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Rewrite of Day 1 to use modern React (#294)
* update to hooks * more class to function * cleanup * finish ts final * update html lesson * add lessons page * clean up * move getters into context * adding type * fix bug * step 5 cleanup * init final pass * text tweak * fix ternaries * readme cleanup * fixed root readme
This commit is contained in:
Micah Godbolt
GitHub
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# Step 1.4 - Introduction to React (Demo)
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In this demo we'll be creating a simple counter that will display a count and increment on click.
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Let's start this demo in [CodePen](https://codepen.io/micahgodbolt/pen/wOWeVb?editors=0010).
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## React Hello World
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```js
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ReactDOM.render(<p>Hello World</p>, document.getElementById('app'));
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```
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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.
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The first parameter to `render()` looks a lot like HTML, but actually, it's [JSX](https://reactjs.org/docs/introducing-jsx.html). There are a few key differences between JSX and HTML:
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- Since `class` is a [reserved word](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Lexical_grammar#Keywords) in JavaScript, you will need to use `className` on your HTML tags: `<div className="foo">`
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- We can use custom HTML tags corresponding to the React components we create: `<div><MyControl>hi</MyControl></div>`
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- Controls can be self-closing: `<MyControl text='hi' />`
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- You can use JavaScript inside of JSX!
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## Writing a React component
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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.
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Components can be created in two ways. The first is method is to use a [JavaScript class](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes), which extends (inherits from) the `React.Component` class.
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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`.
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```jsx
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class App extends React.Component {
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render() {
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return <p>Hello World</p>;
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}
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}
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```
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We could also write this component as a function:
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```jsx
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const App = props => {
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return <p>Hello World</p>;
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};
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```
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Moving our "Hello World" markup into our App's `render` function, we can now update the `ReactDOM.render()` call to look like this:
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```jsx
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ReactDOM.render(<App />, document.getElementById('app'));
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```
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> Note that React components can be reused by writing them in the same way you would an HTML tag.
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### Props
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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`.
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```jsx
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<App text="Hello World" />
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```
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The `text` prop can be accessed inside your component via `props.text` in a function component or `this.props.text` in a class component.
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```jsx
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const App = props => {
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return <p>{props.text}</p>;
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};
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```
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`props` allow your component to be more reusable, since you can create multiple instances of the same component with different props.
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```jsx
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ReactDOM.render(
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<div>
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<App text="Hello World" />
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<App text="How are you doing?" />
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</div>,
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document.getElementById('app')
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);
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```
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> Note that a render function can only return a single element, so our two `App` components need to be wrapped in a `div`.
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```jsx
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const App = props => {
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return <p>{props.text ? props.text : 'oops!'}</p>;
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};
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```
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### Destructuring props
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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.
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```jsx
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const App = props => {
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const text = props.text;
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return <p>{text ? text : 'you missed something'}</p>;
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};
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```
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This works fine for a single prop, but as your component starts to become more complex:
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```jsx
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<MyComponent
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open={false}
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count={5}
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text="Hi there"
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items={['cat', 'dog', 'bird']}
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config={{
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start: 1,
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end: 10,
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autoStart: true
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}}
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/>
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```
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> Note that all non-string values are passed through as JavaScript by wrapping them in `{}`.
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Your code starts to look like this:
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```jsx
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const open = props.open;
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const text = props.text;
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const count = props.count;
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const items = props.items;
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const start = props.config.start;
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const end = props.config.end;
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```
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A common approach to simplify this process is to use a syntax called [destructuring](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Destructuring_assignment#Object_destructuring).
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Destructuring allows you to pull individual pieces of information out of an object in a single statement.
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```jsx
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const {
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open,
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text,
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count,
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items,
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config: { start, end }
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} = props;
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```
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So while this might be overkill right now, it makes it easier to add props down the road.
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### Cleanup
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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.
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Next we'll be creating a `Counter` component. We'll add that to our App now, and then start to write the control.
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```jsx
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const App = props => {
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return <Counter text="chickens" />;
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};
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ReactDOM.render(<App />, document.getElementById('app'));
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```
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> 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.
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## Writing a stateful Counter component
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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.
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> 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`](https://reactjs.org/docs/hooks-intro.html)
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### Adding state
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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`;
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```js
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class Counter extends React.Component {
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constructor(props) {
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super(props);
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this.state = {
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clicks: 0
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};
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}
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}
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```
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- The constructor takes in the component's `props`.
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- The [`super()`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/super) function calls the constructor of the parent class (in this case `React.Component`).
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- Our `clicks` state value can now be accessed via `this.state.clicks`. Later, we can update state by calling `this.setState({ clicks: 1 })`.
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### Rendering our Counter
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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.
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```jsx
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render() {
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const {text} = this.props;
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const {clicks} = this.state;
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return (
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<div>
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{text}: {clicks}
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<button>Click</button>
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</div>
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)
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}
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```
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### Writing our button click handler
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Our next step is to wire up the button to increment the `clicks` in our component state.
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> By convention we place other methods below `render()`, and private methods (those for internal use only) are prefixed with an underscore.
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This function will update our component's state, incrementing the clicks value by 1. (Note that `setState` only modifies the values of keys listed in the object passed as its parameter.)
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```jsx
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_onButtonClick = () => {
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this.setState({
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clicks: this.state.clicks + 1
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});
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};
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```
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> This isn't exactly a method, but a class property that is set to an [arrow function](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions). This mostly works the same as `onButtonClick() { }` but eliminates the need for [extra boilerplate](https://medium.freecodecamp.org/this-is-why-we-need-to-bind-event-handlers-in-class-components-in-react-f7ea1a6f93eb) used to avoid potential "gotchas" with [how `this` works in JavaScript](https://codeburst.io/javascript-the-keyword-this-for-beginners-fb5238d99f85).)
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> Note that the `setState` call could also be written as `this.setState(prevState => ({ clicks: prevState.clicks + 1 }));` to ensure that state is not updated until the previous state has been determined.
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Now that we have a function to increment our count, all that's left is to connect it to our button.
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```jsx
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<button onClick={this._onButtonClick}>Click</button>
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```
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> Also note that each `Counter` maintains its own state! You can modify the state inside of one counter without affecting the others.
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## Try it all out!
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Add a couple `Counter`s to our `App`, each with different text. Notice how they can easy take in different props and maintain their own state.
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## Moving this into our codebase
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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.
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### Module exports and imports
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Open up `step1-04/final/components/Counter.tsx` and look at the `Counter` component.
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```tsx
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export class Counter extends React.Component {
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// ...
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}
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```
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This file exports the Counter component as a **named export**. This means when we import it we do the following:
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```tsx
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import { Counter } from './components/Counter';
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```
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> Note the `{}` wrapped around the import value. This is actually an example of destructuring.
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#### Default exports
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We typically use named exports, but it's also possible export a default value like this:
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```tsx
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export default class Counter extends React.Component {
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// ...
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}
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```
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When we import the component we can call it whatever we want:
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```tsx
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import SomeCounterComponent from './components/Counter';
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```
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## Writing a Button component
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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.
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```jsx
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import React from 'react';
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import './Button.css';
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export const Button = props => {
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return (
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<button className="Button" onClick={props.onClick}>
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{props.children}
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</button>
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);
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};
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```
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@@ -21,7 +21,7 @@
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background: green;
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}
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</pre>
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<pre data-lang="javascript">
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<pre data-lang="typescript">
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ReactDOM.render(
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<div>Hello World</div>,
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document.getElementById('app')
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