Unable to access Rails view helpers

With a js.erb file of the following contents:

const ASSET = "<%= asset_path('my-asset') %>";

The above crashes with the following exception (application name replaced with myapp):

ERROR in ./client/js/constants/AssetConstants.js.erb
Module build failed: Error: Command failed: DISABLE_SPRING=1 /Users/mattwidmann/code/ruby/myapp/bin/rails runner /Users/mattwidmann/code/ruby/myapp/node_modules/rails-erb-loader/erb_transformer.rb __RAILS_ERB_LOADER_DELIMETER__ erb
(erb):24:in `<main>': undefined method `asset_path' for main:Object (NoMethodError)
	from /Users/mattwidmann/.rvm/rubies/ruby-2.3.3/lib/ruby/2.3.0/erb.rb:864:in `eval'
	from /Users/mattwidmann/.rvm/rubies/ruby-2.3.3/lib/ruby/2.3.0/erb.rb:864:in `result'
	from /Users/mattwidmann/code/ruby/myapp/node_modules/rails-erb-loader/erb_transformer.rb:19:in `<top (required)>'
	from /Users/mattwidmann/.rvm/gems/ruby-2.3.3@myapp/gems/railties-4.2.8/lib/rails/commands/runner.rb:60:in `load'
	from /Users/mattwidmann/.rvm/gems/ruby-2.3.3@myapp/gems/railties-4.2.8/lib/rails/commands/runner.rb:60:in `<top (required)>'
	from /Users/mattwidmann/.rvm/gems/ruby-2.3.3@myapp/gems/railties-4.2.8/lib/rails/commands/commands_tasks.rb:123:in `require'
	from /Users/mattwidmann/.rvm/gems/ruby-2.3.3@myapp/gems/railties-4.2.8/lib/rails/commands/commands_tasks.rb:123:in `require_command!'
	from /Users/mattwidmann/.rvm/gems/ruby-2.3.3@myapp/gems/railties-4.2.8/lib/rails/commands/commands_tasks.rb:90:in `runner'
	from /Users/mattwidmann/.rvm/gems/ruby-2.3.3@myapp/gems/railties-4.2.8/lib/rails/commands/commands_tasks.rb:39:in `run_command!'
	from /Users/mattwidmann/.rvm/gems/ruby-2.3.3@myapp/gems/railties-4.2.8/lib/rails/commands.rb:17:in `<top (required)>'
	from /Users/mattwidmann/code/ruby/myapp/bin/rails:9:in `require'
	from /Users/mattwidmann/code/ruby/myap/bin/rails:9:in `<main>'

    at ChildProcess.exithandler (child_process.js:211:12)
    at emitTwo (events.js:106:13)
    at ChildProcess.emit (events.js:191:7)
    at maybeClose (internal/child_process.js:885:16)
    at Socket.<anonymous> (internal/child_process.js:334:11)
    at emitOne (events.js:96:13)
    at Socket.emit (events.js:188:7)
    at Pipe._handle.close [as _onclose] (net.js:501:12)

Same for things like javascript_path, image_path, ect.

I have got this to work with my own rake task separately, heres the code for doing that. The trick is being able to add include Rails.application.routes.url_helpers, which most templating engines do not make very easy (Erubis is terrible as you can see below):

# Copied from https://github.com/rails/rails/blob/master/actionview/test/template/erb/helper.rb
class ViewContext
  include ActionView::Helpers
  include Rails.application.routes.url_helpers

  attr_accessor :output_buffer, :controller

  def protect_against_forgery?() false end
end


filename = './client/js/constants/AssetConstants.js.erb'
result = ActionView::Template::Handlers::Erubis.new(File.read(filename)).evaluate(ViewContext.new)
# `result` is the contents of the file after its been evaluated

[rhys-vdw]

Fair enough. I think the ideal would be to see exactly what Rails does when compiling ERB and use the same code path.

This would require knowledge of this part of Rails’ code base.

I agree this would be nice to support. I am not personally planning to tackle this problem any time soon. I am, however, willing to give guidance and feedback about the approach taken.

I expect this is something that the folks over at rails/webpacker would be interested in having support to achieve parity with Sprockets.

[alassek]

I can’t speak for ERB or Erubi but Erubis provides #evaluate which performs an instance_eval against a context object you pass in.

https://github.com/kwatch/erubis/blob/3bad74be554caaa34527e7407138d4840a71c4f1/lib/erubis/evaluator.rb#L69-L75

    def evaluate(_context=Context.new)
      _context = Context.new(_context) if _context.is_a?(Hash)
      #return _context.instance_eval(@src, @filename || '(erubis)')
      #@_proc ||= eval("proc { #{@src} }", Erubis::EMPTY_BINDING, @filename || '(erubis)')
      @_proc ||= eval("proc { #{@src} }", binding(), @filename || '(erubis)')
      return _context.instance_eval(&@_proc)
    end