Describe Elements with JSX

React for Real — by Ludovico Fischer (12 / 48)

👈 Create a Component | TOC | Match Components and Data 👉

JSX is an alternative syntax for React.createElement that doesn’t introduce any new functionality, but makes the code easier to read.

You may be used to describing the user interface in separate template files. React developers came up with an unorthodox solution: a new syntax to describe user interfaces inside JavaScript files. Instead of writing React.createElement by hand, you write the element description in a syntax that’s almost identical to HTML, called JSX. Then, before the application runs, you process your source with a tool called Babel to replace JSX with React.createElement.

When compared to templates, the advantage to this approach is that there are fewer special rules to learn than with a separate template language, and error messages tend to be easier to interpret. The trade-off is that you need a build step to prepare your code for production. To get started fast, right now we’ll use an in-browser Babel transform: it’s too slow for production use and may sometimes lead to odd runtime errors, but it’s enough to practice the syntax.

Let’s convert our Adder component to use JSX instead of React.createElement. Modify your code so it looks like this:

In this version, we return an <h1> element, as if we’d written it in HTML. We then output the value of the sum inside the <h1> element by surrounding the variable name with braces. You can mix the result of any JavaScript expression into the generated markup by enclosing the JavaScript expression in braces.

Once you convert Adder to JSX, the application won’t work anymore because the browser can’t interpret JSX. We’ll use the Babel standalone transformer to translate on the fly JSX into React.createElement calls. Open index.html. Insert a link to Babel’s standalone transpiler right above your own code, and then change your code’s type to text/babel:

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