JavaScript Syntax Extensions

In the JavaScript landscape, we can use a JavaScript syntax extension. A syntax extension adds new syntax rules that are not a regular feature of the base programming language. Adding new syntax features to programming languages is a large lift and can lead to frustration and confusion amongst the developer community. However, if there is a specific use case that would benefit from new syntax rules, language developers may create a syntax extension to allow a small set of developers to access that beneficial syntax without impacting the base language. One such extension for JavaScript is JSX or JavaScript XML. JSX syntax allows developers to write HTML elements inside JavaScript code without having to handle any of the rendering themselves. React developers use JSX because it efficiently renders components of web applications.

For example, with JSX, we can assign some HTML to a variable like so:

   let newElement = <h1>This is a new element!</h1>;

Behind the scenes, our React app will use this line of code to create a new HTML element that can be passed on to the browser. You will notice that we didn’t have to handle any of this rendering. All we had to do was write a small heading!

Rendering data in JSX

One task that we will need to take on as JavaScript developers is rendering data values in our application. If we turn our attention back to our hiking application, we may want to create a customized welcome page for the users after they log in to their accounts. We can store the value of a user’s name in a variable called hikersName. Let’s try and write some JSX.

   let hikersName = "Mo";
   let greeting = <h1>Welcome, hikersName!</h1>

If we ran this code in our browser, it would just say “Welcome, hikersName!”. We want to render the value of the hikersName variable not the name of the variable. To do so, we need to wrap JavaScript expressions in HTML in curly braces to ensure that when the page is rendered, we are clear on what is HTML and what is JavaScript.

   let hikersName = "Mo";
   let greeting = <h1>Welcome, {hikersName}!</h1>

Now if we were to run this code, the value of hikersName, “Mo”, would be passed to greeting so when the page rendered, you would have an h1 element that said “Welcome, Mo!”. Wrapping the variable in curly braces gives our hiking application the flexibility to display a custom welcome page for the hiker once they log into their account. If hikersName had a different value than the h1 element would render with that value instead.


Now you might be wondering what comments look like in JSX. You can start a comment with {/* and then end a comment with */}.

   {/* Pass value of hiker's name to welcomePage element */}
   let hikersName = "Mo";
   let greeting = <h1>Welcome, {hikersName}!</h1>


Because you have to wrap your JavaScript expressions when writing JSX, there are some things you cannot write the way you used to when you are writing something inside {}:

  1. Loops
  2. Variable declaration
  3. Function declaration
  4. if/else conditions
  5. objects

However, there are some workarounds that you will learn more about as we dive into React further.

Check Your Understanding


True/False: I can write a loop wrapped in a JavaScript expression in JSX.


True/False: I cannot write HTML with JSX. JSX is just JavaScript but with a cooler name.