Ever read online that React is declarative? Yeah, me too. Unfortunately, I had no idea what meant for the longest time. Simply put, React lets you state what you want, not how you want it. That means - instead of creating a DOM element and putting some content in it, you tell React that you want a certain type of element with a certain set of props at some point.

Now, this might not seem like a good developer experience. That's a lot of indirection in order to add a simple <div /> tag to the DOM? Truth is, adding stuff to the DOM is not the most exiting part of the job. We just want to make cool web apps - why care about the details about how stuff actually gets to the screen?

That's the cool part of React - it does all of the heavy lifting for us, and gives us a great way of writing these cool UIs without thinking twice about performance or all the other hard stuff. So how does React do it?

React.createElement

All of React's magic starts with this little function. You might not have seen it before, but you invoke it all of the time. How? With JSX! As I explained in this article, JSX is just a fancy way of writing React.createElement(elementName, props, children). So when you write this snippet:

<button className="button">Clap</button>

, you really write this:

React.createElement('button', { className: 'button' }, 'Clap');

But what does this tool do? Well, simply put, it creates a simple object - React calls them elements - that basically look like this:

{
  type: 'button',
  props: {
    className: 'button',
    children: 'Clap',
  },
}

Don't believe me? Check out this codepen!

These "elements" are created for every single component you render, and is the base for what React calls reconciliation - also known as a "virtual DOM". They are collected, combined and passed to an algorithm that figures out how to translate the current state of the DOM to the next. That algorithm is what React calls Fiber.

How about that Fiber?

Fiber is React's name of a concept it calls a reconciler. It takes an old element tree and a new element tree, and figures out the minimal amount of changes needed to transition one to the other. Thanks to some pretty nifty implementation details, this algorithm can even break up this process into prioritized units of work which can be paused, aborted and resumed at will. This leads to React being able to prioritize stuff you might notice, like animations, and defer stuff that doesn't matter as much (like displaying new data from a data store).

This reconciliation process has nothing to do with rendering stuff to the screen. It simply figures out what elements need to change and how to change them in the smallest amount of operations. The thing that actually does the rendering is what you think of as react-dom or react-native - also known as renderers.

Finally, let's render stuff!

Once React has figured out what needs to change, its result is passed to a renderer of choice. Web developers tend to choose react-dom, which renders to a browser DOM, while mobile developers use react-native, which renders to native iOS or Android views.

The first time you run ReactDOM.render, React will create a completely new DOM tree and insert it into the browser. Whenever the React application changes (due to changes in state or props), ReactDOM (or a renderer of your choice) will make sure to figure out the smallest number of changes possible to update the existing DOM structure to the new one.

And that's it!

Advanced? Heck yes! Luckily we don't have to deal with any of this - we can just write regular React and let the framework deal with the rest. That's the beauty of React - it hides all of this magic for the developer, and lets us focus on writing cool UIs. :)

If you didn't understand any of this - that's fine, too. It doesn't really matter to you as a developer - it's just fun to know what this magic black box called React actually does whenever you call setState.