This code uses respond_to to take the appropriate action depending on the kind of request. (There is no relationship between this respond_to and the respond_to used in the RSpec examples.) The syntax is potentially confusing, and it’s important to understand that in

only one of the lines gets executed (based on the nature of the request)

  In the case of an Ajax request, Rails automatically calls a JavaScript Embedded Ruby (.js.erb) file with the same name as the action, i.e., create.js.erb or destroy.js.erb. As you might guess, the files allow us to mix JavaScript and Embedded Ruby to perform actions on the current page. It is these files that we need to create and edit in order to update the user profile page upon being followed or unfollowed.

       Inside a JS-ERb file, Rails automatically provides the jQuery JavaScript helpers to manipulate the page using the Document Object Model (DOM). The jQuery library provides a large number of methods for manipulating the DOM, but here we will need only two. First, we will need to know about the dollar-sign syntax to access a DOM element based in its unique CSS id. For example, to manipulate the follow_form element, we will use the syntax .

(Recall from Listing 11.23 that this is a div that wraps the form, not the form itself.) This syntax, inspired by CSS, uses the # symbol to indicate a CSS id. As you might guess, jQuery, like CSS, uses a dot . to manipulate CSS classes.

    The second method we’ll need is html, which updates the HTML inside the relevant element with the contents of its argument. For example, to replace the entire follow form with the string "foobar", we would write