Support pushing of feature flags to the frontend

This adds a method to Gitlab::GonHelper called
`push_frontend_feature_flag`. This method can be used to easily expose
the state of a feature flag to Javascript code. For example, using this
method we may write the following controller code:

    before_action do
      push_frontend_feature_flag(:vim_bindings)
    end

    def index
      # ...
    end

    def edit
      # ...
    end

In Javascript we can then check the state of the flag as follows:

    if ( gon.features.vimBindings ) {
      // ...
    }

Fixes https://gitlab.com/gitlab-org/release/framework/issues/17

doc/development/feature_flags.md

@@ -69,6 +69,37 @@ For more information about rolling out changes using feature flags, refer to the
 [Rolling out changes using feature flags](rolling_out_changes_using_feature_flags.md)
 guide.
 
+### Frontend
+
+For frontend code you can use the method `push_frontend_feature_flag`, which is
+available to all controllers that inherit from `ApplicationController`. Using
+this method you can expose the state of a feature flag as follows:
+
+```ruby
+before_action do
+  push_frontend_feature_flag(:vim_bindings)
+end
+
+def index
+  # ...
+end
+
+def edit
+  # ...
+end
+```
+
+You can then check for the state of the feature flag in JavaScript as follows:
+
+```javascript
+if ( gon.features.vimBindings ) {
+  // ...
+}
+```
+
+The name of the feature flag in JavaScript will always be camelCased, meaning
+that checking for `gon.features.vim_bindings` would not work.
+
 ### Specs
 
 In the test environment `Feature.enabled?` is stubbed to always respond to `true`,

lib/gitlab/gon_helper.rb

@@ -30,5 +30,20 @@ module Gitlab
         gon.current_user_avatar_url = current_user.avatar_url
       end
     end
+
+    # Exposes the state of a feature flag to the frontend code.
+    #
+    # name - The name of the feature flag, e.g. `my_feature`.
+    # args - Any additional arguments to pass to `Feature.enabled?`. This allows
+    #        you to check if a flag is enabled for a particular user.
+    def push_frontend_feature_flag(name, *args)
+      var_name = name.to_s.camelize(:lower)
+      enabled = Feature.enabled?(name, *args)
+
+      # Here the `true` argument signals gon that the value should be merged
+      # into any existing ones, instead of overwriting them. This allows you to
+      # use this method to push multiple feature flags.
+      gon.push({ features: { var_name => enabled } }, true)
+    end
   end
 end

spec/lib/gitlab/gon_helper_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe Gitlab::GonHelper do
+  let(:helper) do
+    Class.new do
+      include Gitlab::GonHelper
+    end.new
+  end
+
+  describe '#push_frontend_feature_flag' do
+    it 'pushes a feature flag to the frontend' do
+      gon = instance_double('gon')
+
+      allow(helper)
+        .to receive(:gon)
+        .and_return(gon)
+
+      expect(Feature)
+        .to receive(:enabled?)
+        .with(:my_feature_flag, 10)
+        .and_return(true)
+
+      expect(gon)
+        .to receive(:push)
+        .with({ features: { 'myFeatureFlag' => true } }, true)
+
+      helper.push_frontend_feature_flag(:my_feature_flag, 10)
+    end
+  end
+end