Accessing a "helper" method "foo" defined in a controller, in a .js file


I am having some trouble accessing a method “foo” I defined in a

I was under the impression that if “foo” is in the controller that
bar.html.erb, then bar.html.erb can in fact see a method in this
provided I put the “helper_method: foo” notation in the controller. I
able to verify that foo was in fact accessible in the bar.html.erb file.

The problem is I want to access foo in alpha.js.erb. My application.js
includes alpha.js (alpha.js.erb) implicitely because alpha.js.erb is in
app/assets/javascripts and application.js has “//= require_tree .”.

But alpha.js.erb cannot see foo. I have no idea how to get my custom
functions to become accessible in the js files.

Would appreciate help. I scoured google for a few hours already and
not find much on this.


Hi Neeraj,

You most serious problem you are facing is that js files included in
application.js are not rendered in the context of your request. They are
meant to be precompiled (meaning when you deploy your application they
all compiled only once so that each request for them is very fast) and
loaded separately.

There are a couple of ways around this, the easiest being adding a
javascript block to your application.html.erb or bar.html.erb that
that code instead. Another alternative would be to have your javascript
call an API for that data.

If you aren’t trying to render request-specific data there are other
alternatives. Is that the case?

On Mon, Mar 23, 2015 at 7:34 PM, Neeraj M. [email protected]

The problem is I want to access foo in alpha.js.erb. My application.js

For more options, visit

Daniel Evans

Hi Daniel.

Thanks very much for the response. The fact that they are precompiled at
deployment might explain why I have been finding it excruciatingly hard
figure this out.

If I went with the approach of adding a JS block into my html.erb file,
would be slower, I suspect. But would it mean that all my helpers in
rubyland are now namespace visible? Now if I do this, wouldn’t I have to
move the js file in question out of the folder that application.js is
including all js files from?

My second question is probably more important. The logic needed in the
helper that I need in this js file is logic that is available right at
precompilation time. I know that .js.erb files can in fact use helpers,
how does that work? Are these limited to js.erb files that are not
precompiled, or can precompiled ones also include “special” helpers that
are “executed” at precompilation time? Ideally, application.js is
precompiled and this helper is run at precompilation. If that’s


Oh, and I am not rendering request-specific data. To put it more
the data needed in this helper is in a database table. I just need to
it up in the table and then do some stuff related to it in the js file.
can do this right at precompile time. What alternatives are there?