⏱ A simple counter app built in React

What you will learn in this exercise

• Set the initial state from props
• Update state from its previous state
• Add CSS classes to components
• Publish your app on GitHub Pages

We will be building a simple counter app in React. See the demo of the final app.

Your task is to build a Counter component which displays a counter and contains two buttons - one to increment the counter and the other to decrement the counter.

One way to do this is to first start with HTML and then convert it to JSX using a tool like HTMLtoJSX

Next, we will create functions that will handle when a button is clicked using the onClick attribute.

So far, we have only one component. We can logically split them up into separate components.

Next up, using the className attribute let's dynamically add CSS classes to change the color of the counter. For instance, display the counter as red when it is a negative value and green when it is a positive value.

Finally, we will create multiple Counter components with varying initial count values and render them all to the DOM.

You do not need to call addEventListener to add listeners to a DOM element. Instead, React uses the onClick attribute which accepts a callback to handle events.

<button onClick={this.handleClick}>

There may be times you want to pass arguments to an event handler. Either of the following would work:

<button onClick={(event) => this.deleteRow(id, event)}>Delete Row</button>
<button onClick={this.deleteRow.bind(this, id)}>Delete Row</button>

At this point you may encounter issues with this when passing functions as props to other components. There are several ways to solve this issue.

One way is to add this.handleClick = this.handleClick.bind(this); inside the constructor method.

class Toggle extends React.Component {
  constructor(props) {
    super(props);
    this.state = {isToggleOn: true};
    this.handleClick = this.handleClick.bind(this);
  }

  handleClick() {
    this.setState(state => ({
      isToggleOn: !state.isToggleOn
    }));
  }

  render() {
    return (
      <button onClick={this.handleClick}>
        {this.state.isToggleOn ? 'ON' : 'OFF'}
      </button>
    );
  }
}

Another way is to simply bind this when passing it to onClick like this

<button onClick={this.handleClick.bind(this)}>

Since arrow functions implicitly bind this, you can also wrap it around an arrow function like this

<button onClick={() => this.handleClick}>

Finally, there's also the public class fields syntax

handleClick = () => {
  console.log('this is:', this);
}

To add a CSS class to a React element use the className attribute and then add the CSS rules for that class in a CSS file like style.css. You may optionally use the classnames package to construct CSS classes.

<header className="header">Hello, world!</header>

<main className="my-first-class my-second-class">Some amazing content here!</main>

<footer className={this.props.style}>Built with ❤️ by JumpStarters</footer>

You can also use the style attribute and pass in a JavaScript object. It uses camelCase properties like backgroundColor instead of background-color, which is consistent with the DOM style JavaScript property (e.g. body.style.fontSize)

const divStyle = {
  color: 'blue',
  backgroundImage: 'url(' + imgUrl + ')',
};

function HelloWorldComponent() {
  return <div style={divStyle}>Hello World!</div>;
}

Do note that the official React documentation states that, while convenient, "using the style attribute as the primary means of styling elements is generally not recommended."

CSS classes are generally better for performance than inline styles.

Another option is to use something called CSS-in-JS, where CSS is composed within JavaScript instead of defined in external files. A comparison of numerous third-party libraries that are available can be found here.

Since components are reusable, we can add as many components to our app as we want.

Here's one way you can add multiple components

function App() {
  return (
    <React.Fragment>
      <Hello name="Alice" />
      <Hello name="Bob" />
      <Hello name="Cindy" />
    </React.Fragment>
  )
}

Another way is to first define an array and then use map() to create a new array of React elements

function App() {
  const names = ['Alice', 'Bob', 'Cindy']

  return (
    <React.Fragment>
      {names.map(name => {
        return <Hello name={name} />
      })}
    </React.Fragment>
  )
}

Publish your app by going into settings in your GitHub repository. Under the header GitHub Pages, select the master branch as your source.

Your app should be published here https://<username>.github.io/<repo-name>

• Handling events