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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.
The major features of React are:
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

tag return as JavaScript function to the render function.
jsx harmony class App extends React.Component { render() { return (

{"Welcome to React world!"}

 ); } }
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:
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()
:
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:
Then JSX gets transpiled to a React.createElement()
function tree:
There are two possible ways to create a component.
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:
jsx harmony function Greeting({ message }) { return
{`Hello, ${message}`}
; }
Class Components: You can also use ES6 class to define a component. The above function component can be written as:
jsx harmony class Greeting extends React.Component { render() { return
{`Hello, ${this.props.message}`}
; } }
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.
React.PureComponent
is exactly the same as React.Component
except that it handles the shouldComponentUpdate()
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 whenever shouldComponentUpdate
is called.
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,
```jsx harmony class User extends React.Component { constructor(props) { super(props);
this.state = {
message: "Welcome to React world",
};
}
render() { return ( <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.
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:
this.props.reactProp
inside component’s render()
method.For example, let us create an element with reactProp
property:
jsx harmony <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.
If you try to update state directly then it won’t re-render the component.
Instead use setState()
method. It schedules an update to a component’s state object. When state changes, the component responds by re-rendering.
Note: You can directly assign to the state object either in constructor or using latest javascript’s class field declaration syntax.
setState()
?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.
Whereas in React it follows camelCase convention:
jsx harmony <button onClick={activateLasers}>
false
to prevent default behavior:Whereas in React you must call preventDefault()
explicitly:
()
Whereas in react you should not append ()
with the function name. (refer “activateLasers” function in the first point for example)There are 3 possible ways to achieve this:
class Component extends React.Componenet {
constructor(props) {
super(props);
this.handleClick = this.handleClick.bind(this);
}
handleClick() {
// ...
}
}
jsx harmony handleClick = () => { console.log("this is:", this); };
jsx harmony <button onClick={this.handleClick}>{"Click me"}</button>
jsx harmony <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.
You can use an arrow function to wrap around an event handler and pass parameters:
jsx harmony <button onClick={() => this.handleClick(id)} />
This is an equivalent to calling .bind
:
jsx harmony <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
jsx harmony <button onClick={this.handleClick(id)} />; handleClick = (id) => () => { console.log("Hello, your ticket number is", id); };
SyntheticEvent
is a cross-browser wrapper around the browser’s native event. It’s API is same as the browser’s native event, including stopPropagation()
and preventDefault()
, except the events work identically across all browsers.
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 &&
.
jsx harmony
Hello!
; { messages.length > 0 && !isLogin ? (
You have {messages.length} unread messages.
 ) : (
You don't have unread messages.
 ); }
A key
is 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:
jsx harmony const todoItems = todos.map((todo) =>
{todo.text}
);
When you don’t have stable IDs for rendered items, you may use the item index as a key as a last resort:
jsx harmony const todoItems = todos.map((todo, index) => (
{todo.text}
 ));
Note:
li
tag.key
prop 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.
There are two approaches
React.createRef()
method and attached to React elements via the ref
attribute. In order to use refs throughout the component, just assign the ref to the instance property within constructor.jsx harmony class MyComponent extends React.Component { constructor(props) { super(props); this.myRef = React.createRef(); } render() { return
; } }
jsx harmony 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
Ref forwarding is a feature that lets some components take a ref they receive, and pass it further down to a child.
```jsx harmony const ButtonElement = React.forwardRef((props, ref) => ( ));
// Create ref to the DOM button: const ref = React.createRef();
It is preferred to use callback refs over findDOMNode()
API. Because findDOMNode()
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:
If you worked with React before, you might be familiar with an older API where the ref
attribute is a string, like ref={'textInput'}
, and the DOM node is accessed as this.refs.textInput
. We advise against it because string refs have below issues, and are considered legacy. String refs were removed in React v16.
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.
```jsx harmony 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 (
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.
The Virtual DOM works in three simple steps.
Whenever any underlying data changes, the entire UI is re-rendered in Virtual DOM representation.
Then the difference between the previous DOM representation and the new one is calculated.
Once the calculations are done, the real DOM will be updated with only the things that have actually changed.
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.
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,
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 name
input is accessed using ref.
```jsx harmony 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 ();
} } ```
In most cases, it’s recommend to use controlled components to implement forms.
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. Whereas cloneElement
is 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.
The component lifecycle has three distinct lifecycle phases:
Mounting: The component is ready to mount in the browser DOM. This phase covers initialization from constructor()
, getDerivedStateFromProps()
, render()
, and componentDidMount()
lifecycle methods.
Updating: In this phase, the component get updated in two ways, sending the new props and updating the state either from setState()
or forceUpdate()
. This phase covers getDerivedStateFromProps()
, shouldComponentUpdate()
, render()
, getSnapshotBeforeUpdate()
and componentDidUpdate()
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
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.
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, and componentWillUnmount()
for unmounting.
React 16.3+ Phases (or an interactive version)
Before React 16.3
React 16.3+
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.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.componentDidUpdate()
. This is useful to capture information from the DOM i.e. scroll position.shouldComponentUpdate()
returns false
.Before 16.3
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.shouldComponentUpdate()
which returns true.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.
HOC can be used for many use cases:
You can add/edit props passed to the component using props proxy pattern like this:
```jsx harmony 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} />;
}
}; } ```
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.
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 as children
prop.
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,
; }, });
ReactDOM.render(
The comments in React/JSX are similar to JavaScript Multiline comments but are wrapped in curly braces.
Single-line comments:
jsx harmony
 {/* Single-line comments(In vanilla JavaScript, the single-line comments are represented by double slash(//)) */} {`Welcome ${user}, let's play React`}

Multi-line comments:
jsx harmony
 {/* Multi-line comments for more than one line */} {`Welcome ${user}, let's play React`}

A child class constructor cannot make use of this
reference until super()
method has been called. The same applies for ES6 sub-classes as well. The main reason of passing props parameter to super()
call is to access this.props
in 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.props
is different only within the constructor. It would be the same outside the constructor.
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.
If you are using ES6 or the Babel transpiler to transform your JSX code then you can accomplish this with computed property names.
You need to make sure that function is not being called while passing the function as a parameter.
jsx harmony 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:
jsx harmony render() { // Correct: handleClick is passed as a reference! return <button onClick={this.handleClick}>{'Click Me'}</button> }
No, currently React.lazy
function 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,
// MoreComponents.js
export const SomeComponent = /* ... */;
export const UnusedComponent = /* ... */;
and reexport MoreComponents.js
components in an intermediate file IntermediateComponent.js
Now you can import the module using lazy function as below,
className
over class
attribute?class
is a keyword in JavaScript, and JSX is an extension of JavaScript. That’s the principal reason why React uses className
instead of class
. Pass a string as the className
prop.
jsx harmony render() { return {'Menu'} }
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.
jsx harmony render() { return ( <React.Fragment> <ChildA /> <ChildB /> <ChildC /> </React.Fragment> ) }
There is also a shorter syntax, but it’s not supported in many tools:
jsx harmony render() { return ( <> <ChildA /> <ChildB /> <ChildC /> </> ) }
Portal is a recommended way to render children into a DOM node that exists outside the DOM hierarchy of the parent component.
The first argument is any render-able React child, such as an element, string, or fragment. The second argument is a DOM element.
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 this
keyword altogether.
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
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.number
PropTypes.string
PropTypes.array
PropTypes.object
PropTypes.func
PropTypes.node
PropTypes.element
PropTypes.bool
PropTypes.symbol
PropTypes.any
We can define propTypes
for User
component as below:
```jsx harmony 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 ( <>Welcome, ${this.props.name}
}
<h2>{`Age, ${this.props.age}`}</h2>
</>
);
} } ```
Note: In React v15.5 PropTypes were moved from React.PropTypes
to prop-types
library.
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)
or static getDerivedStateFromError()
:
```jsx harmony 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; } return this.props.children; } } ```
After that use it as a regular component:
jsx harmony <ErrorBoundary> <MyWidget /> </ErrorBoundary>
React v15 provided very basic support for error boundaries using unstable_handleError
method. It has been renamed to componentDidCatch
in React v16.
Normally we use PropTypes library (React.PropTypes
moved to a prop-types
package 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.
react-dom
package?The react-dom
package 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()
react-dom
?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.
The ReactDOMServer
object 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 renderToString
to 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><title>My Page</title>"
);
res.write('
');
res.write(renderToString(<MyPage />));
res.write("
");
res.end();
});
The dangerouslySetInnerHTML
attribute is React’s replacement for using innerHTML
in the browser DOM. Just like innerHTML
, it is risky to use this attribute considering cross-site scripting (XSS) attacks. You just need to pass a __html
object as key and HTML text as value.
In this example MyComponent uses dangerouslySetInnerHTML
attribute for setting HTML markup:
```jsx harmony function createMarkup() { return { __html: “First · Second” }; }
function MyComponent() { return; } ```
The style
attribute 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.
```jsx harmony const divStyle = { color: “blue”, backgroundImage: “url(” + imgUrl + “)”, };
function HelloWorldComponent() { return; } ```
Style keys are camelCased in order to be consistent with accessing the properties on DOM nodes in JavaScript (e.g. node.style.backgroundImage
).
Handling events in React elements has some syntactic differences:
setState()
in constructor?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 use this.state
to initialize variables inside constructor.
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.
jsx harmony { 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.
jsx harmony { todos.map((todo) => <Todo {...todo} key={todo.id} />); }
setState()
in componentWillMount()
method?It is recommended to avoid async initialization in componentWillMount()
lifecycle method. componentWillMount()
is invoked immediately before mounting occurs. It is called before render()
, 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 in componentDidMount()
instead of componentWillMount()
.
jsx harmony componentDidMount() { axios.get(`api/todos`) .then((result) => { this.setState({ messages: [...result.data] }) }) }
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:
```jsx harmony class MyComponent extends React.Component { constructor(props) { super(props);
this.state = {
records: [],
inputValue: this.props.inputValue,
};
}
render() { return; } } ```
Using props inside render method will update the value:
```jsx harmony class MyComponent extends React.Component { constructor(props) { super(props);
this.state = {
record: [],
};
}
render() { return; } } ```
In some cases you want to render different components depending on some state. JSX does not render false
or undefined
, so you can use conditional short-circuiting to render a given part of your component only if a certain condition is true.
jsx harmony const MyComponent = ({ name, address }) => (

{name}
 {address &&
{address}
}
 );
If you need an if-else
condition then use ternary operator.
jsx harmony const MyComponent = ({ name, address }) => (

{name}
 {address ?
{address}
 :
{"Address is not available"}
}
 );
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 ...rest
operator, so it will add only required props. For example,
```jsx harmony const ComponentA = () => (
); ```
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.
```jsx harmony @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.
There are memoize libraries available which can be used on function components. For example moize
library can memoize the component in another component.
```jsx harmony import moize from “moize”; import Component from “./components/Component”; // this module exports a non-memoized component
const MemoizedFoo = moize.react(Component);
const Consumer = () => {{"I will memoize the following entry:"}
<MemoizedFoo />
; }; ```
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.
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.
```jsx harmony import ReactDOMServer from “react-dom/server”; import App from “./App”;
ReactDOMServer.renderToString(
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.
You should use Webpack’s DefinePlugin
method to set NODE_ENV
to production
, 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.
The create-react-app
CLI 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:
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()
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.
getDerivedStateFromProps()
lifecycle method?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, or null
to 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 of componentWillReceiveProps()
.
getSnapshotBeforeUpdate()
lifecycle method?The new getSnapshotBeforeUpdate()
lifecycle method is called right before DOM updates. The return value from this method will be passed as the third parameter to componentDidUpdate()
.
class MyComponent extends React.Component {
getSnapshotBeforeUpdate(prevProps, prevState) {
// ...
}
}
This lifecycle method along with componentDidUpdate()
covers all the use cases of componentWillUpdate()
.
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.
It is recommended to name the component by reference instead of using displayName
.
Using displayName
for naming component:
The recommended approach:
Recommended ordering of methods from mounting to render stage:
static
methodsconstructor()
getChildContext()
componentWillMount()
componentDidMount()
componentWillReceiveProps()
shouldComponentUpdate()
componentWillUpdate()
componentDidUpdate()
componentWillUnmount()
onClickSubmit()
or onChangeDescription()
getSelectReason()
or getFooterContent()
renderNavigation()
or renderProfilePicture()
render()
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 page
prop:
```jsx harmony 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, }; ```
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 after setState()
is called. That means you should not rely on the current state when calling setState()
since you can’t be sure what that state will be. The solution is to pass a function to setState()
, 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 of setState()
.
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.
React.StrictMode
is 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.
```jsx harmony import React from “react”;
function ExampleApplication() { return (
<React.StrictMode>

<ComponentOne />
<ComponentTwo />

</React.StrictMode>

); } ```
In the example above, the strict mode checks apply to <ComponentOne>
and <ComponentTwo>
components only.
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:
isMounted()
an anti-pattern and what is the proper solution?The primary use case for isMounted()
is to avoid calling setState()
after a component has been unmounted, because it will emit a warning.
Checking isMounted()
before calling setState()
does eliminate the warning, but it also defeats the purpose of the warning. Using isMounted()
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 in componentWillUnmount()
, prior to unmounting.
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:
onPointerDown
onPointerMove
onPointerUp
onPointerCancel
onGotPointerCapture
onLostPointerCaptur
onPointerEnter
onPointerLeave
onPointerOver
onPointerOut
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.
jsx harmony 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:
```jsx harmony class myComponent extends Component { render() { return; } }
export default myComponent; ```
While when imported in another file it should start with capital letter:
jsx harmony import MyComponent from "./MyComponent";
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:
jsx harmony <div mycustomattribute={"something"} />
Would render an empty div to the DOM with React v15:
In React v16 any unknown attributes will end up in the DOM:
This is useful for supplying browser-specific non-standard attributes, trying new DOM APIs, and integrating with opinionated third-party libraries.
You should initialize state in the constructor when using ES6 classes, and getInitialState()
method when using React.createClass()
.
Using ES6 classes:
class MyComponent extends React.Component {
constructor(props) {
super(props);
this.state = {
/* initial state */
};
}
}
Using React.createClass()
:
Note: React.createClass()
is deprecated and removed in React v16. Use plain JavaScript classes instead.
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 calling forceUpdate()
.
It is recommended to avoid all uses of forceUpdate()
and only read from this.props
and this.state
in render()
.
super()
and super(props)
in React using ES6 classes?When you want to access this.props
in constructor()
then you should pass props to super()
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 for this.props
.
You can simply use Array.prototype.map
with ES6 arrow function syntax. For example, the items
array of objects is mapped into an array of components:
jsx harmony <tbody> {items.map((item) => ( <SomeComponent key={item.id} name={item.name} /> ))} </tbody>
You can’t iterate using for
loop:
jsx harmony <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 do
expressions which are stage 1 proposal.
React (or JSX) doesn’t support variable interpolation inside an attribute value. The below representation won’t work:
jsx harmony <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:
jsx harmony <img className="image" src={"images/" + this.props.image} />
Using template strings will also work:
jsx harmony <img className="image" src={`images/${this.props.image}`} />
If you want to pass an array of objects to a component with a particular shape then use React.PropTypes.shape()
as an argument to React.PropTypes.arrayOf()
.
You shouldn’t use curly braces inside quotes because it is going to be evaluated as a string.
jsx harmony <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):
jsx harmony <div className={'btn-panel ' + (this.props.visible ? 'show' : 'hidden')}>
Template strings will also work:
jsx harmony <div className={`btn-panel ${this.props.visible ? 'show' : 'hidden'}`}>
The react
package contains React.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. The react-dom
package contains ReactDOM.render()
, and in react-dom/server
we have server-side rendering support with ReactDOMServer.renderToString()
and ReactDOMServer.renderToStaticMarkup()
.
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
, and react-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: react
and react-dom
. This paves the way to writing components that can be shared between the web version of React and React Native.
If you try to render a <label>
element bound to a text input using the standard for
attribute, then it produces HTML missing that attribute and prints a warning to the console.
jsx harmony <label for={'user'}>{'User'}</label> <input type={'text'} id={'user'} />
Since for
is a reserved keyword in JavaScript, use htmlFor
instead.
jsx harmony <label htmlFor={'user'}>{'User'}</label> <input type={'text'} id={'user'} />
You can use spread operator in regular React:
jsx harmony <button style={{ ...styles.panel.button, ...styles.panel.submitButton }}> {"Submit"} </button>
If you’re using React Native then you can use the array notation:
jsx harmony <button style={[styles.panel.button, styles.panel.submitButton]}> {"Submit"} </button>
You can listen to the resize
event in componentDidMount()
and then update the dimensions (width
and height
). You should remove the listener in componentWillUnmount()
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>
);
}
}
setState()
and replaceState()
methods?setState()
the current and previous states are merged. replaceState()
throws out the current state, and replaces it with only what you provide. Usually setState()
is used unless you really need to remove all previous keys for some reason. You can also set state to false
/null
in setState()
instead of using replaceState()
.The following lifecycle methods will be called when state changes. You can compare provided state and props values with current state and props to determine if something meaningful changed.
componentWillUpdate(object nextProps, object nextState)
componentDidUpdate(object prevProps, object prevState)
The better approach is to use Array.prototype.filter()
method.
For example, let’s create a removeItem()
method for updating the state.
It is possible with latest version (>=16.2). Below are the possible options:
jsx harmony render() { return false }
jsx harmony render() { return null }
jsx harmony render() { return [] }
jsx harmony render() { return <React.Fragment></React.Fragment> }
jsx harmony render() { return <></> }
Returning undefined
won’t work.
We can use <pre>
tag so that the formatting of the JSON.stringify()
is retained:
```jsx harmony const data = { name: “John”, age: 42 };
class User extends React.Component { render() { return{JSON.stringify(data, null, 2)}
; } }
React.render(
You can do it by creating ref for input
element and using it in componentDidMount()
:
```jsx harmony class App extends React.Component { componentDidMount() { this.nameInput.focus(); }
render() { return ( <input defaultValue={"Won't focus"} />
<input
ref={(input) => (this.nameInput = input)}
defaultValue={"Will focus"}
/>
</div>
);
} }
ReactDOM.render(
Calling setState()
with an object to merge with state:
Using Object.assign()
to create a copy of the object:
Using spread operator:
Calling setState()
with a function:
setState()
?React may batch multiple setState()
calls into a single update for performance. Because this.props
and this.state
may be updated asynchronously, you should not rely on their values for calculating the next state.
This counter example will fail to update as expected:
The preferred approach is to call setState()
with function rather than object. That function will receive the previous state as the first argument, and the props at the time the update is applied as the second argument.
You can use React.version
to get the version.
```jsx harmony const REACT_VERSION = React.version;
ReactDOM.render(React version: ${REACT_VERSION}
}
, document.getElementById(“app”) ); ```
create-react-app
?Manual import from core-js
:
Create a file called (something like) polyfills.js
and import it into root index.js
file. Run npm install core-js
or yarn add core-js
and import your specific required features.
Using Polyfill service:
Use the polyfill.io CDN to retrieve custom, browser-specific polyfills by adding this line to index.html
:
In the above script we had to explicitly request the Array.prototype.includes
feature as it is not included in the default feature set.
You just need to use HTTPS=true
configuration. You can edit your package.json
scripts section:
or just run set HTTPS=true && npm start
Create a file called .env
in the project root and write the import path:
NODE_PATH=src/app
After that restart the development server. Now you should be able to import anything inside src/app
without relative paths.
Add a listener on the history
object to record each page view:
You need to use setInterval()
to trigger the change, but you also need to clear the timer when the component unmounts to prevent errors and memory leaks.
React does not apply vendor prefixes automatically. You need to add vendor prefixes manually.
jsx harmony <div style={{ transform: "rotate(90deg)", WebkitTransform: "rotate(90deg)", // note the capital 'W' here msTransform: "rotate(90deg)", // 'ms' is the only lowercase vendor prefix }} />
You should use default for exporting the components
```jsx harmony import React from “react”; import User from “user”;
export default class MyProfile extends React.Component { render() { return
With the export specifier, the MyProfile is going to be the member and exported to this module and the same can be imported without mentioning the name in other components.
The component names should start with a uppercase letter but there are few exceptions on this convention. The lowercase tag names with a dot (property accessors) are still considered as valid component names. For example the below tag can be compiled to a valid component,
You can use ES7 static
field to define constant.
Static fields are part of the Class Fields stage 3 proposal.
You could use the ref prop to acquire a reference to the underlying HTMLInputElement
object through a callback, store the reference as a class property, then use that reference to later trigger a click from your event handlers using the HTMLElement.click
method. This can be done in two steps:
Create ref in render method:
jsx harmony <input ref={(input) => (this.inputElement = input)} />
Apply click event in your event handler:
async
/await
in React, you will need Babel and transform-async-to-generator plugin. React Native ships with Babel and a set of transforms.There are two common practices for React project file structure.
Grouping by features or routes:
One common way to structure projects is locate CSS, JS, and tests together, grouped by feature or route.
common/
├─ Avatar.js
├─ Avatar.css
├─ APIUtils.js
└─ APIUtils.test.js
feed/
├─ index.js
├─ Feed.js
├─ Feed.css
├─ FeedStory.js
├─ FeedStory.test.js
└─ FeedAPI.js
profile/
├─ index.js
├─ Profile.js
├─ ProfileHeader.js
├─ ProfileHeader.css
└─ ProfileAPI.js
Grouping by file type:
Another popular way to structure projects is to group similar files together.
api/
├─ APIUtils.js
├─ APIUtils.test.js
├─ ProfileAPI.js
└─ UserAPI.js
components/
├─ Avatar.js
├─ Avatar.css
├─ Feed.js
├─ Feed.css
├─ FeedStory.js
├─ FeedStory.test.js
├─ Profile.js
├─ ProfileHeader.js
└─ ProfileHeader.css
It is recommended to avoid hard coding style values in components. Any values that are likely to be used across different UI components should be extracted into their own modules.
For example, these styles could be extracted into a separate component:
And then imported individually in other components:
eslint-plugin-react
. By default, it will check a number of best practices, with rules checking things from keys in iterators to a complete set of prop types. Another popular plugin is eslint-plugin-jsx-a11y
, which will help fix common issues with accessibility. As JSX offers slightly different syntax to regular HTML, issues with alt
text and tabindex
, for example, will not be picked up by regular plugins.You can use AJAX libraries such as Axios, jQuery AJAX, and the browser built-in fetch
. You should fetch data in the componentDidMount()
lifecycle method. This is so you can use setState()
to update your component when the data is retrieved.
For example, the employees list fetched from API and set local state:
```jsx harmony class MyComponent extends React.Component { constructor(props) { super(props); this.state = { employees: [], error: null, }; }
componentDidMount() { fetch(“https://api.example.com/items”) .then((res) => res.json()) .then( (result) => { this.setState({ employees: result.employees, }); }, (error) => { this.setState({ error }); } ); }
render() { const { error, employees } = this.state; if (error) { return );
}
} } ```
Render Props is a simple technique for sharing code between components using a prop whose value is a function. The below component uses render prop which returns a React element.
jsx harmony <DataProvider render={(data) =>
{`Hello ${data.target}`}
} />
Libraries such as React Router and DownShift are using this pattern.
history
library which handles interaction with the browser’s window.history
with its browser and hash histories. It also provides memory history which is useful for environments that don’t have global history, such as mobile app development (React Native) and unit testing with Node.<Router>
components of React Router v4?React Router v4 provides below 3 <Router>
components:
<BrowserRouter>
<HashRouter>
<MemoryRouter>
The above components will create browser, hash, and memory history instances. React Router v4 makes the properties and methods of the history
instance associated with your router available through the context in the router
object.
push()
and replace()
methods of history
?A history instance has two methods for navigation purpose.
push()
replace()
If you think of the history as an array of visited locations, push()
will add a new location to the array and replace()
will replace the current location in the array with the new one.
There are three different ways to achieve programmatic routing/navigation within components.
Using the withRouter()
higher-order function:
The withRouter()
higher-order function will inject the history object as a prop of the component. This object provides push()
and replace()
methods to avoid the usage of context.
```jsx harmony import { withRouter } from “react-router-dom”; // this also works with ‘react-router-native’
const Button = withRouter(({ history }) => ( <button type=“button” onClick={() => { history.push(“/new-location”); }} > {“Click Me!”} )); ```
Using <Route>
component and render props pattern:
The <Route>
component passes the same props as withRouter()
, so you will be able to access the history methods through the history prop.
```jsx harmony import { Route } from “react-router-dom”;
const Button = () => ( <Route render={({ history }) => ( <button type=“button” onClick={() => { history.push(“/new-location”); }} > {“Click Me!”} )} /> ); ```
Using context:
This option is not recommended and treated as unstable API.
```jsx harmony const Button = (props, context) => ( <button type=“button” onClick={() => { context.history.push(“/new-location”); }} > {“Click Me!”} );
Button.contextTypes = { history: React.PropTypes.shape({ push: React.PropTypes.func.isRequired, }), }; ```
The ability to parse query strings was taken out of React Router v4 because there have been user requests over the years to support different implementation. So the decision has been given to users to choose the implementation they like. The recommended approach is to use query strings library.
const queryString = require("query-string");
const parsed = queryString.parse(props.location.search);
You can also use URLSearchParams
if you want something native:
You should use a polyfill for IE11.
You have to wrap your Route’s in a <Switch>
block because <Switch>
is unique in that it renders a route exclusively.
At first you need to add Switch
to your imports:
Then define the routes within <Switch>
block:
jsx harmony <Router> <Switch> <Route {/* ... */} /> <Route {/* ... */} /> </Switch> </Router>
history.push
method in React Router v4?While navigating you can pass props to the history
object:
this.props.history.push({
pathname: "/template",
search: "?name=sudheer",
state: { detail: response.data },
});
The search
property is used to pass query params in push()
method.
A <Switch>
renders the first child <Route>
that matches. A <Route>
with no path always matches. So you just need to simply drop path attribute as below
jsx harmony <Switch> <Route exact path="/" component={Home} /> <Route path="/user" component={User} /> <Route component={NotFound} /> </Switch>
Create a module that exports a history
object and import this module across the project.
For example, create history.js
file:
You should use the <Router>
component instead of built-in routers. Imported the above history.js
inside index.js
file:
```jsx harmony import { Router } from “react-router-dom”; import history from “./history”; import App from “./App”;
ReactDOM.render(
You can also use push method of history
object similar to built-in history object:
The react-router
package provides <Redirect>
component in React Router. Rendering a <Redirect>
will navigate to a new location. Like server-side redirects, the new location will override the current location in the history stack.
The library provides two ways to format strings, numbers, and dates: react components or an API.
jsx harmony <FormattedMessage id={"account"} defaultMessage={"The amount is less than minimum balance."} />
<FormattedMessage>
as placeholder using React Intl?The <Formatted... />
components from react-intl
return elements, not plain text, so they can’t be used for placeholders, alt text, etc. In that case, you should use lower level API formatMessage()
. You can inject the intl
object into your component using injectIntl()
higher-order component and then format the message using formatMessage()
available on that object.
```jsx harmony import React from “react”; import { injectIntl, intlShape } from “react-intl”;
const MyComponent = ({ intl }) => { const placeholder = intl.formatMessage({ id: “messageId” }); return ; };
MyComponent.propTypes = { intl: intlShape.isRequired, };
export default injectIntl(MyComponent); ```
You can get the current locale in any component of your application using injectIntl()
:
```jsx harmony import { injectIntl, intlShape } from “react-intl”;
const MyComponent = ({ intl }) => (The current locale is ${intl.locale}
}
);
MyComponent.propTypes = { intl: intlShape.isRequired, };
export default injectIntl(MyComponent); ```
The injectIntl()
higher-order component will give you access to the formatDate()
method via the props in your component. The method is used internally by instances of FormattedDate
and it returns the string representation of the formatted date.
```jsx harmony import { injectIntl, intlShape } from “react-intl”;
const stringDate = this.props.intl.formatDate(date, { year: “numeric”, month: “numeric”, day: “numeric”, });
const MyComponent = ({ intl }) => (The formatted date is ${stringDate}
}
);
MyComponent.propTypes = { intl: intlShape.isRequired, };
export default injectIntl(MyComponent); ```
Shallow rendering is useful for writing unit test cases in React. It lets you render a component one level deep and assert facts about what its render method returns, without worrying about the behavior of child components, which are not instantiated or rendered.
For example, if you have the following component:
function MyComponent() {
return (
<div>
<span className={"heading"}>{"Title"}</span>
<span className={"description"}>{"Description"}</span>
</div>
);
}
Then you can assert as follows:
```jsx harmony import ShallowRenderer from “react-test-renderer/shallow”;
// in your test const renderer = new ShallowRenderer(); renderer.render(
const result = renderer.getRenderOutput();
expect(result.type).toBe(“div”); expect(result.props.children).toEqual([ {“Title”}, {“Description”},]); ```
TestRenderer
package in React?This package provides a renderer that can be used to render components to pure JavaScript objects, without depending on the DOM or a native mobile environment. This package makes it easy to grab a snapshot of the platform view hierarchy (similar to a DOM tree) rendered by a ReactDOM or React Native without using a browser or jsdom
.
```jsx harmony import TestRenderer from “react-test-renderer”;
const Link = ({ page, children }) => {children};
const testRenderer = TestRenderer.create( {“Facebook”} );
console.log(testRenderer.toJSON()); // { // type: ‘a’, // props: { href: ‘https://www.facebook.com/’ }, // children: [ ‘Facebook’ ] // } ```
with-addons
package and allow you to perform actions against a simulated DOM for the purpose of unit testing.jsdom
environment. It’s often used for testing components.There are couple of advantages compared to Jasmine:
jsdom
) so that your tests can be run on the command line.Let’s write a test for a function that adds two numbers in sum.js
file:
Create a file named sum.test.js
which contains actual test:
And then add the following section to your package.json
:
Finally, run yarn test
or npm test
and Jest will print a result:
$ yarn test
PASS ./sum.test.js
✓ adds 1 + 2 to equal 3 (2ms)
Flux is an application design paradigm used as a replacement for the more traditional MVC pattern. It is not a framework or a library but a new kind of architecture that complements React and the concept of Unidirectional Data Flow. Facebook uses this pattern internally when working with React.
The workflow between dispatcher, stores and views components with distinct inputs and outputs as follows:
Redux follows three fundamental principles:
Instead of saying downsides we can say that there are few compromises of using Redux over Flux. Those are as follows:
redux-immutable-state-invariant
, Immutable.js, or instructing your team to write non-mutating code.mapStateToProps()
and mapDispatchToProps()
?mapStateToProps()
is a utility which helps your component get updated state (which is updated by some other components):
const mapStateToProps = (state) => {
return {
todos: getVisibleTodos(state.todos, state.visibilityFilter),
};
};
mapDispatchToProps()
is a utility which will help your component to fire an action event (dispatching action which may cause change of application state):
const mapDispatchToProps = (dispatch) => {
return {
onTodoClick: (id) => {
dispatch(toggleTodo(id));
},
};
};
Recommend always using the “object shorthand” form for the mapDispatchToProps
Redux wrap it in another function that looks like (…args) => dispatch(onTodoClick(…args)), and pass that wrapper function as a prop to your component.
You just need to export the store from the module where it created with createStore()
. Also, it shouldn’t pollute the global window object.
These libraries are very different for very different purposes, but there are some vague similarities.
Redux is a tool for managing state throughout the application. It is usually used as an architecture for UIs. Think of it as an alternative to (half of) Angular. RxJS is a reactive programming library. It is usually used as a tool to accomplish asynchronous tasks in JavaScript. Think of it as an alternative to Promises. Redux uses the Reactive paradigm because the Store is reactive. The Store observes actions from a distance, and changes itself. RxJS also uses the Reactive paradigm, but instead of being an architecture, it gives you basic building blocks, Observables, to accomplish this pattern.
You can dispatch an action in componentDidMount()
method and in render()
method you can verify the data.
class App extends Component {
componentDidMount() {
this.props.fetchData();
}
render() {
return this.props.isLoaded ? (
<div>{"Loaded"}</div>
) : (
<div>{"Not Loaded"}</div>
);
}
}
const mapStateToProps = (state) => ({
isLoaded: state.isLoaded,
});
const mapDispatchToProps = { fetchData };
export default connect(mapStateToProps, mapDispatchToProps)(App);
connect()
from React Redux?You need to follow two steps to use your store in your container:
mapStateToProps()
: It maps the state variables from your store to the props that you specify.Connect the above props to your container: The object returned by the mapStateToProps
function is connected to the container. You can import connect()
from react-redux
.
```jsx harmony import React from “react”; import { connect } from “react-redux”;
class App extends React.Component { render() { return; } }
function mapStateToProps(state) { return { containerData: state.data }; }
export default connect(mapStateToProps)(App); ```
You need to write a root reducer in your application which delegate handling the action to the reducer generated by combineReducers()
.
For example, let us take rootReducer()
to return the initial state after USER_LOGOUT
action. As we know, reducers are supposed to return the initial state when they are called with undefined
as the first argument, no matter the action.
const appReducer = combineReducers({
/* your app's top-level reducers */
});
const rootReducer = (state, action) => {
if (action.type === "USER_LOGOUT") {
state = undefined;
}
return appReducer(state, action);
};
In case of using redux-persist
, you may also need to clean your storage. redux-persist
keeps a copy of your state in a storage engine. First, you need to import the appropriate storage engine and then, to parse the state before setting it to undefined and clean each storage state key.
at
symbol in the Redux connect decorator?The **@** symbol is in fact a JavaScript expression used to signify decorators. Decorators make it possible to annotate and modify classes and properties at design time.
Let’s take an example setting up Redux without and with a decorator.
Without decorator:
import React from "react";
import * as actionCreators from "./actionCreators";
import { bindActionCreators } from "redux";
import { connect } from "react-redux";
function mapStateToProps(state) {
return { todos: state.todos };
}
function mapDispatchToProps(dispatch) {
return { actions: bindActionCreators(actionCreators, dispatch) };
}
class MyApp extends React.Component {
// ...define your main app here
}
export default connect(mapStateToProps, mapDispatchToProps)(MyApp);
With decorator:
import React from "react";
import * as actionCreators from "./actionCreators";
import { bindActionCreators } from "redux";
import { connect } from "react-redux";
function mapStateToProps(state) {
return { todos: state.todos };
}
function mapDispatchToProps(dispatch) {
return { actions: bindActionCreators(actionCreators, dispatch) };
}
@connect(mapStateToProps, mapDispatchToProps)
export default class MyApp extends React.Component {
// ...define your main app here
}
The above examples are almost similar except the usage of decorator. The decorator syntax isn’t built into any JavaScript runtimes yet, and is still experimental and subject to change. You can use babel for the decorators support.
You can use redux-thunk
middleware which allows you to define async actions.
Let’s take an example of fetching specific account as an AJAX call using fetch API:
export function fetchAccount(id) {
return (dispatch) => {
dispatch(setLoadingAccountState()); // Show a loading spinner
fetch(`/account/${id}`, (response) => {
dispatch(doneFetchingAccount()); // Hide loading spinner
if (response.status === 200) {
dispatch(setAccount(response.json)); // Use a normal function to set the received state
} else {
dispatch(someError);
}
});
};
}
function setAccount(data) {
return { type: "SET_Account", data: data };
}
The best way to access your store in a component is to use the connect()
function, that creates a new component that wraps around your existing one. This pattern is called Higher-Order Components, and is generally the preferred way of extending a component’s functionality in React. This allows you to map state and action creators to your component, and have them passed in automatically as your store updates.
Let’s take an example of <FilterLink>
component using connect:
import { connect } from "react-redux";
import { setVisibilityFilter } from "../actions";
import Link from "../components/Link";
const mapStateToProps = (state, ownProps) => ({
active: ownProps.filter === state.visibilityFilter,
});
const mapDispatchToProps = (dispatch, ownProps) => ({
onClick: () => dispatch(setVisibilityFilter(ownProps.filter)),
});
const FilterLink = connect(mapStateToProps, mapDispatchToProps)(Link);
export default FilterLink;
Due to it having quite a few performance optimizations and generally being less likely to cause bugs, the Redux developers almost always recommend using connect()
over accessing the store directly (using context API).
Component is a class or function component that describes the presentational part of your application.
Container is an informal term for a component that is connected to a Redux store. Containers subscribe to Redux state updates and dispatch actions, and they usually don’t render DOM elements; they delegate rendering to presentational child components.
Constants allows you to easily find all usages of that specific functionality across the project when you use an IDE. It also prevents you from introducing silly bugs caused by typos – in which case, you will get a ReferenceError
immediately.
Normally we will save them in a single file (constants.js
or actionTypes.js
).
export const ADD_TODO = "ADD_TODO";
export const DELETE_TODO = "DELETE_TODO";
export const EDIT_TODO = "EDIT_TODO";
export const COMPLETE_TODO = "COMPLETE_TODO";
export const COMPLETE_ALL = "COMPLETE_ALL";
export const CLEAR_COMPLETED = "CLEAR_COMPLETED";
In Redux you use them in two places:
During action creation:
Let’s take actions.js
:
In reducers:
Let’s create reducer.js
:
mapDispatchToProps()
?There are a few ways of binding action creators to dispatch()
in mapDispatchToProps()
. Below are the possible options:
The third option is just a shorthand for the first one.
ownProps
parameter in mapStateToProps()
and mapDispatchToProps()
?If the ownProps
parameter is specified, React Redux will pass the props that were passed to the component into your connect functions. So, if you use a connected component:
```jsx harmony import ConnectedComponent from “./containers/ConnectedComponent”;
The ownProps
inside your mapStateToProps()
and mapDispatchToProps()
functions will be an object:
You can use this object to decide what to return from those functions.
Most of the applications has several top-level directories as below:
This structure works well for small and medium size apps.
redux-saga
is a library that aims to make side effects (asynchronous things like data fetching and impure things like accessing the browser cache) in React/Redux applications easier and better.
It is available in NPM:
$ npm install --save redux-saga
redux-saga
is a redux middleware, which means this thread can be started, paused and cancelled from the main application with normal Redux actions, it has access to the full Redux application state and it can dispatch Redux actions as well.call()
and put()
in redux-saga?Both call()
and put()
are effect creator functions. call()
function is used to create effect description, which instructs middleware to call the promise. put()
function creates an effect, which instructs middleware to dispatch an action to the store.
Let’s take example of how these effects work for fetching particular user data.
function* fetchUserSaga(action) {
// `call` function accepts rest arguments, which will be passed to `api.fetchUser` function.
// Instructing middleware to call promise, it resolved value will be assigned to `userData` variable
const userData = yield call(api.fetchUser, action.userId);
// Instructing middleware to dispatch corresponding action.
yield put({
type: "FETCH_USER_SUCCESS",
userData,
});
}
dispatch()
and getState()
as parameters.redux-saga
and redux-thunk
?persistState()
store enhancer, you can persist debug sessions across page reloads.Selectors are functions that take Redux state as an argument and return some data to pass to the component.
For example, to get user details from the state:
You can use applyMiddleware()
.
For example, you can add redux-thunk
and logger
passing them as arguments to applyMiddleware()
:
You need to pass initial state as second argument to createStore:
React is a JavaScript library, supporting both front end web and being run on the server, for building user interfaces and web applications.
React Native is a mobile framework that compiles to native app components, allowing you to build native mobile applications (iOS, Android, and Windows) in JavaScript that allows you to use React to build your components, and implements React under the hood.
You can use console.log
, console.warn
, etc. As of React Native v0.29 you can simply run the following to see logs in the console:
$ react-native log-ios
$ react-native log-android
Follow the below steps to debug React Native app:
Command + D
and a webpage should open up at http://localhost:8081/debugger-ui
.Command + Option + I
to open the Chrome Developer tools, or open it via View
-> Developer
-> Developer Tools
.Reselect is a selector library (for Redux) which uses memoization concept. It was originally written to compute derived data from Redux-like applications state, but it can’t be tied to any architecture or library.
Reselect keeps a copy of the last inputs/outputs of the last call, and recomputes the result only if one of the inputs changes. If the the same inputs are provided twice in a row, Reselect returns the cached output. It’s memoization and cache are fully customizable.
null
, unlike most type systems.The below steps followed to include Font Awesome in React:
font-awesome
:$ npm install --save font-awesome
font-awesome
in your index.js
file:className
:React Developer Tools let you inspect the component hierarchy, including component props and state. It exists both as a browser extension (for Chrome and Firefox), and as a standalone app (works with other environments including Safari, IE, and React Native).
The official extensions available for different browsers or environments.
file://...
) then you must first open Chrome Extensions and check Allow access to file URLs
.Create a Polymer element:
jsx harmony <link rel="import" href="../../bower_components/polymer/polymer.html" />; Polymer({ is: "calender-element", ready: function () { this.textContent = "I am a calender"; }, });
Create the Polymer component HTML tag by importing it in a HTML document, e.g. import it in the index.html
of your React application:
React has the following advantages over Vue.js:
| React | Angular |
|---|---|
| React is a library and has only the View layer | Angular is a framework and has complete MVC functionality |
| React handles rendering on the server side | AngularJS renders only on the client side but Angular 2 and above renders on the server side |
| React uses JSX that looks like HTML in JS which can be confusing | Angular follows the template approach for HTML, which makes code shorter and easy to understand |
| React Native, which is a React type to build mobile applications are faster and more stable | Ionic, Angular’s mobile native app is relatively less stable and slower |
| In React, data flows only in one way and hence debugging is easy | In Angular, data flows both way i.e it has two-way data binding between children and parent and hence debugging is often difficult |
__REACT_DEVTOOLS_GLOBAL_HOOK__
, then React communicates with that hook during initialization. If the website is not using React or if React fails to communicate with DevTools then it won’t show up the tab.styled-components
is a JavaScript library for styling React applications. It removes the mapping between styles and components, and lets you write actual CSS augmented with JavaScript.Lets create <Title>
and <Wrapper>
components with specific styles for each.
import React from "react";
import styled from "styled-components";
// Create a <Title> component that renders an
 which is centered, red and sized at 1.5em
const Title = styled.h1`
font-size: 1.5em;
text-align: center;
color: palevioletred;
`;
// Create a <Wrapper> component that renders a
 with some padding and a papayawhip background
const Wrapper = styled.section`
padding: 4em;
background: papayawhip;
`;
These two variables, Title
and Wrapper
, are now components that you can render just like any other react component.
jsx harmony <Wrapper> <Title>{"Lets start first styled component!"}</Title> </Wrapper>
create-react-app
application?Starting from<EMAIL_ADDRESS>or higher, there is a built-in support for typescript. You can just pass --typescript
option as below
But for lower versions of react scripts, just supply --scripts-version
option as react-scripts-ts
while you create a new project. react-scripts-ts
is a set of adjustments to take the standard create-react-app
project pipeline and bring TypeScript into the mix.
Now the project layout should look like the following:
my-app/
├─ .gitignore
├─ images.d.ts
├─ node_modules/
├─ public/
├─ src/
│ └─ ...
├─ package.json
├─ tsconfig.json
├─ tsconfig.prod.json
├─ tsconfig.test.json
└─ tslint.json
Let’s take calculations and different amounts of a shipment order with the simplified usage of Reselect:
import { createSelector } from "reselect";
const shopItemsSelector = (state) => state.shop.items;
const taxPercentSelector = (state) => state.shop.taxPercent;
const subtotalSelector = createSelector(shopItemsSelector, (items) =>
items.reduce((acc, item) => acc + item.value, 0)
);
const taxSelector = createSelector(
subtotalSelector,
taxPercentSelector,
(subtotal, taxPercent) => subtotal * (taxPercent / 100)
);
export const totalSelector = createSelector(
subtotalSelector,
taxSelector,
(subtotal, tax) => ({ total: subtotal + tax })
);
let exampleState = {
shop: {
taxPercent: 8,
items: [
{ name: "apple", value: 1.2 },
{ name: "orange", value: 0.95 },
],
},
};
console.log(subtotalSelector(exampleState)); // 2.15
console.log(taxSelector(exampleState)); // 0.172
console.log(totalSelector(exampleState)); // { total: 2.322 }
Actions are plain JavaScript objects or payloads of information that send data from your application to your store. They are the only source of information for the store. Actions must have a type property that indicates the type of action being performed.
For example an example action which represents adding a new todo item:
{
type: ADD_TODO,
text: 'Add todo item'
}
No, statics
only works with React.createClass()
:
But you can write statics inside ES6+ classes or writing them outside class as below,
initialValues
get updated from state?You need to add enableReinitialize : true
setting.
const InitializeFromStateForm = reduxForm({
form: "initializeFromState",
enableReinitialize: true,
})(UserEdit);
If your initialValues
prop gets updated, your form will update too.
You can use oneOfType()
method of PropTypes
.
For example, the height property can be defined with either string
or number
type as below:
You can import SVG directly as component instead of loading it as a file. This feature is available with<EMAIL_ADDRESS>and higher.
```jsx harmony import { ReactComponent as Logo } from “./logo.svg”;
const App = () => ({/* Logo is an actual react component */}
<Logo />
); ```
Note: Don’t forget about the curly braces in the import.
If the ref callback is defined as an inline function, it will get called twice during updates, first with null and then again with the DOM element. This is because a new instance of the function is created with each render, so React needs to clear the old ref and set up the new one.
class UserForm extends Component {
handleSubmit = () => {
console.log("Input Value is: ", this.input.value);
};
render() {
return (
<form onSubmit={this.handleSubmit}>
<input type="text" ref={(input) => (this.input = input)} /> //
Access DOM input in handle submit
<button type="submit">Submit</button>
</form>
);
}
}
But our expectation is for the ref callback to get called once, when the component mounts. One quick fix is to use the ES7 class property syntax to define the function
class UserForm extends Component {
handleSubmit = () => {
console.log("Input Value is: ", this.input.value);
};
setSearchInput = (input) => {
this.input = input;
};
render() {
return (
<form onSubmit={this.handleSubmit}>
<input type="text" ref={this.setSearchInput} /> // Access DOM input
in handle submit
<button type="submit">Submit</button>
</form>
);
}
}
There are two main ways of implementing HOCs in React. 1. Props Proxy (PP) and 2. Inheritance Inversion (II). They follow different approaches for manipulating the WrappedComponent.
Props Proxy
In this approach, the render method of the HOC returns a React Element of the type of the WrappedComponent. We also pass through the props that the HOC receives, hence the name Props Proxy.
function ppHOC(WrappedComponent) {
return class PP extends React.Component {
render() {
return <WrappedComponent {...this.props} />;
}
};
}
Inheritance Inversion
In this approach, the returned HOC class (Enhancer) extends the WrappedComponent. It is called Inheritance Inversion because instead of the WrappedComponent extending some Enhancer class, it is passively extended by the Enhancer. In this way the relationship between them seems inverse.
function iiHOC(WrappedComponent) {
return class Enhancer extends WrappedComponent {
render() {
return super.render();
}
};
}
You should be passing the numbers via curly braces({}) where as strings inn quotes
React.render(
<User age={30} department={"IT"} />,
document.getElementById("container")
);
It is up to developer decision. i.e, It is developer job to determine what kinds of state make up your application, and where each piece of state should live. Some users prefer to keep every single piece of data in Redux, to maintain a fully serializable and controlled version of their application at all times. Others prefer to keep non-critical or UI state, such as “is this dropdown currently open”, inside a component’s internal state.
Below are the thumb rules to determine what kind of data should be put into Redux
React creates a service worker for you without any configuration by default. The service worker is a web API that helps you cache your assets and other files so that when the user is offline or on slow network, he/she can still see results on the screen, as such, it helps you build a better user experience, that’s what you should know about service worker’s for now. It’s all about adding offline capabilities to your site.
import React from "react";
import ReactDOM from "react-dom";
import App from "./App";
import registerServiceWorker from "./registerServiceWorker";
ReactDOM.render(<App />, document.getElementById("root"));
registerServiceWorker();
Class components can be restricted from rendering when their input props are the same using PureComponent or shouldComponentUpdate. Now you can do the same with function components by wrapping them in React.memo.
const MyComponent = React.memo(function MyComponent(props) {
/* only rerenders if props change */
});
The React.lazy function lets you render an dynamic import as a regular component. It will automatically load the bundle containing the OtherComponent when the component gets rendered. This must return a Promise which resolves to a module with a default export containing a React component.
const OtherComponent = React.lazy(() => import("./OtherComponent"));
function MyComponent() {
return (

<OtherComponent />

);
}
Note: React.lazy and Suspense is not yet available for server-side rendering. If you want to do code-splitting in a server rendered app, we still recommend React Loadable.
You can compare current value of the state with an existing state value and decide whether to rerender the page or not. If the values are same then you need to return null to stop rerendering otherwise return the latest state value. For example, the user profile information is conditionally rendered as follows,
getUserProfile = (user) => {
const latestAddress = user.address;
this.setState((state) => {
if (state.address === latestAddress) {
return null;
} else {
return { title: latestAddress };
}
});
};
Arrays: Unlike older releases, you don’t need to make sure render method return a single element in React16. You are able to return multiple sibling elements without a wrapping element by returning an array. For example, let us take the below list of developers,
const ReactJSDevs = () => {
return [

John
,

Jackie
,

Jordan
,
];
};
You can also merge this array of items in another array component
const JSDevs = () => {
return (

Brad

Brodge

<ReactJSDevs />

Brandon

);
};
Strings and Numbers: You can also return string and number type from the render method
render() {
return 'Welcome to ReactJS questions';
}
// Number
render() {
return 2018;
}
React Class Components can be made much more concise using the class field declarations. You can initialize local state without using the constructor and declare class methods by using arrow functions without the extra need to bind them. Let’s take a counter example to demonstrate class field declarations for state without using constructor and methods without binding,
class Counter extends Component {
state = { value: 0 };
handleIncrement = () => {
this.setState((prevState) => ({
value: prevState.value + 1,
}));
};
handleDecrement = () => {
this.setState((prevState) => ({
value: prevState.value - 1,
}));
};
render() {
return (

{this.state.value}
<button onClick={this.handleIncrement}>+</button>
<button onClick={this.handleDecrement}>-</button>

);
}
}
Hooks is a new feature that lets you use state and other React features without writing a class. Let’s see an example of useState hook example,
import { useState } from "react";
function Example() {
// Declare a new state variable, which we'll call "count"
const [count, setCount] = useState(0);
return (

You clicked {count} times

<button onClick={() => setCount(count + 1)}>Click me</button>

);
}
You need to follow two rules inorder to use hooks
React team released an ESLint plugin called eslint-plugin-react-hooks that enforces these two rules. You can add this plugin to your project using the below command,
And apply the below config in your ESLint config file,
// Your ESLint configuration
{
"plugins": [
// ...
"react-hooks"
],
"rules": {
// ...
"react-hooks/rules-of-hooks": "error"
}
}
Note: This plugin is intended to use in Create React App by default.
Below are the major differences between Flux and Redux
| Flux | Redux |
|---|---|
| State is mutable | State is immutable |
| The Store contains both state and change logic | The Store and change logic are separate |
| There are multiple stores exist | There is only one store exist |
| All the stores are disconnected and flat | Single store with hierarchical reducers |
| It has a singleton dispatcher | There is no concept of dispatcher |
| React components subscribe to the store | Container components uses connect function |
The componentDidCatch lifecycle method is invoked after an error has been thrown by a descendant component. The method receives two parameters,
The method structure would be as follows
Error boundaries do not catch errors inside event handlers. Event handlers don’t happened or invoked during rendering time unlike render method or lifecycle methods. So React knows how to recover these kind of errors in event handlers. If still you need to catch an error inside event handler, use the regular JavaScript try / catch statement as below
class MyComponent extends React.Component {
constructor(props) {
super(props);
this.state = { error: null };
}
handleClick = () => {
try {
// Do something that could throw
} catch (error) {
this.setState({ error });
}
};
render() {
if (this.state.error) {
return
Caught an error.
;
}
return
Click Me
;
}
}
The above code is catching the error using vanilla javascript try/catch block instead of error boundaries.
Try catch block works with imperative code whereas error boundaries are meant for declarative code to render on the screen. For example, the try catch block used for below imperative code
Whereas error boundaries wrap declarative code as below,
So if an error occurs in a componentDidUpdate method caused by a setState somewhere deep in the tree, it will still correctly propagate to the closest error boundary.
Apart from error messages and javascript stack, React16 will display the component stack trace with file names and line numbers using error boundary concept. For example, BuggyCounter component displays the component stack trace as below,

and user defined elements.The defaultProps are defined as a property on the component class to set the default props for the class. This is used for undefined props, but not for null props. For example, let us create color default prop for the button component,
If props.color is not provided then it will set the default value to ‘red’. i.e, Whenever you try to access the color prop it uses default value
Note: If you provide null value then it remains null value.
This lifecycle method is invoked after an error has been thrown by a descendant component. It receives the error that was thrown as a parameter and should return a value to update state. The signature of the lifecycle method is as follows,
Let us take error boundary use case with the above lifecycle method for demonistration purpose,
class ErrorBoundary extends React.Component {
constructor(props) {
super(props);
this.state = { hasError: false };
}
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
Something went wrong.
;
}
return this.props.children;
}
}
The displayName string is used in debugging messages. Usually, you don’t need to set it explicitly because it’s inferred from the name of the function or class that defines the component. You might want to set it explicitly if you want to display a different name for debugging purposes or when you create a higher-order component. For example, To ease debugging, choose a display name that communicates that it’s the result of a withSubscription HOC.
function withSubscription(WrappedComponent) {
class WithSubscription extends React.Component {
/* ... */
}
WithSubscription.displayName = `WithSubscription(${getDisplayName(
WrappedComponent
)})`;
return WithSubscription;
}
function getDisplayName(WrappedComponent) {
return (
WrappedComponent.displayName || WrappedComponent.name || "Component"
);
}
This method is available from react-dom package and it removes a mounted React component from the DOM and clean up its event handlers and state. If no component was mounted in the container, calling this function does nothing. Returns true if a component was unmounted and false if there was no component to unmount. The method signature would be as follows,
Code-Splitting is a feature supported by bundlers like Webpack and Browserify which can create multiple bundles that can be dynamically loaded at runtime. The react project supports code splitting via dynamic import() feature. For example, in the below code snippets, it will make moduleA.js and all its unique dependencies as a separate chunk that only loads after the user clicks the ‘Load’ button. moduleA.js
App.js
import React, { Component } from "react";
class App extends Component {
handleClick = () => {
import("./moduleA")
.then(({ moduleA }) => {
// Use moduleA
})
.catch((err) => {
// Handle failure
});
};
render() {
return (

<button onClick={this.handleClick}>Load</button>

);
}
}
export default App;
The
The Fragments declared with the explicit <React.Fragment> syntax may have keys. The general usecase is mapping a collection to an array of fragments as below,
function Glossary(props) {
return (

{props.items.map((item) => (
// Without the `key`, React will fire a key warning
<React.Fragment key={item.id}>

{item.term}

{item.description}

</React.Fragment>
))}

);
}
Note: key is the only attribute that can be passed to Fragment. In the future, there might be a support for additional attributes, such as event handlers.
As of React 16, both standard or custom DOM attributes are fully supported. Since React components often take both custom and DOM-related props, React uses the camelCase convention just like the DOM APIs. Let us take few props with respect to standard HTML attributes,

 // Just like node.tabIndex DOM API

 // Just like node.className DOM API
<input readOnly={true} /> // Just like node.readOnly DOM API
These props work similarly to the corresponding HTML attributes, with the exception of the special cases. It also support all SVG attributes.
Higher-order components come with a few caveats apart from its benefits. Below are the few listed in an order
Don’t Use HOCs Inside the render Method: It is not recommended to apply a HOC to a component within the render method of a component.
The above code impact performance by remounting a component that 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 onceStatic Methods Must Be Copied Over: When you apply a HOC to a component 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
You can overcome this by copying the methods onto the container before returning it
Refs Aren’t Passed Through: For HOCs you need to pass through all props to the wrapped component but this does not work for refs. This is because ref is not really a prop similar to key. In this case you need to use the React.forwardRef API
React.forwardRef accepts a render function as parameter and DevTools uses this function to determine what to display for the ref forwarding component. For example, If you don’t name the render function or not using displayName property then it will appear as ”ForwardRef” in the DevTools,
const WrappedComponent = React.forwardRef((props, ref) => {
return <LogProps {...props} forwardedRef={ref} />;
});
But If you name the render function then it will appear as ”ForwardRef(myFunction)”
const WrappedComponent = React.forwardRef(function myFunction(props, ref) {
return <LogProps {...props} forwardedRef={ref} />;
});
As an alternative, You can also set displayName property for forwardRef function,
function logProps(Component) {
class LogProps extends React.Component {
// ...
}
function forwardRef(props, ref) {
return <LogProps {...props} forwardedRef={ref} />;
}
// Give this component a more helpful display name in DevTools.
// e.g. "ForwardRef(logProps(MyComponent))"
const name = Component.displayName || Component.name;
forwardRef.displayName = `logProps(${name})`;
return React.forwardRef(forwardRef);
}
If you pass no value for a prop, it defaults to true. This behavior is available so that it matches the behavior of HTML. For example, below expressions are equivalent,
Note: It is not recommend using this approach because it can be confused with the ES6 object shorthand (example, {name} which is short for {name: name})
You can pass event handlers and other functions as props to child components. It can be used in child component as below,
<button onClick={this.handleClick}>
Yes, You can use. It is often the easiest way to pass parameters to callback functions. But you need to optimize the performance while using it.
class Foo extends Component {
handleClick() {
console.log("Click happened");
}
render() {
return <button onClick={() => this.handleClick()}>Click Me</button>;
}
}
Note: Using an arrow function in render method creates a new function each time the component renders, which may have performance implications
React DOM escapes any values embedded in JSX before rendering them. Thus it ensures that you can never inject anything that’s not explicitly written in your application. Everything is converted to a string before being rendered. For example, you can embed user input as below,
This way you can prevent XSS(Cross-site-scripting) attacks in the application.
You can update UI(represented by rendered element) by passing the newly created element to ReactDOM’s render method. For example, lets take a ticking clock example, where it updates the time by calling render method multiple times,
When you declare a component as a function or a class, it must never modify its own props. Let us take a below capital function,
The above function is called “pure” because it does not attempt to change their inputs, and always return the same result for the same inputs. Hence, React has a single rule saying “All React components must act like pure functions with respect to their props.”
When you call setState() in the component, React merges the object you provide into the current state. For example, let us take a facebook user with posts and comments details as state variables,
Now you can update them independently with separate setState() calls as below,
componentDidMount() {
fetchPosts().then(response => {
this.setState({
posts: response.posts
});
});
fetchComments().then(response => {
this.setState({
comments: response.comments
});
});
}
As mentioned in the above code snippets, this.setState({comments}) updates only comments variable without modifying or replacing posts variable.
During iterations or loops, it is common to pass an extra parameter to an event handler. This can be achieved through arrow functions or bind method. Let us take an example of user details updated in a grid,
<button onClick={(e) => this.updateUser(userId, e)}>Update User details</button>
<button onClick={this.updateUser.bind(this, userId)}>Update User details</button>
In both the approaches, the synthetic argument e is passed as a second argument. You need to pass it explicitly for arrow functions and it forwarded automatically for bind method.
You can prevent component from rendering by returning null based on specific condition. This way it can conditionally render component.
function Greeting(props) {
if (!props.loggedIn) {
return null;
}
return
welcome, {props.name}
;
}
class User extends React.Component {
constructor(props) {
super(props);
this.state = {loggedIn: false, name: 'John'};
}
render() {
return (

//Prevent component render if it is not loggedIn
<Greeting loggedIn={this.state.loggedIn} />
<UserDetails name={this.state.name}>

);
}
In the above example, the greeting component skips its rendering section by applying condition and returning null value.
Keys used within arrays should be unique among their siblings but they don’t need to be globally unique. i.e, You can use the same keys withtwo different arrays. For example, the below book component uses two arrays with different arrays,
function Book(props) {
const index = (

{props.pages.map((page) => (

{page.title}

))}

);
const content = props.pages.map((page) => (

{page.title}

{page.content}

{page.pageNumber}

));
return (

{index}

{content}

);
}
Formik is a form library for react which provides solutions such as validation, keeping track of the visited fields, and handling form submission. In detail, You can categorize them as follows,
It is used to create a scalable, performant, form helper with a minimal API to solve annoying stuff.
Yes, you can use web components in a react application. Even though many developers won’t use this combination, it may require especially if you are using third-party UI components that are written using Web Components. For example, let us use Vaadin date picker web component as below,
If you want to do code-splitting in a server rendered app, it is recommend to use Loadable Components because React.lazy and Suspense is not yet available for server-side rendering. Loadable lets you render a dynamic import as a regular component. Lets take an example,
import loadable from "@loadable/component";
const OtherComponent = loadable(() => import("./OtherComponent"));
function MyComponent() {
return (

<OtherComponent />

);
}
Now OtherComponent will be loaded in a separated bundle
If the module containing the dynamic import is not yet loaded by the time parent component renders, you must show some fallback content while you’re waiting for it to load using a loading indicator. This can be done using Suspense component. For example, the below code uses suspense component,
const OtherComponent = React.lazy(() => import("./OtherComponent"));
function MyComponent() {
return (

<Suspense fallback={
Loading...
}>
<OtherComponent />
</Suspense>

);
}
As mentioned in the above code, Suspense is wrapped above the lazy component.
One of the best place to do code splitting is with routes. The entire page is going to re-render at once so users are unlikely to interact with other elements in the page at the same time. Due to this, the user experience won’t be disturbed. Let us take an example of route based website using libraries like React Router with React.lazy,
import { BrowserRouter as Router, Route, Switch } from "react-router-dom";
import React, { Suspense, lazy } from "react";
const Home = lazy(() => import("./routes/Home"));
const About = lazy(() => import("./routes/About"));
const App = () => (
<Router>
<Suspense fallback={
Loading...
}>
<Switch>
<Route exact path="/" component={Home} />
<Route path="/about" component={About} />
</Switch>
</Suspense>
</Router>
);
In the above code, the code splitting will happen at each route level.
Context is designed to share data that can be considered global for a tree of React components. For example, in the code below lets manually thread through a “theme” prop in order to style the Button component.
//Lets create a context with a default theme value "luna"
const ThemeContext = React.createContext("luna");
// Create App component where it uses provider to pass theme value in the tree
class App extends React.Component {
render() {
return (
<ThemeContext.Provider value="nova">
<Toolbar />
</ThemeContext.Provider>
);
}
}
// A middle component where you don't need to pass theme prop anymore
function Toolbar(props) {
return (

<ThemedButton />

);
}
// Lets read theme value in the button component to use
class ThemedButton extends React.Component {
static contextType = ThemeContext;
render() {
return <Button theme={this.context} />;
}
}
The defaultValue argument is only used when a component does not have a matching Provider above it in the tree. This can be helpful for testing components in isolation without wrapping them. Below code snippet provides default theme value as Luna.
class MyClass extends React.Component {
componentDidMount() {
let value = this.context;
/* perform a side-effect at mount using the value of MyContext */
}
componentDidUpdate() {
let value = this.context;
/* ... */
}
componentWillUnmount() {
let value = this.context;
/* ... */
}
render() {
let value = this.context;
/* render something based on the value of MyContext */
}
}
MyClass.contextType = MyContext;
A Consumer is a React component that subscribes to context changes. It requires a function as a child which receives current context value as argument and returns a react node. The value argument passed to the function will be equal to the value prop of the closest Provider for this context above in the tree. Lets take a simple example,
The context uses reference identity to determine when to re-render, there are some gotchas that could trigger unintentional renders in consumers when a provider’s parent re-renders. For example, the code below will re-render all consumers every time the Provider re-renders because a new object is always created for value.
class App extends React.Component {
render() {
return (
<Provider value={{ something: "something" }}>
<Toolbar />
</Provider>
);
}
}
This can be solved by lifting up the value to parent state,
Refs will not get passed through because ref is not a prop. It handled differently by React just like key. If you add a ref to a HOC, the ref will refer to the outermost container component, not the wrapped component. In this case, you can use Forward Ref API. For example, we can explicitly forward refs to the inner FancyButton component using the React.forwardRef API. The below HOC logs all props,
function logProps(Component) {
class LogProps extends React.Component {
componentDidUpdate(prevProps) {
console.log("old props:", prevProps);
console.log("new props:", this.props);
}
render() {
const { forwardedRef, ...rest } = this.props;
// Assign the custom prop "forwardedRef" as a ref
return <Component ref={forwardedRef} {...rest} />;
}
}
return React.forwardRef((props, ref) => {
return <LogProps {...props} forwardedRef={ref} />;
});
}
Let’s use this HOC to log all props that get passed to our “fancy button” component,
class FancyButton extends React.Component {
focus() {
// ...
}
// ...
}
export default logProps(FancyButton);
Now lets create a ref and pass it to FancyButton component. In this case, you can set focus to button element.
If you don’t use ES6 then you may need to use the create-react-class module instead. For default props, you need to define getDefaultProps() as a function on the passed object. Whereas for initial state, you have to provide a separate getInitialState method that returns the initial state.
var Greeting = createReactClass({
getDefaultProps: function () {
return {
name: "Jhohn",
};
},
getInitialState: function () {
return { message: this.props.message };
},
handleClick: function () {
console.log(this.state.message);
},
render: function () {
return
Hello, {this.props.name}
;
},
});
Note: If you use createReactClass then autobinding is available for all methods. i.e, You don’t need to use .bind(this) with in constructor for event handlers.
Yes, JSX is not mandatory for using React. Actually it is convenient when you don’t want to set up compilation in your build environment. Each JSX element is just syntactic sugar for calling React.createElement(component, props, …children). For example, let us take a greeting example with JSX,
class Greeting extends React.Component {
render() {
return
Hello {this.props.message}
;
}
}
ReactDOM.render(
<Greeting message="World" />,
document.getElementById("root")
);
You can write the same code without JSX as below,
When diffing two trees, React first compares the two root elements. The behavior is different depending on the types of the root elements. It covers the below rules during reconciliation algorithm,
DOM Elements Of The Same Type: When comparing two React DOM elements of the same type, React looks at the attributes of both, keeps the same underlying DOM node, and only updates the changed attributes. Lets take an example with same DOM elements except className attribute,
Recursing On Children: when recursing on the children of a DOM node, React just iterates over both lists of children at the same time and generates a mutation whenever there’s a difference. For example, when adding an element at the end of the children, converting between these two trees works well.
Handling keys: React supports a key attribute. When children have keys, React uses the key to match children in the original tree with children in the subsequent tree. For example, adding a key can make the tree conversion efficient,
Even though the pattern named render props, you don’t have to use a prop named render to use this pattern. i.e, Any prop that is a function that a component uses to know what to render is technically a “render prop”. Lets take an example with the children prop for render props,
Actually children prop doesn’t need to be named in the list of “attributes” in JSX element. Instead, you can keep it directly inside element,
While using this above technique(without any name), explicitly state that children should be a function in your propTypes.
You can implement most higher-order components (HOC) using a regular component with a render prop. For example, if you would prefer to have a withMouse HOC instead of a
function withMouse(Component) {
return class extends React.Component {
render() {
return (
<Mouse
render={(mouse) => <Component {...this.props} mouse={mouse} />}
/>
);
}
};
}
This way render props gives the flexibility of using either pattern.
The falsy values such as false, null, undefined, and true are valid children but they don’t render anything. If you still want to display them then you need to convert it to string. Let’s take an example on how to convert to a string,
In React, the value attribute on form elements will override the value in the DOM. With an uncontrolled component, you might want React to specify the initial value, but leave subsequent updates uncontrolled. To handle this case, you can specify a defaultValue attribute instead of value.
render() {
return (
<form onSubmit={this.handleSubmit}>

User Name:
<input
defaultValue="John"
type="text"
ref={this.input} />

<input type="submit" value="Submit" />
</form>
);
}
The same applies for select
and textArea
inputs. But you need to use defaultChecked for checkbox
and radio
inputs.
Below are the main differences between Real DOM and Virtual DOM,
| Real DOM | Virtual DOM |
|---|---|
| Updates are slow | Updates are fast |
| DOM manipulation is very expensive. | DOM manipulation is very easy |
| You can update HTML directly. | You Can’t directly update HTML |
| It causes too much of memory wastage | There is no memory wastage |
| Creates a new DOM if element updates | It updates the JSX if element update |
Bootstrap as Dependency: If you are using a build tool or a module bundler such as Webpack, then this is the preferred option for adding Bootstrap to your React application
Below are the top 10 websites
using React as their front-end framework,
The effect hook called useEffect
is used to fetch the data with axios from the API and to set the data in the local state of the component with the state hook’s update function. Let’s take an example in which it fetches list of react articles from the API
import React, { useState, useEffect } from "react";
import axios from "axios";
function App() {
const [data, setData] = useState({ hits: [] });
useEffect(async () => {
const result = await axios(
"http://hn.algolia.com/api/v1/search?query=react"
);
setData(result.data);
}, []);
return (

{data.hits.map((item) => (

{item.title}

))}

);
}
export default App;
Remember we provided an empty array as second argument to the effect hook to avoid activating it on component updates but only for the mounting of the component. i.e, It fetches only for component mount.
useState
?When we declare a state variable with useState
, it returns a pair — an array with two items. The first item is the current value, and the second is a function that updates the value. Using [0] and [1] to access them is a bit confusing because they have a specific meaning. This is why we use array destructuring instead. For example, the array index access would look as follows:
var userStateVariable = useState("userProfile"); // Returns an array pair
var user = userStateVariable[0]; // Access first item
var setUser = userStateVariable[1]; // Access second item
Whereas with array destructuring the variables can be accessed as follows:
Redux Thunk, Redux Promise, Redux Saga
.react-scripts
package is a set of scripts from the create-react-app starter pack which helps you kick off projects without configuring. The react-scripts start
command sets up the development environment and starts a server, as well as hot module reloading.ReactDOMServer#renderToNodeStream
method is used to generate HTML on the server and send the markup down on the initial request for faster page loads. It also helps search engines to crawl your pages easily for SEO purposes. Note: Remember this method is not available in the browser but only server.MobX is a simple, scalable and battle tested state management solution for applying functional reactive programming (TFRP). For reactJs application, you need to install below packages,
Below are the main differences between Redux and MobX,
| Topic | Redux | MobX |
|---|---|---|
| Definition | It is a javascript library for managing the application state | It is a library for reactively managing the state of your applications |
| Programming | It is mainly written in ES6 | It is written in JavaScript(ES5) |
| Data Store | There is only one large store exist for data storage | There is more than one store for storage |
| Usage | Mainly used for large and complex applications | Used for simple applications |
| Performance | Need to be improved | Provides better performance |
| How it stores | Uses JS Object to store | Uses observable to store the data |
No, you don’t have to learn es2015/es6 to learn react. But you may find many resources or React ecosystem uses ES6 extensively. Let’s see some of the frequently used ES6 features,
// in es 5
var someData = this.props.someData;
var dispatch = this.props.dispatch;
// in es6
const { someData, dispatch } = this.props;
// in es 5
<SomeComponent someData={this.props.someData} dispatch={this.props.dispatch} />
// in es6
<SomeComponent {...this.props} />
The Concurrent rendering makes React apps to be more responsive by rendering component trees without blocking the main UI thread. It allows React to interrupt a long-running render to handle a high-priority event. i.e, When you enabled concurrent Mode, React will keep an eye on other tasks that need to be done, and if there’s something with a higher priority it will pause what it is currently rendering and let the other task finish first. You can enable this in two ways,
Yes, you can use javascript: URLs but it will log a warning in the console. Because URLs starting with javascript: are dangerous by including unsanitized output in a tag like and create a security hole.
const companyProfile = {
website: "javascript: alert('Your website is hacked')",
};
// It will log a warning
More details;
Remember that the future versions will throw an error for javascript URLs.
The imperative way of doing this would be:
```javascript
if( user.likes() ) {
if( hasBlue() ) {
removeBlue();
addGrey();
} else {
removeGrey();
addBlue();
}
}```
Basically, you have to check what is currently on the screen and handle all the changes necessary to redraw it with the current state, including undoing the changes from the previous state. You can imagine how complex this could be in a real-world scenario.
In contrast, the declarative approach would be:
```javascript
if( this.state.liked ) {
return <blueLike />;
} else {
return <greyLike />;
}```
Because the declarative approach separates concerns, this part of it only needs to handle how the UI should look in a sepecific state, and is therefore much simpler to understand.