React - ReactDOM.render

About

ReactDOM.render ¹⁾ is the main function that renders an element into a DOM element.

It will be:

the root element in a single page application
or another react element called a secondary root, when you want to integrate react at several place in another HTML application (php, ruby,…)

Note that the rendering is updated when the state changes. See this page for more information about re-rendering: React - Rendering (Lifecycle)

Example

Javascript

In Native Javascript, you need to create the element of the virtual dom

First node of the tree: An html react element that represents an heading2 with the React.createElement function.

// equivalent to `<h2>It is ...</h2>`
let h2 = React.createElement('h2', {key:2}, `It is ${new Date().toLocaleTimeString()}`);

Second Node of the tree: A user element called also component (ie a function with a single object parameters called a props that returns an react element

let welcomeComponent = function (props) { return React.createElement(
    'h1', 
    {key:props.key,className: 'greeting'}, 
    `Hello, ${props.name}!`
    ); 
};

The root of the tree with its two children node

// equivalent to `<div><h1><h2> ...</h2></h1></div>`
let root = React.createElement('div',{},
                            [
                               welcomeComponent({key:1,name:"Foo"}), // the user component
                               h2 // the html element
                            ]
); 

// render
ReactDOM.render(root, document.getElementById('root'));

The HTML

<div id="root">
<!-- called the "root" DOM node because the rendering of the root node is done inside -->
</div>

Result:

Rendered by WebCode

When using jsx, the text and the javascript variable of the user component are split into two elements making potential update more granular

Jsx

jsx is a language that compiles to React Javascript and permits to create easily react element in an HTML declarative way.

Demo:

Rendering an element (jsx) at interval every second - The tree is created each time to fake an update of the state

function tick() {
  const element = (
    <div>
      <h1>Hello, world!</h1>
      <h2>It is {new Date().toLocaleTimeString()}.</h2>
    </div>
  );
  ReactDOM.render(
    element,
    document.getElementById('root')
  );
}

setInterval(tick, 1000);

<div id="root">
<!-- called the "root" DOM node because everything inside it will be managed by React DOM -->
</div>

Below React will modify only the time part

Rendered by WebCode

Note that the text It is is not modified as React split them in different element.

Multiple root

See the dedicated page

Documentation / Reference

¹⁾

https://reactjs.org/docs/react-dom.html#render