React is a JavaScript library (MIT License) made by Facebook to create interactive UIs. It's used to create dynamic and reusable components. The most powerful thing about React is that can be used in the client, server, mobile applications, and even VR applications. 

In the modern web, we need to manipulate the DOM constantly; the problem is that doing this a lot may affect the performance of our application seriously. React uses a Virtual DOM, which means that all updates occur in memory (this is faster than manipulating the real DOM directly). The learning curve of React is short in comparison with other JavaScript frameworks such as Angular, Vue, or Backbone, mainly because the React code is mostly written with modern JavaScript (classes, arrow functions, string templates, and so on) and does not have too many patterns used to write code, like Dependency Injection, or a template system, like in Angular.

Companies such as Airbnb, Microsoft, Netflix, Disney, Dropbox, Twitter, PayPal, Salesforce, Tesla, and Uber are extensively using React in their projects. In this book, you will learn how to develop your React applications in the way they do, using best practices.

As I said in the introduction, React is mainly written with modern JavaScript (ES6, ES7, and ES8). If you want to take advantage of React, there are some modern JS features that you should master to get the best results for your React applications. In this first recipe, we are going to cover the essential JS features so you are ready and can start working on your first React application.

In this section, we will see how to use the most important JS features in React:

1. let and const: The new way to declare variables in JavaScript is by using let or const. You can use let to declare variables that can change their value but in block scope. The difference between let and var is that let is a block scoped variable that cannot be global, and with var, you can declare a global variable, for example:

    var name = 'Carlos Santana';
    let age = 30;

    console.log(window.name); // Carlos Santana
    console.log(window.age);  // undefined

2. The best way to understand "block scope" is by declaring a for loop with var and let. First, let's use var and see its behavior:

    for (var i = 1 ; i <= 10; i++) {
      console.log(i); // 1, 2, 3, 4... 10
    }

    console.log(i); // Will print the last value of i: 10

3. If we write the same code, but with let, this will happen:

    for (let i = 1 ; i <= 10; i++) {
      console.log(i); // 1, 2, 3, 4... 10
    }

    console.log(i); // Uncaught ReferenceError: i is not defined

4. With const, we can declare constants, which means the value can't be changed (except for arrays and objects):

    const pi = 3.1416;
    pi = 5; // Uncaught TypeError: Assignment to constant variable.

5. If we declare an array with const, we can manipulate the array elements (add, remove, or modify elements):

    const cryptoCurrencies = ['BTC', 'ETH', 'XRP'];

    // Adding ERT: ['BTC', 'ETH', 'XRP', 'ERT'];
    cryptoCurrencies.push('ERT'); 

    // Will remove the first element: ['ETH', 'XRP', 'ERT'];
    cryptoCurrencies.shift();
    
    // Modifying an element
    cryptoCurrencies[1] = 'LTC'; // ['ETH', 'LTC', 'ERT'];

6. Also, using objects, we can add, remove, or modify the nodes:

    const person = {
      name: 'Carlos Santana',
      age: 30,
      email: 'carlos@milkzoft.com'
    };
    
    // Adding a new node...
    person.website = 'https://www.codejobs.com';

    // Removing a node...
    delete person.email;

    // Updating a node...
    person.age = 29;

7. Spread operator: The spread operator (...) splits an iterable object into individual values. In React, it can be used to push values into another array, for example when we want to add a new item to a Todo list by utilizing setState (this will be explained in the next chapter):

    this.setState({
      items: [
        ...this.state.items, // Here we are spreading the current items
        {
      task: 'My new task', // This will be a new task in our Todo list.
        }
      ]
    });

8. Also, the Spread operator can be used in React to spread attributes (props) in JSX:

    render() {
      const props = {};

      props.name = 'Carlos Santana';
      props.age = 30;
      props.email = 'carlos@milkzoft.com';
        
      return <Person {...props} />;
    }

9. Rest parameter: The rest parameter is also represented by .... The last parameter in a function prefixed with ... is called the rest parameter. The rest parameter is an array that will contain the rest of the parameters of a function when the number of arguments exceeds the number of named parameters:

    function setNumbers(param1, param2, ...args) {
      // param1 = 1
      // param2 = 2
      // args = [3, 4, 5, 6];
      console.log(param1, param2, ...args); // Log: 1, 2, 3, 4, 5, 6
    }
  
    setNumbers(1, 2, 3, 4, 5, 6);

10. Destructuring: The destructuring assignment feature is the most used in React. It is an expression that allows us to assign the values or properties of an iterable object to variables. Generally, with this we can convert our component props into variables (or constants):

    // Imagine we are on our <Person> component and we are 
    // receiving the props (in this.props): name, age and email.
    render() {
      // Our props are: 
      // { name: 'Carlos Santana', age: 30, email: 
       'carlos@milkzoft.com' }
      console.log(this.props); 
      const { name, age, email } = this.props;
    
      // Now we can use the nodes as constants...
      console.log(name, age, email);

      return (
        <ul>
          <li>Name: {name}</li>
          <li>Age: {age}</li>
          <li>Email: {email}</li>
        </ul>
      );
    }

    // Also the destructuring can be used on function parameters
    const Person = ({ name, age, email }) => (
      <ul>
        <li>Name: {name}</li>
        <li>Age: {age}</li>
        <li>Email: {email}</li>
      </ul>
    );

11. Arrow functions: ES6 provides a new way to create functions using the => operator. These functions are called arrow functions. This new method has a shorter syntax, and the arrow functions are anonymous functions. In React, arrow functions are used as a way to bind the this object in our methods instead of binding it in the constructor:

    class Person extends Component {
      showProps = () => {
        console.log(this.props); // { name, age, email... }
      }

      render() {
        return (
          <div>
            Consoling props: {this.showProps()}
          </div>
        );
      }
    }

12. Template literals: The template literal is a new way to create a string using backticks (` `) instead of single quotes (' ')   or double quotes (" "). React use template literals to concatenate class names or to render a string using a ternary operator:

    render() {
      const { theme } = this.props;

      return (
        <div 
          className={`base ${theme === 'dark' ? 'darkMode' : 
         'lightMode'}`}
        >
          Some content here...
        </div>
      );
    }

13. Map: The map() method returns a new array with the results of calling a provided function on each element in the calling array. Map use is widespread in React, and is mainly used to render multiple elements inside a React component; for example, it can be used to render a list of tasks:

    render() {
      const tasks = [
        { task: 'Task 1' },
        { task: 'Task 2' },
        { task: 'Task 3' }
      ];

      return (
        <ul>
          {tasks.map((item, key) => <li key={key}>{item.task}</li>}
        </ul>
      );
    }

14. Object.assign(): The Object.assign() method is used to copy the values of all enumerable own properties from one or more source objects to a target object. It will return the target object. This method is used mainly with Redux to create immutable objects and return a new state to the reducers (Redux will be covered in Chapter 5, Mastering Redux):

    export default function coinsReducer(state = initialState, action) {
      switch (action.type) {
        case FETCH_COINS_SUCCESS: {
          const { payload: coins } = action;

          return Object.assign({}, state, {
            coins
          });
        }

        default:
         return state;
      }
    };

15. Classes: JavaScript classes, introduced in ES6, are mainly a new syntax for the existing prototype-based inheritance. Classes are functions and are not hoisted. React uses classes to create class Components:

    import React, { Component } from 'react';

    class Home extends Component {
      render() {
        return <h1>I'm Home Component</h1>;
      }
    }

    export default Home;

16. Static methods: Static methods are not called on instances of the class. Instead, they're called on the class itself. These are often utility functions, such as functions to create or clone objects. In React, they can be used to define the PropTypes in a component:

    import React, { Component } from 'react';
    import PropTypes from 'prop-types';
    import logo from '../../images/logo.svg';

    class Header extends Component {
      static propTypes = {
        title: PropTypes.string.isRequired,
        url: PropTypes.string
      };

      render() {
        const { 
          title = 'Welcome to React', 
          url = 'http://localhost:3000' 
        } = this.props;

        return (
          <header className="App-header">
            <a href={url}>
              <img src={logo} className="App-logo" alt="logo" />
            </a>
            <h1 className="App-title">{title}</h1>
          </header>
        );
      }
    }

    export default Header;

17. Promises: The Promise object represents the eventual completion (or failure) of an asynchronous operation and its resulting value. We will use promises in React to handle requests by using axios or fetch; also, we are going to use Promises to implement the server-side rendering (this will be covered in Chapter 11, Implementing Server-Side Rendering).
18. async/await: The async function declaration defines an asynchronous function, which returns an AsyncFunction object. This also can be used to perform a server request, for example using axios:

    Index.getInitialProps = async () => {
      const url = 'https://api.coinmarketcap.com/v1/ticker/';
      const res = await axios.get(url);

      return {
        coins: res.data
      };
    };

This paragraph was written on August 14, 2018, and the latest version of React was 16.4.2. The React 16 version has a new core architecture named Fiber. 

In this recipe, we will see the most important updates in this version that you should be aware of to get the most out of React.

Let's see the new updates:

1. Components can now return arrays and strings from render: Before, React forced you to return an element wrapped with a <div> or any other tag; now it is possible to return an array or string directly:

    // Example 1: Returning an array of elements.
    render() {
      // Now you don't need to wrap list items in an extra element
      return [
        <li key="1">First item</li>,
        <li key="2">Second item</li>,
        <li key="3">Third item</li>,
      ];
    }

    // Example 2: Returning a string
    render() {
      return 'Hello World!';
    }

2. Also, React now has a new feature called Fragment, which also works as a special wrapper for elements. It can be specified with empty tags (<></>) or directly using React.Fragment:

    // Example 1: Using empty tags <></>
    render() {
      return (
        <>
          <ComponentA />
          <ComponentB />
          <ComponentC />
        </>
      );
    }

    // Example 2: Using React.Fragment
    render() {
      return (
        <React.Fragment>
          <h1>An h1 heading</h1>
          Some text here.
          <h2>An h2 heading</h2>
          More text here.
          Even more text here.
        </React.Fragment>
      );
    }

    // Example 3: Importing Fragment
    import React, { Fragment } from 'react';
    ...
    render() {
      return (
        <Fragment>
          <h1>An h1 heading</h1>
          Some text here.
          <h2>An h2 heading</h2>
          More text here.
          Even more text here.
        </Fragment>
      );
    }

3. Error boundaries with from the official website:

    class ErrorBoundary extends React.Component {
      constructor(props) {
        super(props);
        
        this.state = { 
          hasError: false 
        };
      }

      componentDidCatch(error, info) {
        // Display fallback UI
        this.setState({ 
          hasError: true 
        });
        
        // You can also log the error to an error reporting service
        logErrorToMyService(error, info);
      }

      render() {
        if (this.state.hasError) {
          // You can render any custom fallback UI
          return <h1>Something went wrong.</h1>;
        }
        
        return this.props.children;
      }
    }

    // Then you can use it as a regular component:
    render() {
      <ErrorBoundary>
        <MyComponent />
      </ErrorBoundary>
    }

4. Better server-side rendering with from the official site:

5. Reduced file size with from the official site: "Despite all these additions, React 16 is actually smaller compared to 15.6.1.
   • react is 5.3 kb (2.2 kb gzipped), down from 20.7 kb (6.9 kb gzipped)
   • react-dom is 103.7 kb (32.6 kb gzipped), down from 141 kb (42.9 kb gzipped)
   • react + react-dom is 109 kb (34.8 kb gzipped), down from 161.7 kb (49.8 kb gzipped)

That amounts to a combined 32% size decrease compared to the previous version (30% post-gzip)."

I'm not a big fan of Windows for development since it's kind of problematic to configure sometimes. I will always prefer Linux or Mac, but I'm aware that a lot of people who are reading this book will use Windows. In this recipe, I'll show you the most common problems you may have when you try to follow the recipes in this book using Windows.

We'll now see the most common problems using Windows for development:

1. Terminal: The first problem you will face is to use the Windows terminal (CMD) because it does not support Unix commands (like Linux or Mac). The solution is to install a Unix Terminal; the most highly recommended is to use the Git Bash Terminal, which is included with Git when you install it (https://git-scm.com), and the second option is to install Cygwin, which is a Linux Terminal in Windows (https://www.cygwin.com).
2. Environment variables: Another common problem using Windows is to set environment variables. Generally, when we write npm scripts, we set environment variables such as NODE_ENV=production or BABEL_ENV=development, but to set those variables in Windows, you use the SET command, which means you need to do SET NODE_ENV=production or SET BABEL_ENV=development. The problem with this is that if you are working with other people that use Linux or Mac, they will have problems with the SET command, and probably you will need to ignore this file and modify it only for your local environment. This can be tedious. The solution to this problem is to use a package called cross-env; you can install it by doing npm install cross-env, and this will work in Windows, Mac, and Linux:

   "scripts": {
      "start": "cross-env NODE_ENV=development webpack-dev-server --
       mode development --open",
      "start-production": "cross-env NODE_ENV=production webpack-dev-
       server --mode production"
    }

3. Case-sensitive files or directories: In reality, this also happens on Linux, but sometimes it is very difficult to identify this problem, for example, if you create a component in the components/home/Home.jsx directory but in your code you're trying to import the component like this:

    import Home from './components/Home/Home';

4. Paths: Windows uses a backslash (\) to define a path, while in Mac or Linux they use a forward slash (/). This is problematic because sometimes we need to define a path (in Node.js mostly) and we need to do something like this:

    // In Mac or Linux
    app.use(
      stylus.middleware({
        src: __dirname + '/stylus',
        dest: __dirname + '/public/css',
        compile: (str, path) => {
          return stylus(str)
            .set('filename', path)
            .set('compress', true);
        }
      })
    );

    // In Windows
    app.use(
      stylus.middleware({
        src: __dirname + '\stylus',
        dest: __dirname + '\public\css',
        compile: (str, path) => {
          return stylus(str)
            .set('filename', path)
            .set('compress', true);
        }
      })
    );

    // This can be fixed by using path
    import path from 'path';
    
   // path.join will generate a valid path for Windows or Linux and Mac
    app.use(
      stylus.middleware({
        src: path.join(__dirname, 'stylus'),
        dest: path.join(__dirname, 'public', 'css'),
        compile: (str, path) => {
          return stylus(str)
            .set('filename', path)
            .set('compress', config().html.css.compress);
        }
      })
    );