What Is JSX?

JSX is a React-specific XML/HTML-like syntax that extends ECMAScript to allow for the coexistence of XML/HTML-like text and JavaScript/React code. Preprocessors (i.e., transpilers like Babel) will employ this syntax to convert HTML-like content present in JavaScript files into standard JavaScript objects that a JavaScript engine will parse.
JSX allows you to write compact HTML/XML-like structures (e.g., DOM-like tree structures) in the same file as JavaScript code, and Babel will later convert these expressions to JavaScript code. Instead of putting JavaScript into HTML as in the past, JSX allows us to put HTML into JavaScript.
By using JSX one can write the following JSX/JavaScript code:

var nav = (
    <ul id="nav">
      <li><a href="#">Home</a></li>
      <li><a href="#">About</a></li>
      <li><a href="#">Clients</a></li>
      <li><a href="#">Contact Us</a></li>
    </ul>
);

And Babel will transform it into this:

var nav = React.createElement(
   "ul",
   { id: "nav" },
   React.createElement(
      "li",
      null,
      React.createElement(
         "a",
         { href: "#" },
         "Home"
      )
   ),
   React.createElement(
      "li",
      null,
      React.createElement(
         "a",
         { href: "#" },
         "About"
      )
   ),
   React.createElement(
      "li",
      null,
      React.createElement(
         "a",
         { href: "#" },
         "Clients"
      )
   ),
   React.createElement(
      "li",
      null,
      React.createElement(
         "a",
         { href: "#" },
         "Contact Us"
      )
   )
);

You can think of JSX as a shorthand for calling React.createElement().
The idea of mixing HTML and JavaScript in the same file can be a rather contentious topic. Ignore the debate. Use it if you find it helpful. If not, write the React code required to create React nodes. Your choice. My opinion is that JSX provides a concise and familiar syntax for defining a tree structure with attributes that does not require learning a templating language or leaving JavaScript. Both of which are can be a win when building large applications