This post is part of the series 30 Days of React.

In this series, we're starting from the very basics and walk through everything you need to know to get started with React. If you've ever wanted to learn React, this is the place to start!

What is JSX?

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Now that we know what React is, let's take a look at a few terms and concepts that will come up throughout the rest of the series.

In our previous article, we looked at what React is and discussed at a high-level how it works. In this article, we're going to look at one part of the React ecosystem: ES6 and JSX.

JSX/ES5/ES6 WTF??!

In any cursory search on the Internet looking for React material, no doubt you have already run into the terms JSX, ES5, and ES6. These opaque acronyms can get confusing quickly.

ES5 (the ES stands for ECMAScript) is basically "regular JavaScript." The 5th update to JavaScript, ES5 was finalized in 2009. It has been supported by all major browsers for several years. Therefore, if you've written or seen any JavaScript in the recent past, chances are it was ES5.

ES6 is a new version of JavaScript that adds some nice syntactical and functional additions. It was finalized in 2015. ES6 is almost fully supported by all major browsers. But it will be some time until older versions of web browsers are phased out of use. For instance, Internet Explorer 11 does not support ES6 but has about 12% of the browser market share.

In order to reap the benefits of ES6 today, we have to do a few things to get it to work in as many browsers as we can:

  1. We have to transpile our code so that a wider range of browsers understand our JavaScript. This means converting ES6 JavaScript into ES5 JavaScript.
  2. We have to include a shim or polyfill that provides additional functionality added in ES6 that a browser may or may not have.

We'll see how we do this a bit later in the series.

Most of the code we'll write in this series will be easily translatable to ES5. In cases where we use ES6, we'll introduce the feature at first and then walk through it.

As we'll see, all of our React components have a render function that specifies what the HTML output of our React component will be. JavaScript eXtension, or more commonly JSX, is a React extension that allows us to write JavaScript that looks like HTML.

Although in previous paradigms it was viewed as a bad habit to include JavaScript and markup in the same place, it turns out that combining the view with the functionality makes reasoning about the view straight-forward.

To see what this means, imagine we had a React component that renders an h1 HTML tag. JSX allows us to declare this element in a manner that closely resembles HTML:

class Header extends React.Component {
  render() {
    return (
      <h1 className='large'>Hello World</h1>
    );
  }
}

The render() function in the HelloWorld component looks like it's returning HTML, but this is actually JSX. The JSX is translated to regular JavaScript at runtime. That component, after translation, looks like this:

class HelloWorld extends React.Component {
  render() {
    return (
      React.createElement(
        'h1',
        {className: 'large'},
        'Hello World'
      )
    );
  }
}

While JSX looks like HTML, it is actually just a terser way to write a React.createElement() declaration. When a component renders, it outputs a tree of React elements or a virtual representation of the HTML elements this component outputs. React will then determine what changes to make to the actual DOM based on this React element representation. In the case of the HelloWorld component, the HTML that React writes to the DOM will look like this:

<h1 class='large'>Hello World</h1>

The class extends syntax we used in our first React component is ES6 syntax. It allows us to write objects using a familiar Object-Oriented style. In ES6, the class syntax might be translated as:

var HelloWorld = function() {}
Object.extends(HelloWorld, React.Component)
HelloWorld.prototype.render = function() {}

Because JSX is JavaScript, we can't use JavaScript reserved words. This includes words like class and for.

React gives us the attribute className. We use it in HelloWorld to set the large class on our h1 tag. There are a few other attributes, such as the for attribute on a label that React translates into htmlFor as for is also a reserved word. We'll look at these when we start using them.

If we want to write pure JavaScript instead of rely on a JSX compiler, we can just write the React.createElement() function and not worry about the layer of abstraction. But we like JSX. It's especially more readable with complex components. Consider the following JSX:

<div>
  <img src="profile.jpg" alt="Profile photo" />
  <h1>Welcome back Ari</h1>
</div>

The JavaScript delivered to the browser will look like this:

React.createElement("div", null, 
  React.createElement("img", {src: "profile.jpg", alt: "Profile photo"}),
  React.createElement("h1", null, "Welcome back Ari")
);

Again, while you can skip JSX and write the latter directly, the JSX syntax is well-suited for representing nested HTML elements.

Now that we understand JSX, we can start writing our first React components. Join us tomorrow when we jump into our first React app.


Ari Lerner

Hi, I'm Ari. I'm an author of Fullstack React and ng-book and I've been teaching Web Development for a long time. I like to speak at conferences and eat spicy food. I technically got paid while I traveled the country as a professional comedian, but have come to terms with the fact that I am not funny.

Connect with Ari on Twitter at @auser.