The Ajax API
There's a lot we can do on the client side without having to write a single line of JavaScript code.
The Drupal Ajax API is a robust system that allows us to define client-side interactions via PHP. We most commonly use Ajax when we interact with forms—triggering certain actions that change the DOM without having to reload the page. We will demonstrate how all this works by expanding a bit more on the importer functionality we built in Chapter 7, Your Own Custom Entity and Plugin Types. Before that, though, let's take a quick look at a simpler use case of Ajax in Drupal.
Ajax links
The simplest way to interact with Drupal's Ajax API is to add the use-ajax class to any link. This will cause the link to make an Ajax request to the path of the link rather than moving the browser to it. A similar thing can be done with the submit button of a form using the use-ajax-submit class. This makes the form submit via Ajax to the path defined in the form...
