React newcomers have a hard time coming to grips with the idea that components mix markup in with their JavaScript in order to declare UI structures. 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. This principle states that different concerns, such as logic and presentation, should be separate from one another. 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). A component renders content by returning some JSX. The JSX itself is usually HTML markup, mixed with custom tags for React components. The specifics don't matter at this point; we'll go into detail in the coming chapters. What's groundbreaking about the declarative JSX approach 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. This is called imperative programming, and it's problematic for UI development. While this example of changing the class of an element 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. This is why JSX is 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. Once you've declared your UI structure, you need to specify how it changes over time.
