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• Click the button to launch the GitPod IDE.

Create React App is a fantastic tool to spin up a basic React App with all necessary dependencies.

✅ Step 2a: Run npx create-react-app my-app

✅ Step 2b: Run cd my-app

And that's it! We now have a basic React App. We can go ahead and run the app right away.

✅ Step 2b: Run npm start

As the message states, we can make changes to App.js to update the page. Let's make a few changes.

✅ Step 2c: Edit src/App.js

import logo from './logo.svg';
import './App.css';

function App() {
  return (
    <div className="App">
      <header className="App-header">
        <img src={logo} className="App-logo" alt="logo" />
        <p>
          Hello World!
        </p>
      </header>
    </div>
  );
}

export default App;

So we've removed some unnecessary links, and changed the text. Let's save and update the site.

Cool right?

For our Todo App, we are going to want to have a list of Todos, so let's go over making a new component.

• Create a folder, src/components
• Create a new file, src/components/List.js

✅ Step 3a: Edit List.js

import React from 'react'

function List() {
    return (
        
    );
}

export default List;

Here's our basic component. We are importing our necessary React library, defining our List component, and exporting it for use in App.js.

Let's think about this a bit. We are going to have a list of items, so let's define an array to hold that list, and make up some entries.

const itemList = ["Get milk", "Buy Amazon", "Take over the world"];

Now let's render this list by mapping each item in the array to a <p> tag.

✅ Step 3b: Edit List.js

import React from 'react'

function List() {
    const itemList = ["Get milk", "Buy Amazon", "Take over the world"];

    return (
        itemList.map((item) => (
            <p>{item}</p>
        ))
    );
}

export default List;

Great! We've created our first custom component! Now we need to import it into App.js and use it.

✅ Step 3c: Edit App.js

import logo from './logo.svg';
import './App.css';

import List from './components/List';

function App() {
  return (
    <div className="App">
      <header className="App-header">
        <img src={logo} className="App-logo" alt="logo" />
        <List />
      </header>
    </div>
  );
}

export default App;

Now save, and reload the site.

Awesome! But that's just the start. We can make our component more dynamic using props.

Okay, so we have our List.js component, but it's not very dynamic. It just takes the array we specified and spits out each item as text. We can move the array out of the List component and provide it as a prop instead.

✅ Step 4a: Edit List.js

import React from 'react'

function List(props) {
    const { itemList } = props;

    return (
        itemList.map((item) => (
            <p>{item}</p>
        ))
    );
}

export default List;

Now our List component is going to be looking for a prop called itemList that contains an array. We will provide that prop when we use the component.

<List itemList={["Get milk", "Buy Amazon", "Take over the world"]}/>

Let's add this to App.js.

✅ Step 4b: Edit App.js

import logo from './logo.svg';
import './App.css';

import List from './components/List';

function App() {
  return (
    <div className="App">
      <header className="App-header">
        <img src={logo} className="App-logo" alt="logo" />
        <List itemList={["Get milk", "Buy Amazon", "Take over the world"]}/>
      </header>
    </div>
  );
}

export default App;

What this allows us to do is reuse our List component for any given array. Let's add another List component.

<List itemList={["Get bread", "Get eggs"]} />

Now our component is dynamic! All we need to do is get a list of items as an array, and we can use our component to list the items out.

...But we can go deeper

Let's make another new component

✅ Step 5a: Create src/components/Item.js

import React from 'react';

function Item(props) {
    const { item } = props;

    return (
        <p>{item}</p>
    );
}

export default Item;

And import and use this component in place of <p>{item}</p> in our List component.

✅ Step 5b: Edit List.js

import React from 'react';
import Item from './Item';

function List(props) {
    const { itemList } = props;

    return (
        itemList.map((item) => (
            <Item item={item} />
        ))
    );
}

export default List;

Now we have an Item component that we can customize. For example, we can give the item a different background when the user mouses over.

✅ Step 5c: Edit Item.js

import React from 'react';

function Item(props) {
    const { item } = props;
    const [bg, setBg] = React.useState('');

    const handleMouseOver = () => {
        setBg('red');
    }
    const handleMouseOut = () => {
        setBg('');
    }

    return (
        <p 
            onMouseOver={handleMouseOver} 
            onMouseOut={handleMouseOut} 
            style={{backgroundColor: bg}}
        >
            {item}
        </p>
    );
}

export default Item;

Or we can add input to each item.

✅ Step 5d: Edit Item.js

import React from 'react';

function Item(props) {
    const { item } = props;
    const [isChecked, setIsChecked] = React.useState(false);

    return (
        <div>
            <input
                type="checkbox"
                checked={isChecked}
                onChange={() => setIsChecked(true)}
            />
            <label>{item}</label>
        </div>
    );
}

export default Item;

Voila!

Wait. There's some extra stuff in there. What is this React.useState() stuff!?!

Before we talk about State, let's briefly talk about how React renders things.

Ok, so by now you should be familiar with the return() block. In our latest version of Item.js it looks like this:

return (
    <div>
        <input
            type="checkbox"
            checked={isChecked}
            onChange={() => setIsChecked(true)}
        />
        <label>{item}</label>
    </div>
);

This contains the HTML that is injected into the virtual DOM when this component is used. However, this is initially done when the component first 'mounts' or initilizes and that's it. Unless there is a prop or state change. So in the case of our Item.js component. If the item prop is updated from the parent component, it will trigger a re-render. But what if we don't want to rely on the parent component updating the prop? We can use state.

So in the case of our Item.js component. We want to re-render the component whenever the checkbox is checked, so we can actually see the checked checkbox. For this we are using a state variable called isChecked and we initialize it like this:

const [isChecked, setIsChecked] = React.useState(false);

This is called a React Hook. Hooks are built in methods that we can use to do certain things, in this case create and manage a state variable.

So how does it work?

Well, the useState() hook takes an array of 2 variables, the first will be our state variable - isChecked and the second will be the function we use to update that state variable - setIsChecked. Then we initialize the useState() hook with the variable's initial value, in this case false since the checkbox will start out unchecked.

Now when we call setIsChecked(true) whenever the checkbox changes, the state variable will change to true, triggering a re-render and making our checkbox checked.

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