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JavaScript Features

Features is an opinionated framework for structured, unobtrusive, jQuery-based JavaScript in Ruby on Rails apps. If you follow a few conventions, lots of things will "just work", just like they do in Rails.

Using it

  1. Add the gem to your Gemfile: gem 'javascript_features'

  2. Add the middleware to your Rack stack: use Rack::JavascriptFeatures

  3. Include the JavaScript in your layout, e.g. in ERB:

      <body class="<%= javascript_feature_classes %>">
        <%= include_javascript_features %>
  4. Add any jQuery plugins you want to use. These should live in /public/javascripts/lib

  5. Start writing JavaScript features. Each should have its own file in /public/javascripts/main and should look like this:

    Features.my_feature = {
      init: function() {
        // This code will be run on pages that
        // explicitly activate this feature
      global_init: function() {
        // This code will be run on every page
  6. Within your templates, activate specific features by called <% use_javascript_for :my_feature %> to make sure the init function is called. You can also specify several features at once: <% use_javascript_for :first_feature, :second_feature %>.

Working with AJAX

If you load some HTML via AJAX and inject it into your page, the relevant JavaScript features will still be initialised for the asynchronously loaded content, as long as the call to use_javascript_for is part of the view or partial that is rendered in response to the AJAX request. However, to make this work correctly you need to make sure that the same bit of markup doesn't get enhanced twice. There are two ways to do this:

  1. Write defensive init functions

    You can add a class to the markup you are enhancing, and only enhance markup that doesn't already have that class. For example:

    Features.my_feature = {
      init: function() {
        // Find all elements with class "my_feature" but not class "enhanced"
        $(".my_feature:not(.enhanced)").each(function() {
          // Set up your feature…
          // Make sure this element isn't enhanced again
  2. Use jQuery's AJAX context option

    If you take this approach you will need to set a context when you make AJAX requests:

      url: "/some/path",
      context: $("div#ajax_target"),
      success: function(data) {

    The context will then be passed to your init functions:

    Features.my_feature = {
      init: function(context) {
        context.find(".my_feature").each(function() {
          // Set up your feature…

    When the init function is called on DOM ready, the whole document will be passed as context.


The JavascriptFeatures::TestCase class is used by the gem to test it's own JavaScript. If you want to test your JavaScript within a minimal HTML page that is isolated from your application then you might also want to use this in you applications. Of course, just because your JavaScript works with a minimal doesn't mean you application's markup hasn't changed and introduced a regression. In most cases it's probably more sensible to test your JavaScript in the context of a real page in your application with something like HolyGrail or Selenium.

See javascript-features/test/javacript_test.rb and xhr_test.rb for examples.

Other bits and bobs

  • You can created multiple sets of features. Just put them in different sub-folders and then pass the name of the folder to include_javascript_features, e.g. you can include the features from /public/javascripts/foo by calling include_javascript_features('foo')


You may use, copy and redistribute this library under the same terms as Ruby itself or under the MIT license.


Structured, unobtrusive JavaScript in Rails apps.






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