React - ReactDOM.render


ReactDOM.render 1) is the main function that renders initialy an element.

It will be:

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



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

// 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(
    {key:props.key,className: 'greeting'}, 
    `Hello, ${}!`
  • 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 -->
  • Result:

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


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


  • 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 = (
      <h1>Hello, world!</h1>
      <h2>It is {new Date().toLocaleTimeString()}.</h2>

setInterval(tick, 1000);
<div id="root">
<!-- called the "root" DOM node because everything inside it will be managed by React DOM -->

Below React will modify only the time part

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

Multiple root

See the dedicated page

Documentation / Reference

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