Rendering Elements

As we mentioned before, a React.element describes what you see on the screen:

RES
let element = <h1> {React.string("Hello World")} </h1>

Unlike browser DOM elements, React elements are plain objects, and are cheap to create. React DOM takes care of updating the DOM to match the React elements.

Rendering Elements to the DOM

Let's assume we've got an HTML file that contains a div like this:

HTML
<div id="root"/>

We call this a “root” DOM node because everything inside it will be managed by React DOM.

Plain React applications usually have a single root DOM node. If you are integrating React into an existing app, you may have as many isolated root DOM nodes as you like.

To render our React application into the root div, we need to do two things:

• Find our DOM node with ReactDOM.querySelector

• Render our React element to our queried Dom.element with ReactDOM.render

Here is a full example rendering our application in our root div:

// Dom access can actually fail. ReScript
// is really explicit about handling edge cases!
switch(ReactDOM.querySelector("#root")){
| Some(root) => ReactDOM.render(<div> {React.string("Hello Andrea")} </div>, root)
| None => () // do nothing
}

React elements are immutable. Once you create an element, you can’t change its children or attributes. An element is like a single frame in a movie: it represents the UI at a certain point in time.

At this point we would need to understand a few more concepts, such as React components and state management to be able to update the rendered elements after the initial ReactDOM.render. For now, just imagine your React application as a tree of elements, whereby some elements may get replaced during the lifetime of your application.

React will automatically recognize any element changes and will only apply the DOM updates necessary to bring the DOM to the desired state.