React es una biblioteca de JavaScript para crear interfaces de usuario. Fue votado como el más querido en la categoría "Marcos, bibliotecas y otras tecnologías" de la Encuesta para desarrolladores de 2017 de Stack Overflow.

React es una biblioteca de JavaScript y las aplicaciones de React creadas en ella se ejecutan en el navegador, NO en el servidor. Las aplicaciones de este tipo solo se comunican con el servidor cuando es necesario, lo que las hace muy rápidas en comparación con los sitios web tradicionales que obligan al usuario a esperar a que el servidor vuelva a renderizar páginas enteras y las envíe al navegador.

React se utiliza para crear interfaces de usuario: lo que el usuario ve en su pantalla y con lo que interactúa para usar su aplicación web. Esta interfaz se divide en componentes, en lugar de tener una página enorme, la divide en partes más pequeñas conocidas como componentes. En términos más generales, este enfoque se denomina modularidad.

• Es declarativo: React usa un paradigma declarativo que hace que sea más fácil razonar sobre su aplicación.
• Es eficiente: React calcula el conjunto mínimo de cambios necesarios para mantener su DOM actualizado.
• Y es flexible: React funciona con las bibliotecas y marcos que ya conoce.

Los mejores tutoriales para aprender a reaccionar

freeCodeCamp tiene un tutorial de React en YouTube que le enseñará todos los conceptos básicos en solo 5 horas.

También tenemos un tutorial intermedio de React más detallado que le enseña cómo crear una aplicación React de redes sociales completa usando Firebase. Tiene una duración de 12 horas, y si lo sigue, aprenderá un montón de las complejidades de React.

¿Por qué aprender React?

React involucra Composición, es decir, muchos componentes que envuelven las funcionalidades en un contenedor encapsulado.

Muchos sitios web populares usan React implementando el patrón arquitectónico MVC. Facebook (Parcialmente), Instagram (Completamente), Khan Academy (Parcialmente), New York Times (Parcialmente), Yahoo Mail (Completamente), la nueva aplicación de galería de fotos y videos de Dropbox Carousel (Completamente) son los sitios web populares que se sabe que usan React.

¿Cómo se crean estas grandes aplicaciones con React? La respuesta simple es construir pequeñas aplicaciones o componentes. Ejemplo:

const Component2 = () => { return ( ); }; const Component3 = () => { return ( ); }; const Component1 = () => { return ( ); }; ReactDOM.render( , document.getElementById("app") );

React es declarativo, en su mayor parte, lo que significa que nos preocupa más qué hacer que cómo hacer una tarea específica.

La programación declarativa es un paradigma de programación que expresa la lógica de un cálculo sin describir su flujo de control. La programación declarativa viene con ciertas ventajas, como efectos secundarios reducidos (ocurre cuando modificamos cualquier estado o mutamos algo o hacemos una solicitud de API), mutabilidad minimizada (ya que se abstrae gran parte de ella), legibilidad mejorada y menos errores.

React también tiene un flujo de datos unidireccional. La interfaz de usuario en React es en realidad la función del estado. Esto significa que a medida que el estado se actualiza, también actualiza la interfaz de usuario. Entonces, nuestra interfaz de usuario progresa a medida que cambia el estado.

Ventajas de React

Algunas razones para usar React son:

1. Rápido. Las aplicaciones creadas en React pueden manejar actualizaciones complejas y aún así sentirse rápidas y receptivas.
2. Modular. En lugar de escribir archivos de código grandes y densos, puede escribir muchos archivos más pequeños y reutilizables. La modularidad de React puede ser una hermosa solución a los problemas de mantenimiento de JavaScript.
3. Escalable. Los programas grandes que muestran una gran cantidad de datos cambiantes son donde React se desempeña mejor.
4. Flexible. Puede usar React para proyectos interesantes que no tienen nada que ver con la creación de una aplicación web. La gente todavía está descubriendo el potencial de React. Hay espacio para explorar.

DOM virtual

La magia de React proviene de su interpretación del DOM y su estrategia para crear UI.

React usa Virtual DOM para representar un árbol HTML virtualmente primero. Luego, cada vez que cambia un estado y obtenemos un nuevo árbol HTML que debe llevarse al DOM del navegador, en lugar de escribir el árbol completamente nuevo, React solo escribirá la diferencia entre el árbol nuevo y el árbol anterior (ya que React tiene ambos árboles en la memoria). Este proceso se conoce como Reconciliación de árbol.

Reconciliación

React tiene un algoritmo de diferenciación inteligente que usa para regenerar solo en su nodo DOM lo que realmente necesita ser regenerado mientras mantiene todo lo demás como está. Este proceso diferente es posible debido al DOM virtual de React.

Usando el DOM virtual, React mantiene la última versión del DOM en la memoria. Cuando tenga una nueva versión de DOM para llevar al navegador, esa nueva versión de DOM también estará en la memoria, por lo que React puede calcular la diferencia entre la nueva y la antigua versión.

React le indicará al navegador que actualice solo la diferencia calculada y no todo el nodo DOM. No importa cuántas veces regeneremos nuestra interfaz, React llevará al navegador solo las nuevas actualizaciones "parciales".

Reaccionar desde cero

¿Le gustaría comenzar a aprender los conceptos básicos de reaccionar sin atascarse en la creación de un entorno de desarrollo? Lo más probable es que si eres nuevo en el desarrollo web, configurar un entorno de desarrollo puede dejarte un poco intimidado cuando solo estás tratando de aprender a React.

En este artículo veremos cómo podemos comenzar con React usando solo un editor de texto y un navegador y nada más.

1 - Configurar el código de la placa de la caldera con Emmet

Comencemos con el paso 1. Comenzaremos con un archivo en nuestro navegador llamado "index.html". Comenzaremos con el código HTML de la placa de la caldera. Para un comienzo rápido, recomiendo usar Emmet con cualquier editor de texto que tenga. En la primera línea, escriba y html:5luego presione la tecla Mayús para obtener el código a continuación. O puede seguir adelante y copiar y pegar el código de abajo.

html:5

Esto dará como resultado el siguiente código:

      Document    

Podemos completar el título de “¡Es hora de reaccionar!”.

Este contenido no aparecerá en su página web. Cualquier cosa en la sección principal del archivo HTML serán metadatos que nuestro navegador utilizará para interpretar nuestro código en la sección del cuerpo. Este título será lo que aparezca en la pestaña de nuestra página, no realmente en la página.

2 - Obtenga etiquetas de script para aprovechar el poder de las bibliotecas React y Babel

Ok, item one is checked off of our list. Let’s look at item two. We are going to set up our developer environment by using script tags to bring in React and Babel.

This is not a real life developer environment. That would be quite an elaborate setup. It would also leave us with a lot of boiler plate code and libraries that would take us off subject of learning React basics. The goal of this series is to go over the basic syntax of React and get right into coding. We are going to use tags to bring in the React Library, the React DOM library (why), and the Babel library.

 ...       ... Time to React! 

You are free to use more updated versions of these libraries as they come out. They should not create any breaking changes for the content we are covering.

What are we doing here? The HTML element is used to embed or reference an executable script. The “src” attribute points to the external script files for the React library, ReactDOM library and Babel library.

This is like if you have an electric razor. It is literally no good to you no matter how fancy the electric razor unless you can plug it into the wall and gain access to electricity. Our React code we will write will be no good to us if our browser can’t plug into these libraries to understand and interpret what we are going.

This is how our application is going to gain the power of React, it is going to be how we insert React into the Dom. We have React and ReactDOM as two different libraries because there are use cases such as React Native where rendering to the DOM isn’t needed for mobile development so the library was split so people could decide what they needed depending on the project they were working on.

Because we will need our React to make it to the DOM we’ll use both scripts. Babel is how we take advantage of ECMA script beyond ES5 and deal with something called JSX (JavaScript as XML) that we will use in React. We’ll take a deeper look at the magic of Babel in an upcoming section :)

Alright, we have completed steps 1 and 2. We have set up our boiler plate code and set up our developer environment.

3 - Render React to the DOM

Our next two steps will be to choose our location within the DOM that we want to render our React content. And we'll use another script tag for our React content within the body. Generally, as a good separations of concerns practice, this would be in its own file then linked to this html document. We’ll do that later in upcoming sections. For now, we’ll let this dwell within the body of the html document we are currently in.

Now we are going to look at how simple it is to choose a place on the DOM to render our React content. We’ll go within the body. And best practice isn’t just to throw React into the body tag to be displayed but to create a separate element, often a div, that you can treat as a root element to insert your React content.

 React has not rendered yet 

We’ll create a simple element and give it an id of “app”. We are going to be able to target this location to insert our React content much the same way you might use CSS to target an id for styling of your choice. Any react content will be rendered within the div tags with the id of app. In the meantime we’ll leave some text saying that “React has not rendered yet”. If we see this when we preview our page it means that somewhere we missed rendering React.

Now, let’s go ahead and create a script tag within our body where we will create with React for the first time. The syntax we are going to need for our script tag is to add an attribute of “type”. This specifies the media type of the script. Above in our head we used an src attribute that pointed to the external script files for the React library, ReactDOM library and Babel library.

 React has not rendered yet 

The “type” of script that we are using will be wrapped in quotes and set to "text/babel". We’ll need the ability to use babel right away as we work with JSX.

First, we are going to render React to the DOM. We will use the ReactDOM.render() method to do this. This will be a method, and remember a method is just a function attached to an object. This method will take two arguments.

 React has not rendered yet ReactDOM.render(React What, React Where);  

The first argument is the “what” of React. The second argument is the “where” of the location you want it to be placed in the DOM. Let’s start by calling our ReactDOM.render() method. Our first argument is going to be our JSX.

 React has not rendered yet ReactDOM.render( 
Hello World
, React Where );  

The official react docs state: “This funny tag syntax is neither a string nor HTML. It is called JSX, and it is a syntax extension to JavaScript. We recommend using it with React to describe what the UI should look like. JSX may remind you of a template language, but it comes with the full power of JavaScript. JSX produces React “elements.”

Often times, JSX freaks people out who have been developers for a while because it looks like HTML. At a very early age developers are taught separation of concerns. HTML has its place, CSS has its place and JavaScript has its place. JSX seems to blur the lines. You are using what looks like HTML but as Facebook says it comes with the full power of JavaScript.

This can freak out veterans, so many React tutorials start without JSX which can be quite complex. We won’t do that. Because this article is directed towards those who are very young in their careers you may not bring those red flags when you see this syntax.

And JSX is just really intuitive. You can probably quite easily read this code and see that this is going to be the largest header tag displaying the text “Hello World”. No mystery and pretty straightforward. Now, let’s look at what our second argument would be.

 React has not rendered yet ReactDOM.render( 
Hello World
, document.getElementById("app") );  

This is where we want our React content rendered to the DOM. You’ve probably done this quite a few times in the past. We’ll just type in document.getElementById(). And we’ll pass into the argument of the id of app. And that is it. We will now target the div with the id of app to insert our React content.

We want to make sure our content is saved. Go ahead and open this up in the browser and you should see “Hello World”. As you can probably guess, using React is not the quickest or best way to create a Hello World app. We aren’t quite seeing the benefits of it yet. But now, we know that everything is working.

Go ahead and open up the console and look at the “elements”. You can do that on a Mac with command + shift + j or on Windows and Linux: Ctrl + Shift + J

If you click on the head tag, we can see our script libraries we included. Then we can go down to the body of our document. Let’s click on our div with the id of “app”. And when we do we see our

tag with the content “Hello World”.

View Entire Code Here.

Recap

So let’s do a quick recap. In our head tag we grabbed the script tags for React, ReactDOM and Babel. These are the tools our browser needs in its meta data to read our React code and JSX in specific.

We then located the position within the DOM that we wanted to insert our React by creating an element div with the id of “app”.

Next, we created a script tag to input our React code. We used the ReactDOM.render() method that takes two arguments. The “what” of the React content, in this case our JSX, and the second argument is the “where” that you want to insert the React content into the DOM. In this case it is the location with the id of “app”.

As an alternative to JSX, you can use ES6 and Javascript’s compiler like Babel. //babeljs.io/

Installing React

Creating a new React project

You could just embed the React library in your webpage like so2:

Smart programmers want to take the more practical and productive way: Create React App

npm install -g create-react-app create-react-app my-app cd my-app npm start

This will set up your development environment so that you can use the latest JavaScript features, provide a nice developer experience, and optimize your app for production.

npm start will start up a development server which allows live reloading3.

After you finish your project and are ready to deploy your App to production, you can just use npm run build to create an optimized build of your app in the build folder.

Your first React App

Installation

As specified in the previous section (Installation), run the Create React App tool. After everything has finished, cd into the folder of your application and run npm start. This will start a development server and you are all set to start developing your app!

npm install -g react-create-app create-react-app my-first-app cd my-first-app npm start

Editing the code

Start up your editor or IDE of choice and edit the App.js file in the src folder. When created with the react-create-app tool, there will already be some code in this file.

The code will consist of these parts:

imports

import React, { Component } from 'react'; import logo from './logo.svg'; import './App.css';

This is used by webpack to import all required modules so that your code can use them. This code imports 3 modules:

1. React and Component, which allow us to use React as it should be used. (With components)
2. logo, which allows us to use logo.svg in this file.
3. ./App.css, which imports the stylesheet for this file.

classes/components

class App extends Component { render() { return ( 
Welcome to React
 To get started, edit src/App.js and save to reload. 
 ); } }

React is a library that makes use of Components, which let you split up your UI into independent, reusable pieces, and think about each piece in isolation. There is already 1 component created, the App component. If you used the create-react-app tool, this component is the main component in the project and you should build around this central class.

We will look at components in more detail shortly.

exports

When creating a class in React, you should export them after declaration, which allows you to use the component in another file by using the import keyword. You can use default after the export keyword to tell React that this is the main class of this file.

export default App;

View the results!

When you’ve started the development server by issuing the npm start command, you can view the changes you add to your project live in your browser. After issuing the command, npm should open a browser automatically displaying your app.

React - Components

Components are reusable in React. You can inject value into props as given below:

function Welcome(props) { return 
Hello, {props.name}
; } const element = ; ReactDOM.render( element, document.getElementById('root') );

name="Faisal Arkan" will give value into {props.name} from the  function Welcome(props) and will return the component that has given value by name="Faisal Arkan". After that react will render the element into HTML.

Other ways to declare components

There are many ways to declare components when using React. There are two kinds of components, stateless components and stateful components.

Stateful

Class Type Components

class Cat extends React.Component { constructor(props) { super(props); this.state = { humor: 'happy' } } render() { return( 
{this.props.name}
 {this.props.color} 
 ); } }

Stateless Components

Functional Components (Arrow Function from ES6)

const Cat = props => { return ( 
{props.name}
{props.color}
 ; ); };

Implicit Return Components

const Cat = props =>
{props.name}
{props.color}
 ;