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React Components

render() Method

React class components must have a render() method. This method should return some React elements created with JSX.

class MyComponent extends React.Component { render() { return <h1>Hello from the render method!</h1>; } }

React Component Base Class

React class components need to inherit from the React.Component base class and have a render() method. Other than that, they follow regular JavaScript class syntax.

This example shows a simple React class component.

class MyComponent extends React.Component { render() { return <h1>Hello world!</h1>; } }

Importing React

In order to use React, we must first import the React library. When we import the library, it creates an object that contains properties needed to make React work, including JSX and creating custom components.

import React from 'react';

React Components

A React component is a reusable piece of code used to define the appearance, behavior, and state of a portion of a web app’s interface. Components are defined as functions or as classes. Using the component as a factory, an infinite number of component instances can be created.

import React from 'react'; function MyFunctionComponent() { return <h1>Hello from a function component!</h1>; } class MyClassComponent extends React.Component { render() { return <h1>Hello from a class component!</h1>; } }

JSX Capitalization

React requires that the first letter of components be capitalized. JSX will use this capitalization to tell the difference between an HTML tag and a component instance. If the first letter of a name is capitalized, then JSX knows it’s a component instance; if not, then it’s an HTML element.

// This is considered a component by React. <ThisComponent /> // This is considered a JSX HTML tag. <div>

ReactDOM.render()

ReactDOM.render()‘s first argument is a component instance. It will render that component instance.

In this example, we will render an instance of MyComponent.

import React from 'react'; import ReactDOM from 'react-dom'; class MyComponent extends React.Component { render() { return <h1>Hello world!</h1>; } } ReactDOM.render(<MyComponent />, document.getElementById('app'));

Multi-line JSX Expressions

Parentheses are used when writing a multi-line JSX expression. In the example, we see that the component’s render() method is split over multiple lines. Therefore it is wrapped in parentheses.

render() { return ( <blockquote> <p>Be the change you wish to see in the world.</p> <cite> <a target="_blank" href="https://en.wikipedia.org/wiki/Mahatma_Gandhi" > Mahatma Gandhi </a> </cite> </blockquote> ); }

Code in render()

A React component can contain JavaScript before any JSX is returned. The JavaScript before the return statement informs any logic necessary to render the component.

In the example code, we see JavaScript prior to the return statement which rounds the value to an integer.

class Integer extends React.Component { render() { const value = 3.14; const asInteger = Math.round(value); return <p>{asInteger}</p>; } }

Object Properties As Attribute Values

In React, JSX attribute values can be set through data stored in regular JavaScript objects. We see this in the example block of code.

In our code example we first see our JavaScript object seaAnemones and the values stored with this image. We then see how these stored values are used to set the <img> attributes in our JSX expression for the SeaAnemones component.

const seaAnemones = { src: 'https://commons.wikimedia.org/wiki/Category:Images#/media/File:Anemones_0429.jpg', alt: 'Sea Anemones', width: '300px', }; class SeaAnemones extends React.Component { render() { return ( <div> <h1>Colorful Sea Anemones</h1> <img src={seaAnemones.src} alt={seaAnemones.alt} width={seaAnemones.width} /> </div> ); } }

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