React Core

In this section, we will discuss the core API of React.

JSX

You do not need JSX to use React, as I’ve shown you in previous section. However, JSX would make your code a bit more readable.

Let’s convert Movie component to using JSX. It will look like this:

src/movie.js
jsx
import * as React from 'react';
const Movie = (props) => (
  <div className="movie-container">
    <h1>{props.name}</h1>
    <h2>{props.releaseDate}</h2>
  </div>
);
export default Movie;

src/movie.js
jsx
import * as React from 'react';
const Movie = (props) => (
  <div className="movie-container">
    <h1>{props.name}</h1>
    <h2>{props.releaseDate}</h2>
  </div>
);
export default Movie;

Personally, I feel this is more readable. You may feel uncomfortable to introduce HTML in Javascript, I invite you to give it a shot until the end of this workshop.
Comparing with the previous code, now you know what actually JSX does — it is just translating those HTML tags into React.createElement calls. That’s it.
Note the strange {props.name} syntax: this is how you output Javascript expression. It you take away {}, it will literally output the string props.name.
Notice you still have to import React despite React not being explicitly used. As JSX is compiled to React.createElement, anywhere you use JSX, you need to import React. However, ESLint doesn’t understand this relationship by default, thus it is yelling at you that “React is defined but never used”. We will fix this later.

So now JSX is demystified a bit, let’s go convert App and index.js.

src/app.js
jsx
class App extends React.Component {
  render() {
    return (
      <div>
        <div className="title-bar">
          <h1>React Movie App</h1>
        </div>
        <Movie name="Aquaman" releaseDate="2018-12-07" />
        <Movie name="Bumblebee" releaseDate="2018-12-15" />
        <Movie name="Fantastic Beasts: The Crimes of Grindelwald" releaseDate="2018-11-14" />
      </div>
    );
  }
}

src/app.js
jsx
class App extends React.Component {
  render() {
    return (
      <div>
        <div className="title-bar">
          <h1>React Movie App</h1>
        </div>
        <Movie name="Aquaman" releaseDate="2018-12-07" />
        <Movie name="Bumblebee" releaseDate="2018-12-15" />
        <Movie name="Fantastic Beasts: The Crimes of Grindelwald" releaseDate="2018-11-14" />
      </div>
    );
  }
}

src/index.js
jsx
ReactDOM.render(<App />, document.getElementById('root'));

src/index.js
jsx
ReactDOM.render(<App />, document.getElementById('root'));

Notice that why first letter of Movie is capitalized. It must be. If you make it lowercase, it will try to have movie as a web component and not a React component.
We now pass props down as we add attributes to an HTML tag.

Configure Babel to compile JSX

If you try to compile the code by running npm run build now, it will throw SyntaxError. This is because Babel does not recognize JSX by default since JSX is not part of Javascript — it’s just a syntatic sugar introduced to make React code more readable.

Therefore, we need to configure Babel so that it will recognize JSX and compile them to React.createElement calls.

run npm install -D @babel/preset-react

add @babel/preset-react into .babelrc presets.

.babelrc
json
{
  "presets": ["@babel/preset-env", "@babel/preset-react"]
}

.babelrc
json
{
  "presets": ["@babel/preset-env", "@babel/preset-react"]
}

Now when you start webpack-dev-server, the compilation should succeeds and your app should work as before.

Exercise

Update index.js, app.js, and movie.js to use JSX as described above.
Configure Babel to compile JSX.
Restart webpack-dev-server and verify that the application still works as before.

Commit: 070-jsx-config

React States and Lifecycle Methods

Our current app is too simple, as it’s just rendering a list of movies. In an actual webapp, our application need to be more complex, e.g.:

hide some information by default to declutter your page, but allow user click to show more
loading data from backend api, and showing loading indicator while waiting for response

We will do that by using React states and lifecycle methods

Use React States to track UI states e.g. show/hide

Let’s assume the design of our app is to display “React Movie App” title and a “Show Movies” button only by default. When user clicks the button, the movies will be shown.

To achieve that, let’s modify our App component to:

src/app.js
jsx
class App extends React.Component {
  constructor(props) {
    super(props);
    this.state = {
      showMovies: false,
    };
    this.showMovies = this.showMovies.bind(this);
  }
  showMovies() {
    this.setState({
      showMovies: true,
    });
  }
  render() {
    let movies;
    if (this.state.showMovies) {
      movies = (
        <React.Fragment>
          <Movie name="Aquaman" releaseDate="2018-12-07" />
          <Movie name="Bumblebee" releaseDate="2018-12-15" />
          <Movie name="Fantastic Beasts: The Crimes of Grindelwald" releaseDate="2018-11-14" />
        </React.Fragment>
      );
    }
    return (
      <div>
        <div className="title-bar">
          <h1>React Movie App</h1>
        </div>
        <div className="button-container">
          <button onClick={this.showMovies} className="button">
            Show Movies
          </button>
        </div>
        {movies}
      </div>
    );
  }
}

src/app.js
jsx
class App extends React.Component {
  constructor(props) {
    super(props);
    this.state = {
      showMovies: false,
    };
    this.showMovies = this.showMovies.bind(this);
  }
  showMovies() {
    this.setState({
      showMovies: true,
    });
  }
  render() {
    let movies;
    if (this.state.showMovies) {
      movies = (
        <React.Fragment>
          <Movie name="Aquaman" releaseDate="2018-12-07" />
          <Movie name="Bumblebee" releaseDate="2018-12-15" />
          <Movie name="Fantastic Beasts: The Crimes of Grindelwald" releaseDate="2018-11-14" />
        </React.Fragment>
      );
    }
    return (
      <div>
        <div className="title-bar">
          <h1>React Movie App</h1>
        </div>
        <div className="button-container">
          <button onClick={this.showMovies} className="button">
            Show Movies
          </button>
        </div>
        {movies}
      </div>
    );
  }
}

We declare constructor for our App and initialize our state with this.state = { showMovies: false }. Note that I’ve told you previously that class component is more powerful, and state is one of the functionality that only available for class component (at least for now).
React state should always be a plain Object, while the value of the properties it totally up to you.
Because this is dynamic in Javascript, we need to bind this keywords in showMovies methods to the component with the following code:
```
javascript
this.showMovies = this.showMovies.bind(this);
```
```
javascript
this.showMovies = this.showMovies.bind(this);
```
(This binding is required for each custom method that you define for React component. I will show you how to avoid those tedious repetitions slightly later.)
We declare a showMovies methods, which will call this.setState. setState is a method that is available to all React class component (the component inherit this method via extends React.Component), and it’s only way for you to update states. If you update state directly (this.state.showMovies = true), React will not be notified that the state has been change and thus will not re-render your component, then what is displayed will be incorrect.
When we call setState, React will merge the object we provide it with its current state, then it will rerender the component.
In the render method, we create a button element which will call showMovies method when it is clicked.
Besides, we declare movies variable in render method, which be a list of movies if this.state.showMovies is true. React.Fragment is a container that renders nothing but allows you to wrap a list of React elements.

Before we proceed, there is some cleanup that I want to suggest:

Get rid of movies variable and just inline it with && expression.
You can replace React.Fragment with <>, Babel would understand that <> is actually React.Fragment. I prefer this way, as this expresses what React.Fragment really is — an empty container.

The App component should be as below now:

src/app.js
jsx
class App extends React.Component {
  constructor(props) {
    super(props);
    this.state = {
      showMovies: false,
    };
    this.showMovies = this.showMovies.bind(this);
  }
  showMovies() {
    this.setState({
      showMovies: true,
    });
  }
  render() {
    return (
      <div>
        <div className="title-bar">
          <h1>React Movie App</h1>
        </div>
        <div className="button-container">
          <button onClick={this.showMovies} className="button">
            Show Movies
          </button>
        </div>
        {this.state.showMovies && (
          <>
            <Movie name="Aquaman" releaseDate="2018-12-07" />
            <Movie name="Bumblebee" releaseDate="2018-12-15" />
            <Movie name="Fantastic Beasts: The Crimes of Grindelwald" releaseDate="2018-11-14" />
          </>
        )}
      </div>
    );
  }
}

src/app.js
jsx
class App extends React.Component {
  constructor(props) {
    super(props);
    this.state = {
      showMovies: false,
    };
    this.showMovies = this.showMovies.bind(this);
  }
  showMovies() {
    this.setState({
      showMovies: true,
    });
  }
  render() {
    return (
      <div>
        <div className="title-bar">
          <h1>React Movie App</h1>
        </div>
        <div className="button-container">
          <button onClick={this.showMovies} className="button">
            Show Movies
          </button>
        </div>
        {this.state.showMovies && (
          <>
            <Movie name="Aquaman" releaseDate="2018-12-07" />
            <Movie name="Bumblebee" releaseDate="2018-12-15" />
            <Movie name="Fantastic Beasts: The Crimes of Grindelwald" releaseDate="2018-11-14" />
          </>
        )}
      </div>
    );
  }
}

Use Class Properties Syntax

It is tedious and error-prone to remember and write the binding for each of your component methods. Luckily, you could use the class properties syntax.

Convert your App component as below:

src/app.js
jsx
class App extends React.Component {
  state = {
    showMovies: false
  };
  showMovies = () => {
    this.setState({
      showMovies: true
    });
  }
  render() {
    ...
  }
}

src/app.js
jsx
class App extends React.Component {
  state = {
    showMovies: false
  };
  showMovies = () => {
    this.setState({
      showMovies: true
    });
  }
  render() {
    ...
  }
}

This is much more terse than previous code. However, you would see some compilation error now because class properties is still at proposal phase and it is not included as part of preset-env nor preset-react. To make babel understand additonal syntax, we need to install plugin to “teach” it to parse the syntax.

Install the babel plugin:

bash
npm install -D @babel/plugin-proposal-class-properties

bash
npm install -D @babel/plugin-proposal-class-properties

Update .babelrc as below:

json
{
  "presets": ["@babel/preset-env", "@babel/preset-react"],
  "plugins": ["@babel/plugin-proposal-class-properties"]
}

json
{
  "presets": ["@babel/preset-env", "@babel/preset-react"],
  "plugins": ["@babel/plugin-proposal-class-properties"]
}

Restart webpack-dev-server.

Your code should works now, but with terser code.

React DevTools

One last thing before you write your code, install browser extension for React DevTools. The extension will add a new tab in your browser Devtools with title “React”, which you can used to inspect your React component props and states.

React DevTools

Exercise

Install React DevTools.
Configure babel to parse class properties syntax.
Modify your App component to show movies only when clicked as described above.
Verify that the application works as expected.

Commit: 080-hide-movies

Configure ESLint for React App

Currently ESLint is having parsing error. Let’s configure ESLint before proceed further.

install required packages

bash
npm install -D eslint-plugin-react babel-eslint

bash
npm install -D eslint-plugin-react babel-eslint

update .eslintrc:

.eslintrc
json
{
  "extends": ["eslint:recommended", "plugin:react/recommended", "prettier", "prettier/react"],
  "plugins": ["react"],
  "rules": {
    "react/prop-types": 0
  },
  "parser": "babel-eslint",
  "parserOptions": {
    "ecmaVersion": 2016,
    "sourceType": "module",
    "ecmaFeatures": {
      "jsx": true
    }
  },
  "env": {
    "es6": true,
    "browser": true,
    "node": true
  },
  "settings": {
    "react": {
      "version": "16.7"
    }
  }
}

.eslintrc
json
{
  "extends": ["eslint:recommended", "plugin:react/recommended", "prettier", "prettier/react"],
  "plugins": ["react"],
  "rules": {
    "react/prop-types": 0
  },
  "parser": "babel-eslint",
  "parserOptions": {
    "ecmaVersion": 2016,
    "sourceType": "module",
    "ecmaFeatures": {
      "jsx": true
    }
  },
  "env": {
    "es6": true,
    "browser": true,
    "node": true
  },
  "settings": {
    "react": {
      "version": "16.7"
    }
  }
}

plugin:react/recommended are set of rules included as part of eslint-plugin-react.
react is added to the plugins, in which ESLint will lookup eslint-plugin-react. This plugin “teach” ESLint to recognize all the JSX are using React.createElement under the hood, so it doesn’t show React is not being used error.
babel-eslint is specified as the parser. This allows eslint to use babel to parse the code, which will process the code based on our .babelrc config.
React version is added to settings, in which eslint-plugin-react depends on to suggest applicable rules.
react/prop-types rules is overwritten, as we’re not going to use prop-types in this workshop. I personally think that prop-types does not worth the investment — I recommend Typescript if you want to introduce type-checking in your project.

Exercise

Configure ESLint as described above.
Run npm run lint and verify that no more linting error is shown.

Commit: 090-eslint-react

More on setState

Our current hide-show functionality only allow us to show, but we can’t hide it after that. Let’s enhance it.

Modify App to be following:

src/app.js
jsx
class App extends React.Component {
  state = {
    showMovies: false,
  };
  toggleMovies = () =>
    this.setState((prevState) => ({
      showMovies: !prevState.showMovies,
    }));
  render() {
    return (
      <div>
        <div className="title-bar">
          <h1>React Movie App</h1>
        </div>
        <div className="button-container">
          <button onClick={this.toggleMovies} className="button">
            {this.state.showMovies ? 'Hide' : 'Show'} Movies
          </button>
        </div>
        {this.state.showMovies && (
          <>
            <Movie name="Aquaman" releaseDate="2018-12-07" />
            <Movie name="Bumblebee" releaseDate="2018-12-15" />
            <Movie name="Fantastic Beasts: The Crimes of Grindelwald" releaseDate="2018-11-14" />
          </>
        )}
      </div>
    );
  }
}

src/app.js
jsx
class App extends React.Component {
  state = {
    showMovies: false,
  };
  toggleMovies = () =>
    this.setState((prevState) => ({
      showMovies: !prevState.showMovies,
    }));
  render() {
    return (
      <div>
        <div className="title-bar">
          <h1>React Movie App</h1>
        </div>
        <div className="button-container">
          <button onClick={this.toggleMovies} className="button">
            {this.state.showMovies ? 'Hide' : 'Show'} Movies
          </button>
        </div>
        {this.state.showMovies && (
          <>
            <Movie name="Aquaman" releaseDate="2018-12-07" />
            <Movie name="Bumblebee" releaseDate="2018-12-15" />
            <Movie name="Fantastic Beasts: The Crimes of Grindelwald" releaseDate="2018-11-14" />
          </>
        )}
      </div>
    );
  }
}

Button content has been enhanced to show different text based on the state. As explained previously, we need {} to code JS expression, else it would be treated as string.
showMovies method is renamed to toggleMovies and its content changed. Now the parameters to setState is no longer an object but a function. This is because React may batch multiple setState calls into a single update for performance. Therefore, if our setState is dependent on previous value of state, the correct way to call setState is to pass it a function, in which the parameters of the function would be the previous state (prevState).

In short, you can pass setState either of the following:

the change that you wish React to apply (use this if the new value doesn’t depends on previous value), OR
```
javascript
this.setState({ show: true });
```
```
javascript
this.setState({ show: true });
```

a function that returns the change (use this if the new value depends on previous value)

javascript
this.setState((prevState) => ({ show: !prevState.show }));

javascript
this.setState((prevState) => ({ show: !prevState.show }));

Another things about setState that may surprise React beginner is that it is asynchronous, which means that when you call setState, the state will not be updated straight-away (you can verify this by adding console.log(this.state) after setState call). If you want to perform some action whenever state is updated, you can provide a callback as second parameter to setState:

javascript
this.setState({ show: true }, () => {
  // this will only be called after React apply the state change
  console.log(this.state);
});

javascript
this.setState({ show: true }, () => {
  // this will only be called after React apply the state change
  console.log(this.state);
});

Exercise

Enhance your App component to show/hide movies when button is clicked and display different button text, as described above.
Verify that the application works as expected.

Commit: 100-toggle-movies

Getting Data from Backend API

Currently our movies data are hardcoded in our App:

src/app.js
jsx
...
<>
  <Movie name="Aquaman" releaseDate="2018-12-07" />
  <Movie name="Bumblebee" releaseDate="2018-12-15" />
  <Movie
    name="Fantastic Beasts: The Crimes of Grindelwald"
    releaseDate="2018-11-14"
  />
</>
...

src/app.js
jsx
...
<>
  <Movie name="Aquaman" releaseDate="2018-12-07" />
  <Movie name="Bumblebee" releaseDate="2018-12-15" />
  <Movie
    name="Fantastic Beasts: The Crimes of Grindelwald"
    releaseDate="2018-11-14"
  />
</>
...

Let’s load these data from backend API instead.

React doesn’t dictate how you get data from backend, it’s up to you to decide what library you want to use the make AJAX call to your backend API.

Common options:

window.fetch - no library required as it is supported by all modern browsers. However, polyfill required to support older browser
Axios - Promise based HTTP client.
jQuery AJAX - good option if JQuery is already included in your page, else bad option as it doesn’t make sense to include whole JQuery library for one function only.

For this workshop, I will use Axios as example, but feel free to use other library that you prefer.

I’ve created a backend API. Open this link to see its content. We will utilize this API for this exercise.

To load data from backend API:

install the ajax library of your choice, e.g. npm install axios.

create a file and name it as api.js with the following content: (actual code may differs based on the ajax library that you use)

src/api.js
js
import axios from 'axios';
export const loadMovies = () =>
  axios('https://react-intro-movies.herokuapp.com/movies').then((res) => res.data);

src/api.js
js
import axios from 'axios';
export const loadMovies = () =>
  axios('https://react-intro-movies.herokuapp.com/movies').then((res) => res.data);

modify your App constructor and define a componentDidMount method as below:
```
src/app.js
jsx
import * as React from 'react';
import Movie from './movie';
import { loadMovies } from './api'; // highlight-line
class App extends React.Component {
  state = {
      showMovies: false,
      movies: [] // highlight-line
  };
 // highlight-start
  componentDidMount() {
    loadMovies().then(movies => this.setState({ movies }));
  }
  // highlight-end
  ...
```
```
src/app.js
jsx
import * as React from 'react';
import Movie from './movie';
import { loadMovies } from './api'; // highlight-line
class App extends React.Component {
  state = {
      showMovies: false,
      movies: [] // highlight-line
  };
 // highlight-start
  componentDidMount() {
    loadMovies().then(movies => this.setState({ movies }));
  }
  // highlight-end
  ...
```
- componentDidMount is one of the React component lifecycle methods. componentDidMount will be called immediately after a component is mounted (inserted into DOM).
- There are a few lifecycle methods that you can define to invoke functions at certain lifecycles of React component, e.g. componentWillUnmount (which will be called before component is removed) and componentDidUpdate (which will be called when the props/states of a component has been changed). Read through the docs to get a rough idea what each method does, and refer back whenever you are unsure.
- Verify in “Network” tab of your browser devtools an API call has been made. Inspect the state of your App with React DevTools, and you should able to see the movies states is loaded with the data from the API.

Update render method of your App:

jsx
class App extends React.Component {
  ...
   render() {
     return (
       <div>
         <div className="title-bar">
           <h1>React Movie App</h1>
         </div>
         <div className="button-container">
           <button onClick={this.toggleMovies} className="button">
             {this.state.showMovies ? 'Hide' : 'Show'} Movies
           </button>
         </div>
         {this.state.showMovies &&
           this.state.movies.map(movie => (
             <Movie
               name={movie.name}
               releaseDate={movie.releaseDate}
               key={movie.id}
             />
           ))}
       </div>
     );
   }
}

jsx
class App extends React.Component {
  ...
   render() {
     return (
       <div>
         <div className="title-bar">
           <h1>React Movie App</h1>
         </div>
         <div className="button-container">
           <button onClick={this.toggleMovies} className="button">
             {this.state.showMovies ? 'Hide' : 'Show'} Movies
           </button>
         </div>
         {this.state.showMovies &&
           this.state.movies.map(movie => (
             <Movie
               name={movie.name}
               releaseDate={movie.releaseDate}
               key={movie.id}
             />
           ))}
       </div>
     );
   }
}

as this.state.movies is an array, we use Array.map method to loop through the array and render a list of Movie component instances.
when you render a dynamic list of component, you need to provide a special props, key. key is used by React to identify a specific component instance so that it can decide whether an item need to be unmount or just reorder the dom whenever the list change.

Touching Up UI by Adding Loading Indicator

Our app is able to load data from backend API now, which is great. However, there is some problem that is not obvious to us.

Once your page load and you quickly click “Show Movies” button, you may see no movies is displayed if that API is slow. For that, we should display a loading indicator if we waiting for the API response.

To show loading indicator when waiting API response:

create a file busy-container.js in src folder with the following contents:

src/busy-container.js
jsx
import * as React from 'react';
export const BusyContainer = ({ isLoading, children }) => (
  <div>
    {isLoading && <span>loading...</span>}
    {children}
  </div>
);

src/busy-container.js
jsx
import * as React from 'react';
export const BusyContainer = ({ isLoading, children }) => (
  <div>
    {isLoading && <span>loading...</span>}
    {children}
  </div>
);

BusyContainer is a simple component that will render “loading…” text when its isLoading props is true. In an actual application, you may want to add some fancy spinner svg here.
children is a special props for React components. It’s the contents within the JSX tags (if you remember, the third parameters to React.createElement).

update App component:

src/app.js
jsx
import * as React from 'react';
import { loadMovies } from './api';
import { BusyContainer } from './busy-container'; // highlight-line
import Movie from './movie';
class App extends React.Component {
  state = {
    showMovies: false,
    isLoading: true, // highlight-line
    movies: [],
  };
  componentDidMount() {
    loadMovies().then((movies) => this.setState({ movies, isLoading: false })); // highlight-line
  }
  toggleMovies = () => {
    this.setState((prevState) => ({
      showMovies: !prevState.showMovies,
    }));
  };
  render() {
    return (
      <div>
        <div className="title-bar">
          <h1>React Movie App</h1>
        </div>
        <div className="button-container">
          <button onClick={this.toggleMovies} className="button">
            {this.state.showMovies ? 'Hide' : 'Show'} Movies
          </button>
        </div>
        {this.state.showMovies && (
          // highlight-next-line
          <BusyContainer isLoading={this.state.isLoading}>
            {this.state.movies.map((movie) => (
              <Movie name={movie.name} releaseDate={movie.releaseDate} key={movie.id} />
            ))}
            {/* highlight-next-line */}
          </BusyContainer>
        )}
      </div>
    );
  }
}

src/app.js
jsx
import * as React from 'react';
import { loadMovies } from './api';
import { BusyContainer } from './busy-container'; // highlight-line
import Movie from './movie';
class App extends React.Component {
  state = {
    showMovies: false,
    isLoading: true, // highlight-line
    movies: [],
  };
  componentDidMount() {
    loadMovies().then((movies) => this.setState({ movies, isLoading: false })); // highlight-line
  }
  toggleMovies = () => {
    this.setState((prevState) => ({
      showMovies: !prevState.showMovies,
    }));
  };
  render() {
    return (
      <div>
        <div className="title-bar">
          <h1>React Movie App</h1>
        </div>
        <div className="button-container">
          <button onClick={this.toggleMovies} className="button">
            {this.state.showMovies ? 'Hide' : 'Show'} Movies
          </button>
        </div>
        {this.state.showMovies && (
          // highlight-next-line
          <BusyContainer isLoading={this.state.isLoading}>
            {this.state.movies.map((movie) => (
              <Movie name={movie.name} releaseDate={movie.releaseDate} key={movie.id} />
            ))}
            {/* highlight-next-line */}
          </BusyContainer>
        )}
      </div>
    );
  }
}

we initiate state with additional props, isLoading and set it as true.
when ajax call returns, we set isLoading to false.
isLoading is passed to BusyContainer as props.

Exercise

Get the movie data for your App from backend API instead of hand-coding them.
Create BusyContainer as described and use it in your App component to show loading indicator.
Verify that the application works as expected.

Commit: 110-ajax-calls