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wrapped up step4
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Micah Godbolt
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# Step 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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- `ReactDOM.render()` - This function is how our code gets on the page. The function takes two parameters, the content to place on the page, and the location that you want it placed.
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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 as well as 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 extendable 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 that takes in `props` and returns [JSX](https://reactjs.org/docs/introducing-jsx.html)
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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()` function 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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Either way you write the component, the component can take in additional props using the same syntax as HTML attribute 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 prop can then be accessed by your component via `props.text` in our function or `this.props.text` in the class.
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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 as you can use 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 tedius. 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 technique 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 a complex data type in a single line.
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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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```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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### Cleanup
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Before we move on, we'll reset our `ReactDom.render` to just return our App. This render function typically includes just the single component with no props.
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Next we'll be creating a `Counter` component, so we'll add that to our App now, and 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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## React State
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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 uses 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 of `clicks` 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(props)`](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 `counter` state value can now be accessed via `this.state.counter`. Later, we can update state by calling `this.setState({ counter: 1 })`.
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### Creating 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 `counter` 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 counter 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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> Note that this 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
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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 out codebase
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### Module Exports
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### Module Imports
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## Using 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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- All components need to import React (don't worry, only one copy ever gets into your app)
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- CSS files imported into the component are only loaded if the component is used
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- React components can be created as a class **or** as a function. In this function component, props are passed in as a function parameter.
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> Until recently, you could only access state in class-based components. But with the advent of [hooks](https://reactjs.org/docs/hooks-intro.html) you can create stateful function components.
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- Since this is a function, we don't have any methods, including `render()`. Just return your JSX as you would in the render function of a class-based component.
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- `props.children` contains anything passed between the opening and closing tags: `<Button>I'm in children</Button>`
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