Question

When does Rails compile Coffeescript resources to JavaScript? Does it happen on-demand or at startup?

Answer 1

CoffeeScript compiles to JavaScript on demand when a request is made to a view that includes a CoffeeScript resource.

However, you can use the following to have CoffeeScript compile to JavaScript ahead of time:

rake assets:precompile

See this link for more information on the Rails asset pipeline.

From the link:

The file extensions used on an asset determine what
preprocessing is applied. When a controller or a scaffold is generated
with the default Rails gemset, a CoffeeScript file and a SCSS file are
generated in place of a regular JavaScript and CSS file. The example
used before was a controller called “projects”, which generated an
app/assets/javascripts/projects.js.coffee and an
app/assets/stylesheets/projects.css.scss file.

When these files are requested, they are processed by the processors
provided by the coffee-script and sass-rails gems and then sent back
to the browser as JavaScript and CSS respectively.

…

Assets are compiled and cached on the first request after the server
is started. Sprockets sets a must-revalidate Cache-Control HTTP header
to reduce request overhead on subsequent requests — on these the
browser gets a 304 (Not Modified) response.

If any of the files in the manifest have changed between requests, the
server responds with a new compiled file.

…

Rails comes bundled with a rake task to compile the asset manifests
and other files in the pipeline to the disk.

Compiled assets are written to the location specified in
config.assets.prefix. The default setting will use the public/assets
directory.

You must use this task either during deployment or locally if you do
not have write access to your production filesystem.

The rake task is:

bundle exec rake assets:precompile

UPDATE:
For those interested in precompiling assets for development, set the RAILS_ENV variable to development first (from here):

RAILS_ENV=development bundle exec rake assets:precompile