Declarative UI structure

React newcomers have a hard time coming to grips with the idea that components mix markup in with their JavaScript. If you've looked at React examples and had the same adverse reaction, don't worry. Initially, we're all skeptical of this approach, and I think the reason is that we've been conditioned for decades by the separation of concerns principle. Now, whenever we see things mixed together, we automatically assume that this is bad and shouldn't happen.

The syntax used by React components is called JSX (JavaScript XML). The idea is actually quite simple. A component renders content by returning some JSX. The JSX itself is usually HTML markup, mixed with custom tags for the React components. The specifics don't matter at this point; we'll get into details in the coming chapters. What's absolutely groundbreaking here is that we don't have to perform little micro-operations to change the content of a component.

For example, think about using something like jQuery to build your application. You have a page with some content on it, and you want to add a class to a paragraph when a button is clicked. Performing these steps is easy enough, but the challenge is that there are steps to perform at all. This is called imperative programming, and it's problematic for UI development. While this example of changing the class of an element in response to an event is simple, real applications tend to involve more than three or four steps to make something happen.

React components don't require executing steps in an imperative way to render content. This is why JSX is so central to React components. The XML-style syntax makes it easy to describe what the UI should look like. That is, what are the HTML elements that this component is going to render? This is called declarative programming, and is very well suited for UI development.