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

1 files changed, 31 insertions, 0 deletions

diff --git a/doc/development/feature_flags.md b/doc/development/feature_flags.md
index 417298205f5..0f1f079bdb4 100644
--- a/doc/development/feature_flags.md
+++ b/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`,