De bedste reaktionsvejledninger

React er et JavaScript-bibliotek til opbygning af brugergrænseflader. Det blev valgt til den mest elskede i kategorien "Frameworks, Libraries, and Other Technologies" i Stack Overflow's 2017 Developer Survey.

React er et JavaScript-bibliotek, og React-applikationer, der er bygget på det, kører i browseren, IKKE på serveren. Applikationer af denne art kommunikerer kun med serveren, når det er nødvendigt, hvilket gør dem meget hurtige sammenlignet med traditionelle websteder, der tvinger brugeren til at vente på serveren til at gengive hele sider igen og sende dem til browseren.

React bruges til at opbygge brugergrænseflader - hvad brugeren ser på deres skærm og interagerer med for at bruge din webapp. Denne grænseflade er opdelt i komponenter, i stedet for at have en enorm side opdeler du den i mindre stykker kendt som komponenter. I mere generelle vendinger kaldes denne tilgang Modularitet.

  • Det er deklarativt: React bruger et deklarativt paradigme, der gør det lettere at begrunde din ansøgning.
  • Det er effektivt: React beregner det minimale sæt af ændringer, der er nødvendige for at holde din DOM opdateret.
  • Og det er fleksibelt: React fungerer med de biblioteker og rammer, som du allerede kender.

De bedste selvstudier til læring reagere

freeCodeCamp har en React-tutorial på YouTube, der lærer dig alt det grundlæggende på bare 5 timer.

Vi har også en mere detaljeret mellemliggende React-tutorial, der lærer dig, hvordan du bygger en hel social media React-app ved hjælp af Firebase. Det er 12 timer langt, og hvis du følger med, lærer du masser af indviklingen i React.

Hvorfor lære at reagere?

React involverer sammensætning, det vil sige mange komponenter, der indpakker funktionaliteterne i en indkapslet beholder.

Mange populære websteder bruger React til at implementere MVC-arkitektoniske mønstre. Facebook (delvist), Instagram (fuldstændigt), Khan Academy (delvist), New York Times (delvist), Yahoo Mail (fuldstændigt), Dropbox's nye foto- og videogalleri-app Carousel (helt) er de populære hjemmesider, der vides at bruge React.

Hvordan er disse store applikationer bygget ved hjælp af React? Det enkle svar er ved at opbygge små applikationer eller komponenter. Eksempel:

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

Reagerer er for det meste erklærende, hvilket betyder, at vi er mere bekymrede over hvad vi skal gøre snarere end hvordan vi skal udføre en bestemt opgave.

Deklarativ programmering er et programmeringsparadigme, der udtrykker logikken i en beregning uden at beskrive dens kontrolflow. Deklarativ programmering har visse fordele, såsom reducerede bivirkninger (opstår, når vi ændrer en hvilken som helst tilstand eller muterer noget eller fremsætter en API-anmodning), minimeret mutabilitet (da meget af det er abstrakt), forbedret læsbarhed og færre fejl.

React har også et ensrettet dataforløb. UI i React er faktisk statens funktion. Dette betyder, at når staten opdaterer, opdaterer den også brugergrænsefladen. Så vores brugergrænseflade skrider frem, når staten ændrer sig.

Fordele ved at reagere

Nogle grunde til at bruge React er:

  1. Hurtig. Apps lavet i React kan håndtere komplekse opdateringer og stadig føle sig hurtige og lydhøre.
  2. Modulær. I stedet for at skrive store, tætte kodefiler, kan du skrive mange mindre, genanvendelige filer. React's modularitet kan være en smuk løsning på JavaScript's vedligeholdelsesproblemer.
  3. Skalerbar. Store programmer, der viser mange skiftende data, er, hvor React fungerer bedst.
  4. Fleksibel. Du kan bruge React til interessante projekter, der ikke har noget at gøre med at oprette en webapp. Folk finder stadig ud af React's potentiale. Der er plads til at udforske.

Virtuel DOM

React's magi kommer fra dets fortolkning af DOM og dens strategi til oprettelse af brugergrænseflader.

React bruger Virtual DOM til at gengive et HTML-træ næsten først. Hver gang en tilstand ændres, og vi får et nyt HTML-træ, der skal føres til browserens DOM, i stedet for at skrive det helt nye træ, vil React kun skrive forskellen mellem det nye træ og det forrige træ (da React har begge træer i hukommelsen). Denne proces er kendt som træafstemning.

Forsoning

React har en smart forskellig algoritme, som den bruger til kun at regenerere i sin DOM-knude, hvad der faktisk skal regenereres, mens den holder alt andet som det er. Denne forskellige proces er mulig på grund af React's virtuelle DOM.

Ved hjælp af den virtuelle DOM gemmer React den sidste DOM-version i hukommelsen. Når den har en ny DOM-version til browseren, vil den nye DOM-version også være i hukommelsen, så React kan beregne forskellen mellem den nye og den gamle version.

React vil derefter instruere browseren om kun at opdatere den beregnede diff og ikke hele DOM-noden. Uanset hvor mange gange vi regenererer vores grænseflade, tager React kun de nye “delvise” opdateringer til browseren.

Reager fra Scratch

Ønsker du at komme i gang med at lære det grundlæggende i at reagere uden at gå i stå ved at skabe et udviklingsmiljø? Chancerne er, at hvis du er ny inden for webudvikling, kan opsætning af et udviklingsmiljø lade dig føle dig lidt skræmt, når du bare prøver at lære at reagere.

I denne artikel skal vi se på, hvordan vi kan komme i gang med React ved kun at bruge en teksteditor og en browser og intet andet.

1 - Opsæt kedelpladekode med Emmet

Lad os komme i gang med trin 1. Vi begynder med en fil i vores browser kaldet “index.html”. Vi begynder med kedelpladens HTML-kode. For en hurtig start anbefaler jeg at bruge Emmet med den teksteditor, du har. Indtast på den første linje, html:5og tryk derefter på shift-tasten for at få koden nedenfor. Eller du kan gå videre og kopiere og indsætte koden nedenfra.

html:5

Dette vil resultere i følgende kode:

      Document    

Vi kan udfylde titlen "Tid til at reagere!".

Dette indhold vises ikke på din webside. Alt i hovedafsnittet af HTML-filen vil være metadata, som vores browser bruger til at fortolke vores kode i body-sektionen. Denne titel bliver det, der vises på fanen for vores side, ikke faktisk på siden.

2 - Få script-tags til at udnytte styrken i React og Babel-biblioteker

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}

; ); };

Implicitte returkomponenter

const Cat = props =>

{props.name}

{props.color}

;