Let's start by writing pure React. No compile step. No JSX. No Babel. No Webpack or Parcel. Just some JavaScript on a page.

Let's start your project. Create your project directory. I'm going to call mine adopt-me since we're going to be building a pet adoption app throughout this course. Create an index.html and put it into a src/ directory inside of your project folder. In index.html put:

<!DOCTYPE html>
<html lang="en">

  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <link rel="stylesheet" href="./style.css">
  <title>Adopt Me</title>

  <div id="root">not rendered</div>
  <script src="https://unpkg.com/react@18.2.0/umd/react.development.js"></script>
  <script src="https://unpkg.com/react-dom@18.2.0/umd/react-dom.development.js"></script>
    // Your code is going to go here


What's new between React 17 and React 18? A few things, here and there, but almost entirely additive and few things to change. We'll cover them over the arc of this course.

Now open this file in your browser. On Mac, hit ⌘ (command) + O in your favorite browser, and on Windows and Linux hit CTRL + O to open the Open prompt. Navigate to wherever you saved the file and open it. You should see a line of text saying "not rendered".

  • Pretty standard HTML5 document. If this is confusing, I teach another course called Intro to Web Dev that can help you out.
  • We're adding a root div. We'll render our React app here in a sec. It doesn't have to be called root, just a common practice.
  • We have two script tags.
    • The first is the React library. This library is the interface of how to interact with React; all the methods (except one) will be via this library. It contains no way of rendering itself though; it's just the API.
    • The second library is the rendering layer. Since we're rendering to the browser, we're using React DOM. There are other React libraries like React Native, React 360 (formerly React VR), A-Frame React, React Blessed, and others. You need both script tags. The order is not important.
  • The last script tag is where we're going to put our code. You don't typically do this but I wanted to start as simple as possible. This script tag must come after the other two.

Let's add some style! Click here to get the stylesheet for this course. Make a file called style.css in src/ and paste the previous file there. If you follow along with the course and use the same class names, the styles will be applied for you automatically. This isn't a course on CSS so I make no assertion it's any good!

In the last script tag, put the following.

const App = () => {
  return React.createElement(
    React.createElement("h1", {}, "Adopt Me!")

const container = document.getElementById("root");
const root = ReactDOM.createRoot(container);

This is about the simplest React app you can build.

  • The first thing we do is make our own component, App. React is all about making components. And then taking those components and making more components out of those.
  • There are two types of components, function components and class components. This is a function component. We'll see class components shortly.
  • A function component must return markup (which is what React.createElement generates.)
  • These component render functions have to be fast. This function is going to be called a lot. It's a hot code path.
  • Inside of the render function, you cannot modify any sort of state. Put in functional terms, this function must be pure. You don't know how or when the function will be called so it can't modify any ambient state.
  • React.createElement creates one instance of some component. If you pass it a string, it will create a DOM tag with that as the string. We used h1 and div, those tags are output to the DOM. If we put x-custom-date-picker, it'll output that (so web components are possible too.)
  • The second empty object (you can put null too) is attributes we're passing to the tag or component. Whatever we put in this will be output to the element (like id or style.)
  • First we're using document.getElementById to grab an existing div out of the HTML document. Then we take that element (which we called container) and pass that into ReactDOM.createRoot. This is how we signal to React where we want it to render our app. Note later we can root.render again to change what the root of our React app looks like (I rarely need to do that.)
  • Notice we're using React.createElement with App as a parameter to root.render. We need an instance of App to render out. App is a class of components and we need to render one instance of a class. That's what React.createElement does: it makes an instance of a class. An analogy is that App as a class of components is like Honda has a line of cars called Civics. It's a whole line of cars with various different options and parameters. An instance of a Civic would be one individual car. It's a concrete instance of the Civic car line.

ReactDOM.createRoot is a new API as of React v18. The old ReactDOM.render is still available (and deprecated) but it'll render your app in "legacy" mode which won't use all the fun new features packed into React v18