Rails asset pipeline and digest values

Question

Does anyone know how exactly the asset digest value is calculated? If I have two JS files which contain various other included JS scripts then will each file maintain the same digest hash if none of the inner scripts have been changed? Or is a new digest value calculated each time the assets:precompile operation is run?

Answer 1

The accepted answer is not quite true. We build static assets for our staging, demo and production servers, and the same asset is given different digest values in each case.

It turns out that the Rails environment is also being taken into consideration. Sprockets creates the digest as follows:

# Sprockets::Environment::initialize
@digest_class = ::Digest::MD5

# Sprockets::Base::digest
@digest ||= digest_class.new.update(VERSION).update(version.to_s)

# Sprockets::Base::file_digest(path)
digest.file(path.to_s)

# Sprockets::Asset::initialize
@digest = environment.file_digest(pathname).hexdigest

Rails hooks into Sprockets as follows:

# Sprockets::Railtie
app.assets = Sprockets::Environment.new(app.root.to_s) do |env|
  env.version = ::Rails.env + "-#{config.assets.version}"
  ...
end

So rails is creating a Sprockets environment that has a version equal to the Rails environment, and Sprockets uses the version when creating the digest class.