frontend-bootcamp/step1-04/demo/README.md

Step 1.4 - Introduction to React (Demo)

In this demo we'll be creating a simple counter that will display a count and increment on click.

Let's start this demo in CodePen.

React Hello World

ReactDOM.render(<p>Hello World</p>, document.getElementById('app'));

Calling ReactDOM.render() is how our code gets on the page. The function takes two parameters: the content to place on the page, and the element in which you want it placed.

The first parameter to render() looks a lot like HTML, but actually, it's JSX. There are a few key differences between JSX and HTML:

• Since class is a reserved word in JavaScript, you will need to use className on your HTML tags: <div className="foo">
• We can use custom HTML tags corresponding to the React components we create: <div><MyControl>hi</MyControl></div>
• Controls can be self-closing: <MyControl text='hi' />
• You can use JavaScript inside of JSX!

Writing a React component

A React component is a piece of code that returns a portion of your application. This can include HTML markup, CSS styles, and JavaScript driven functionality.

Components can be created in two ways. The first is method is to use a JavaScript class, which extends (inherits from) the React.Component class.

Classes in JavaScript provide a way to collect methods (functions) and properties (values) in an extensible container. We extend React.Component because it provides us with several built-in methods, including render.

class App extends React.Component {
  render() {
    return <p>Hello World</p>;
  }
}

We could also write this component as a function:

const App = props => {
  return <p>Hello World</p>;
};

Moving our "Hello World" markup into our App's render function, we can now update the ReactDOM.render() call to look like this:

ReactDOM.render(<App />, document.getElementById('app'));

Note that React components can be reused by writing them in the same way you would an HTML tag.

Props

Whether you write the component as a class or a function, it can take in additional props using the same syntax as HTML attributes like id or href.

<App text="Hello World" />

The text prop can be accessed inside your component via props.text in a function component or this.props.text in a class component.

const App = props => {
  return <p>{props.text}</p>;
};

props allow your component to be more reusable, since you can create multiple instances of the same component with different props.

ReactDOM.render(
  <div>
    <App text="Hello World" />
    <App text="How are you doing?" />
  </div>,
  document.getElementById('app')
);

Note that a render function can only return a single element, so our two App components need to be wrapped in a div.

const App = props => {
  return <p>{props.text ? props.text : 'oops!'}</p>;
};

Destructuring props

Writing props.text over and over in a function (or this.props.text in a class) can be quite tedious. Since this is all JavaScript, you could create a new variable for this text using variable assignment.

const App = props => {
  const text = props.text;
  return <p>{text ? text : 'you missed something'}</p>;
};

This works fine for a single prop, but as your component starts to become more complex:

<MyComponent
  open={false}
  count={5}
  text="Hi there"
  items={['cat', 'dog', 'bird']}
  config={{
    start: 1,
    end: 10,
    autoStart: true
  }}
/>

Note that all non-string values are passed through as JavaScript by wrapping them in {}.

Your code starts to look like this:

const open = props.open;
const text = props.text;
const count = props.count;
const items = props.items;
const start = props.config.start;
const end = props.config.end;

A common approach to simplify this process is to use a syntax called destructuring.

Destructuring allows you to pull individual pieces of information out of an object in a single statement.

const {
  open,
  text,
  count,
  items,
  config: { start, end }
} = props;

So while this might be overkill right now, it makes it easier to add props down the road.

Cleanup

Before we move on, we'll modify our ReactDOM.render call to just include our App. This render call typically includes just a single component with no props.

Next we'll be creating a Counter component. We'll add that to our App now, and then start to write the control.

const App = props => {
  return <Counter text="chickens" />;
};

ReactDOM.render(<App />, document.getElementById('app'));

Note the capitalization of Counter. HTML might not be case-sensitive, but JSX is! A common practice is to use the capitalized names of HTML elements to name corresponding React components: Button, Select, Label, Form, etc.

Writing a stateful Counter component

React allows each control to specify its own data store, called state. We can reference values in state when we render our UI, and we can also update state over the lifetime of our application.

Most stateful components you'll see today will be class based. It is just recently possible to add state to function components through the use of hooks

Adding state

JavaScript classes use a constructor method to instantiate each copy of a class, along with any applicable state. Let's create a new component called Counter and give it a state containing a clicks property with a default value of 0;

class Counter extends React.Component {
  constructor(props) {
    super(props);
    this.state = {
      clicks: 0
    };
  }
}

• The constructor takes in the component's props.
• The super() function calls the constructor of the parent class (in this case React.Component).
• Our counter state value can now be accessed via this.state.counter. Later, we can update state by calling this.setState({ counter: 1 }).

Rendering our Counter

For our Counter component, the goal is to be able to track how many times the counter's button is clicked. We'll use the following markup.

render() {
  const {text} = this.props;
  const {clicks} = this.state;
  return (
    <div>
      {text}: {clicks}
      <button>Click</button>
    </div>
  )
}

Writing our button click handler

Our next step is to wire up the button to increment the counter in our component state.

By convention we place other methods below render(), and private methods (those for internal use only) are prefixed with an underscore.

This function will update our component's state, incrementing the counter value by 1. (Note that setState only modifies the values of keys listed in the object passed as its parameter.)

_onButtonClick = () => {
  this.setState({
    clicks: this.state.clicks + 1
  });
};

This isn't exactly a method, but a class property that is set to an arrow function. This mostly works the same as onButtonClick() { } but eliminates the need for extra boilerplate used to avoid potential "gotchas" with how this works in JavaScript.)

Note that the setState call could also be written as this.setState(prevState => ({ counter: prevState.counter + 1 })); to ensure that state is not updated until the previous state has been determined.

Now that we have a function to increment our count, all that's left is to connect it to our button.

<button onClick={this._onButtonClick}>Click</button>

Also note that each Counter maintains its own state! You can modify the state inside of one counter without affecting the others.

Try it all out!

Add a couple Counters to our App, each with different text. Notice how they can easy take in different props and maintain their own state.

Moving this into our codebase

To scale our application, we'll need to break up the file into smaller, reusable pieces. In this part of the demo we'll look at the final folder and how the JavaScript module system allows us to break up our components into a collection of files exporting their functionality.

Module exports and imports

Open up step1-04/final/components/Counter.tsx and look at the Counter component.

export class Counter extends React.Component {
  // ...
}

This file exports the Counter component as a named export. This means when we import it we do the following:

import { Counter } from './components/Counter';

Note the {} wrapped around the import value. This is actually an example of destructuring.

Default exports

We typically use named exports, but it's also possible export a default value like this:

export default class Counter extends React.Component {
  // ...
}

When we import the component we can call it whatever we want:

import SomeCounterComponent from './components/Counter';

Writing a Button component

Buttons are among the most commonly written components. Custom buttons help abstract common styling, add icons or other decorations, and increase functionality (menu buttons etc). Let's take a quick look at a custom button component to see how it comes together.

import React from 'react';
import './Button.css';

export const Button = props => {
  return (
    <button className="Button" onClick={props.onClick}>
      {props.children}
    </button>
  );
};