React.createElement()

Time to install React! Find your open terminal: yarn add react - and we also need a second package called react-dom:

yarn add react react-dom --dev

react is... um... React. react-dom is the library we'll use to render our React app onto the page, the DOM. These are separate libraries because you can actually use React to render things in a non-browser environment.

Anyways, when that finishes, go back to rep_log_react.js. Like with all libraries, to use React, we need to require or import it. I'll use the newer, far more hipster import syntax in this tutorial to import React from react.

Elements & the Virtual DOM

Here's the idea behind React: it's actually really beautiful: we create React element objects that represent HTML elements, like an h1 tag. These don't actually live on the page, they're just objects that describe HTML elements.

But then, we can tell React to look at our element objects, and use them to create real HTML elements on the page, or, on the DOM. This means that we will have a tree of element objects in React and a tree of element objects on the page. To say it a different way, we will have a virtual DOM in React and the real DOM on the page.

And... that's it! Of course, the magic is that, when we change some data on a React element object, React will update the corresponding DOM element automatically.

Creating React Elements

So... let's create some React elements! To really master this, yea... we're going to do things the hard way first. But, it will be totally worth it.

Create a new const el set to React.createElement(). Make this an h2 tag: we're building the title on top of the app. Pass null for the second argument: but this is where you could pass an array of any HTML attributes for the element. These are called "props" in React - but more on that later. For the third argument, pass whatever you want to put inside, like text: "Lift History!".

Cool! Let's console.log(el): I want you to see that this is just a simple object. Go refresh the page. The element is not, yet, being rendered to the screen in any way. It's just a React element that describes a potential HTML element.

Rendering React to the DOM

Now, go back to index.html.twig. To render the element onto the page, we need a target. The existing app is rendered in this js-rep-log-table element. Above this, add a new <div class="row"> and inside, an empty div we can render into with id="", how about, lift-stuff-app. Then, for a bit more separation, add a bunch of line breaks and an <hr>.

Awesome! Copy the id of the div. To render React to the DOM, we need to use that other package we installed: import ReactDom from react-dom. Then, just, ReactDom.render() to render our el into document.getElementById('lift-stuff-app').

That's it! Step 1: create a React element object and, step 2, use ReactDom and some boring, raw JavaScript to render it onto the page.

Let's go try it! Move over and refresh! Ha! We have our very first, but I, know very simple, React app. We deserve balloons!

Nested Elements

Of course, in a real app, we're going to have more than just one element. Heck, we're going to have a big nested tree of elements inside of other elements, just like we do in normal HTML.

So... how could we put an element inside of our h2? First, break things onto multiple lines to keep our sanity. The answer is... by adding more and more arguments to the end of React.createElement(). Each argument - starting with the third argument - becomes a new child that lives inside the h2. For example, to create a nested span element, use React.createElement() with span, null and a heart Emoji.

Let's log el again. Then, flip over and... refresh!

Ha! There it is! Inspect the element: yep, the h2 tag with a span inside. Check out the logged Element: it now has two "children", which is a React term: the text and another React element object.

Awesome! But... you've probably already noticed a problem. Building a real app with many nested elements is going to get really ugly... really quickly. This React "element" idea is great in theory.... but in practice, it's a nightmare! We need another tool to save us. That tool is love. I mean, JSX.