React is all about keeping your front-end code modular and reusable. Components play a crucial role in making React modular. Using the right kind of components can make sure that your front end code is reusable and scalable. Components are the building blocks of React. Using the right type of component will make a lot of difference to your codes’ reusability and performance. There are two types of components in react functional and class based components. In this article we will explore what are the differences between functional components and class based components, advantages and disadvantages of these two types of components and thumb rules to decide when to use what. What are functional components? Let us say we need to create a component for printing the welcome message. Let us create a component called In its simplest form the component would like following welcomeMessage const = () => {return "Hello there"}; welcomeMessage export default ; welcomeMessage This code can be easily read and understood by any Javascript developer, even one without any background in React. You have a function definition and then the function is being exported. That is all you need to have in the file that contains a component. Everything else is optional. You should observe that we are not even having which is a common sight in React. This is because we are just returning some text from this function. If you are using JSX you will need to import React class. It is just a harmless, easily readable function. import React from "react"; While this will qualify as a React component, in most of our scenarios we will generally be returning some JSX, based on the props received by our component. It is this ability to create expected output based on the input props which makes components reusable. So let us modify our to take props as input and output the welcome message based on that. welcomeMessage import React from "react"; const = props => {return (<div><p>Hello <b>{props.name}</b> from </p></div>);}; welcomeMessage welcomeMessage export default ; welcomeMessage As you can see this is just a pure function that takes as the input and returns as the output based on the input. props JSX We are using the ES6 standard in the above example. The same code can also be written as following. function (props) {return (<div><p>Hello <b>{props.name}</b> from </p></div>);}; welcomeMessage welcomeMessage But I wouldn’t suggest using this format. React uses of ES6 for a reason and hence it makes sense that we also stick to the same. Thanks to babel setup in Create React App(CRA) the backward compatibility of our code is taken by the default setup. As an added advantage your code will blend in well with React code when you use ES6. It is better to align ourselves to the future direction of the language. Now we can pass the props to our component within the in the following manner. welcomeMessage App.js import React, { Component } from "react";import logo from "./logo.svg";import "./App.css"; import from "./components/W "; welcomeMessage elcomeMessage class App extends Component {render() {let numberName = 5;return (<div className="App"><header className="App-header"><img src={logo} className="App-logo" alt="logo" /><h1 className="App-title">Welcome to React</h1></header><p className="App-intro">**<**W elcomeMessage name="React enthusiast"/> </p>  
  </div>  
); }} export default App; Advantages of functional components They help keep your code simple, readable and reusable. Since they are pure functions it is easier to test them. Since they do not have access to you are forced to think in terms of passing which will make your components reusable. If you think your component can be used outside of your project you can even consider publishing your component to npm. state props Should I stick to functional components So does that mean it is always better to use functional components? Not really. If you really feel the need, you can convert them to class based components. In the next part we will explore when it makes sense to use class based components and what are the advantages of using them. But they are always a great starting point for your components. What are class based components? A class based component is a JavaScript class that extends The only required method in is the method. React.Component React.Component render So our functional component which was like the following helloUserFunctionalComponent import React from "react"; //functional Component const W = props => {return (<div><p>Hello <b>{props.name}</b> from W </p></div>);}; elcomeMessage elcomeMessage export default W ; elcomeMessage would now become like this. import React, { Component } from "react"; class W extends Component {render() {return (<div><p>Hello {this.props.name} from W .</p></div>);}} elcomeMessage elcomeMessage export default W ; elcomeMessage As mentioned earlier functional components make a great starting point. I generally start out with functional components and then switch to class based components only in one of the following scenarios. Need to make use of state There will be some scenarios where you need to keep track of the state of your component . In such scenarios it is better to make use of the . An example can be accordion where you need to maintain whether the accordion is clicked or not in . This corresponds to the state of the UI component. There is one more scenarios where we use state to save external data which we will discuss next. state state Need to fetch data Another scenarios is where you need to fetch data. In such scenarios it is again better to use class based components. In a container you generally do the data fetching and store the fetched data in . Then you pass the relevant data from to a functional component as , which will just render the data provided. The components that just take data as props and render them can be considered similar to the templates. They are also called dumb components. Instead of using a single class based component for both fetching data and rendering data, it is better to use an approach called containers. state state props Need some lifecycle hooks. Class based components provide various lifecycle hooks which allow us to hook into various stages of component lifecycle and perform actions based on that. While there are many lifecycle hooks, will be the most frequently used one. For example in the previous section we discussed about data fetching, which generally happens in the hook. Let us look at the code for a scenario where we need to fetch data from an API and then render it. componentDidMount componentDidMount() We will use a container to do the API call and then we will use dump component to do the rendering. Container — A special purpose class based component. class extends React.Component {state = { users: [] };componentDidMount() {let users = await fetchUsers();this.setState({ : users });}render() {return < ={this.state.users} />;}} UserListContainer users UserList users A dumb component for rendering our user list. const = props =><ul>{ (user => (<li><a href="{ }">{ }</a></li>))}</ul> UserList props.users.map user.profile_link user.name As you can see since we created a separate functional component, we can reuse it to render a Userlist from any source as long as the passed prop has and attributes in the user object. UserList users profile_link name Need performance optimisations provided by Component based class If you have worked on the front end optimisation you would have observed once the page is loaded and the state changes most of the time in front end is spent in re-rendering the components. Time taken for javascript processing seems to be negligible when compared to the render times. Making sure that you only re-render components when required can greatly improve the performance of your front end app. Class based components have an optional method called which decides whether you need to re-render a component or not. This method takes as inputs using which you can return a . If is returned it means component will re-render and if a is returned it means there is no need to re-render the component. If you meddle with this function without understanding the consequences you might end up with more issues than what you began with. There is also an easier alternative. React provides In most of the scenarios it would suffice to extend instead of . It is equivalent to implementing with a shallow comparison of current and previous props and state. shouldComponentUpdate nextProps and nextState boolean true false React.PureComponent. React.PureComponent React.Component shouldComponentUpdate() Summary As a rule of thumb start with functional components and transition to class based components only when you see the need. Since functional components do not have access to state it prevents you from over using the state unnecessarily. Making use of state can make you lazy. Starting with a functional component makes you think if you really need to use state. Functional components keep your code simple, readable and reusable. Functional components are easy to test. Whenever you should make use of , create class based components. state Use class based components whenever you want to use performance optimisations provided by Component base class. Separate data fetching and rendering. Or in other terms keep your business logic/context separate from presentation. For data fetching use container pattern as it will help you follow the paradigm of separation of concerns. Containers are used for data fetching and in turn use other components for rendering. I remember reading somewhere . That is a good base rule and everything else stems from it. “Don’t over burden your components with application logic or context specific details” I wrote this post as a sample for Mosh. You can checkout his website on and he also has a course on React which you can check out on https://programmingwithmosh.com/ https://codewithmosh.com/p/mastering-react You may also like the following articles. _I have had a kind of “love and hate” relationship with JavaScript. But nevertheless JavaScript was always intriguing…_hackernoon.com Understanding promises in JavaScript _Async and Await are extensions of promises. So if you are not clear about the basics of promises please get comfortable…_hackernoon.com Understanding async-await in Javascript _I recently read a medium post where the author claimed that using async-await is better than using promises. While this…_hackernoon.com Should I use Promises or Async-Await _I think npm was one of the reasons for quick adoption of nodejs. As of writing this article there are close 7,00,000…_hackernoon.com Understanding npm in Nodejs

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