render an empty div to the DOM with React v15:
<div />
In React v16 any unknown attributes will end up in the DOM:
<div mycustomattribute='something' />
This is useful for supplying browser-specific non-standard attributes, trying new DOM APIs, and integrating with opinionated third-party libraries.
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You should initialize state in the constructor when using ES6 classes, and
getInitialState()method when usingReact.createClass().Using ES6 classes:
class MyComponent extends React.Component { constructor(props) { super(props) this.state = { /* initial state */ } } }
Using
React.createClass():const MyComponent = React.createClass({ getInitialState() { return { /* initial state */ } } })
Note:
React.createClass()is deprecated and removed in React v16. Use plain JavaScript classes instead.
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By default, when your component's state or props change, your component will re-render. If your
render()method depends on some other data, you can tell React that the component needs re-rendering by callingforceUpdate().component.forceUpdate(callback)
It is recommended to avoid all uses of
forceUpdate()and only read fromthis.propsandthis.stateinrender().
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When you want to access
this.propsinconstructor()then you should pass props tosuper()method.Using
super(props):class MyComponent extends React.Component { constructor(props) { super(props) console.log(this.props) // { name: 'John', ... } } }
Using
super():class MyComponent extends React.Component { constructor(props) { super() console.log(this.props) // undefined } }
Outside
constructor()both will display same value forthis.props.
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You can simply use
Array.prototype.mapwith ES6 arrow function syntax. For example, theitemsarray of objects is mapped into an array of components:<tbody> {items.map(item => <SomeComponent key={item.id} name={item.name} />)} </tbody>
You can't iterate using
forloop:<tbody> for (let i = 0; i < items.length; i++) { <SomeComponent key={items[i].id} name={items[i].name} /> } </tbody>
This is because JSX tags are transpiled into function calls, and you can't use statements inside expressions. This may change thanks to
doexpressions which are stage 1 proposal.
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React (or JSX) doesn't support variable interpolation inside an attribute value. The below representation won't work:
<img className='image' src='images/{this.props.image}' />
But you can put any JS expression inside curly braces as the entire attribute value. So the below expression works:
<img className='image' src={'images/' + this.props.image} />
Using template strings will also work:
<img className='image' src={`images/${this.props.image}`} />
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If you want to pass an array of objects to a component with a particular shape then use
React.PropTypes.shape()as an argument toReact.PropTypes.arrayOf().ReactComponent.propTypes = { arrayWithShape: React.PropTypes.arrayOf(React.PropTypes.shape({ color: React.PropTypes.string.isRequired, fontSize: React.PropTypes.number.isRequired })).isRequired }
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You shouldn't use curly braces inside quotes because it is going to be evaluated as a string.
<div className="btn-panel {this.props.visible ? 'show' : 'hidden'}">
Instead you need to move curly braces outside (don't forget to include spaces between class names):
<div className={'btn-panel ' + (this.props.visible ? 'show' : 'hidden')}>
Template strings will also work:
<div className={`btn-panel ${this.props.visible ? 'show' : 'hidden'}`}>
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The
reactpackage containsReact.createElement(),React.Component,React.Children, and other helpers related to elements and component classes. You can think of these as the isomorphic or universal helpers that you need to build components. Thereact-dompackage containsReactDOM.render(), and inreact-dom/serverwe have server-side rendering support withReactDOMServer.renderToString()andReactDOMServer.renderToStaticMarkup().
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The React team worked on extracting all DOM-related features into a separate library called ReactDOM. React v0.14 is the first release in which the libraries are split. By looking at some of the packages,
react-native,react-art,react-canvas, andreact-three, it has become clear that the beauty and essence of React has nothing to do with browsers or the DOM. To build more environments that React can render to, React team planned to split the main React package into two:reactandreact-dom. This paves the way to writing components that can be shared between the web version of React and React Native.
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If you try to render a
<label>element bound to a text input using the standardforattribute, then it produces HTML missing that attribute and prints a warning to the console.<label for={'user'}>{'User'}</label> <input type={'text'} id={'user'} />
Since
foris a reserved keyword in JavaScript, usehtmlForinstead.<label htmlFor={'user'}>{'User'}</label> <input type={'text'} id={'user'} />
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You can use spread operator in regular React:
<button style={{...styles.panel.button, ...styles.panel.submitButton}}>{'Submit'}</button>
If you're using React Native then you can use the array notation:
<button style={[styles.panel.button, styles.panel.submitButton]}>{'Submit'}</button>
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You can listen to the
resizeevent incomponentDidMount()and then update the dimensions (widthandheight). You should remove the listener incomponentWillUnmount()method.class WindowDimensions extends React.Component { constructor(props){ super(props); this.updateDimensions = this.updateDimensions.bind(this); } componentWillMount() { this.updateDimensions() } componentDidMount() { window.addEventListener('resize', this.updateDimensions) } componentWillUnmount() { window.removeEventListener('resize', this.updateDimensions) } updateDimensions() { this.setState({width: window.innerWidth, height: window.innerHeight}) } render() { return <span>{this.state.width} x {this.state.height}</span> } }