Javascript helper in rails projects
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Gem provides a helper asset_path in javascript.


Add gem to your Gemfile

gem 'js_assets'

And run bundle install.

The current version supports only Sprockets 3.x. For Sprockets 2.x use version 0.0.10

gem 'js_assets', '<= 0.0.10'


In your application.js

//= require app_assets

This directive adds the method asset_path in the global scope.

Get the path to the template app/assets/javascripts/rubrics/views/index.html in javascript:

var path = asset_path('rubrics/views/index.html')
// the function will return for development:
// /assets/rubrics/views/index.html
// and for production
// /assets/rubrics/views/index-5eb3bb250d5300736006c8944e436e3f.html

You can look at the raw hash of assets by inspecting JsAssets::List.fetch on the Rails console or the global JavaScript variable, project_assets, in your browser's console.

Updating after changes

js_assets would be pretty slow if it had to calculate the assets on every request! For performance reasons, js_assets therefore keeps a cache of assets and the cache will survive app restarts, even if you restart Spring during development. If you change js_assets' configuration or add/remove any asset files, you should delete tmp/cache under Rails root.

To automatically update app_assets.js when adding new files in app/assets, you can use the guard tool - it monitors the file system and performs actions when certain events occur. This can be useful during development.

To setup Guard, Add to Gemfile:

group :development do
  gem 'guard'
  gem 'guard-shell'

Add to Guardfile:

guard :shell do
    watch(%r{^app/assets/.*}) { `rm -rf tmp/cache` }

Run the command bundle exec to install the gems. Before starting to develop run guard:

$ bundle exec guard

Warning! This may adversely affect the rate of return assets list in the development environment. Since they will be compiled at each change.

Example: Reference Slim-generated HTML assets from JavaScript

For example we want to use templating Slim in AngularJS app. Let our templates will be in app/assets/webapp/. We make the following settings:

In general, the standard config to generate HTML from Slim would look something like this:

# config/application.rb
config.assets.paths << Rails.root.join('app', 'assets', 'webapp')

# config/initizlizers/assets_engine.rb
Rails.application.assets.register_engine('.slim', Slim::Template)

# config/environments/production.rb
config.assets.precompile += ['*.html']

By default, js_assets will make all "*.html" assets available, so we don't have to do any special js_assets config. To reference these templates in JavaScript, just include app_assets in application.js.

//= require app_assets

And now we get a path for these Slim-generated assets in our JavaScript! e.g. to reference the template app/assets/webapp/blogs/edit.html.slim:

var path = asset_path('blogs/edit.html')
// the function will return for development:
// /assets/blogs/edit.html
// and for production
// /assets/blogs/edit-5eb3bb250d5300736006c8944e436e3f.html

The asset_path function will be available to any JavaScript, including those you've generated from other syntaxes such as CoffeeScript.


Using JSAssets::List, you can specify, for example in the initializer, which assets will be available via the asset_path helper, and which should be excluded. By default, "*.html" is included.

You can configure this in application.rb or create an initialiser file like config/intializers/js_assets.rb and configure it there.

To add a file pattern to the list, use:

JsAssets::List.allow << '*.png'

To exclude:

JsAssets::List.exclude << '*.png'

Initially, the list is taken asset falling within the filter app/config/environments/production.rb

config.assets.precompile += ['*.html']

The default settings are:

JsAssets::List.exclude = ["application.js"]
JsAssets::List.allow = ["*.html"]

And remember to delete tmp/cache.

Be careful! If the list of available JsAssets::List.allow get a file that is inserted directive require app_assets, recursion will occur as sprockets will calculate the md5-based content. Generally, if you are using files like "application.js" with a list of "require" directives, you should exclude them using the .exclude setting above.

To determine if filename will be used with md5 hashes, js_assets will use the Rails config:

# Generate digests for assets URLs.
config.assets.digest = true

You can use the directive Rails asset_host by setting ActionController::Base.asset_host in the application configuration, typically in config/environments/production.rb:

config.action_controller.asset_host = ""


Copyright © 2013 Zaur Abasmirzoev <>

JsAssets is distributed under an MIT-style license. See LICENSE for details.