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Judicious execution of Javascript in a Rails app
Headshot 128 Ben, May 23, 2012
When you start a new rails app, you get a new javascript file for each controller that you scaffold. Assuming that you're using the asset pipeline, these will all get concatenated into a single file called application.js. This results in all of your javascript being executed by the browser each time the file is pulled down. In many cases, I would find myself tossing controller-specific JS into jQuery's document-ready event. Many times this isn't really an issue, but there were numerous times that it would cause me problems. One example would be if I wanted a jQuery plugin applied to certain elements on one page, but only after a certain action on other pages. On top of that, I'm a slave to the left brain and it bothered me that code for TripsController was executing on a page in the PlansController.

While building Minimundo Travel, I came up with a solution that I felt was pretty solid. There are probably other similar solutions out there, but here's mine.

I started by putting all controller-specific JS into a CoffeeScript class. For example, the JS file for my PlansController starts with:
class MMT.Plans
Then, if I have JS that I want executed for every action in that controller, I give the class an init method.
class MMT.Plans
init: ()->
/* do some controller-wide initialization here */
Next, for each action that has relevant JS, I give the class an init method for that particular action.
class MMT.Plans
init: ()->
/* do some controller-wide initialization here */

initIndex: ()->
/* do some index-action-specific initialization here */
In order to inform the JS of the current controller and action, I added data attributes to the body tag in my application layout.
<body data-controller=\"<%=params[:controller].camelize%>\" data-action=\"<%=params[:action].camelize%>\">
Lastly, I created another file called base.js.coffee that ties everything together and fires on document-ready. All of my classes are namespaced under MMT, so I create a new instance of the controller's class, if it exists. Then, I call the relevant init functions to ultimately fire the JS corresponding to the given action.
$ ()->
controller = $("body").data("controller")
action = $("body").data("action")

if MMT[controller]?
instance = new MMT[controller]()
instance["init"]() if typeof(instance["init"]) == "function"
instance["init#{action}"]() if typeof(instance["init#{action}"]) == "function"

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