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, 32 insertions, 0 deletions

diff --git a/spec/lib/gitlab/gon_helper_spec.rb b/spec/lib/gitlab/gon_helper_spec.rb
new file mode 100644
index 00000000000..c6f09ca2112
--- /dev/null
+++ b/spec/lib/gitlab/gon_helper_spec.rb
@@ -0,0 +1,32 @@
+# 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