adding spec for devise_helper

spec/helpers/devise_helper_spec.rb

@@ -0,0 +1,47 @@
+require 'rails_helper'
+
+RSpec.describe DeviseHelper, type: :helper do
+  describe '#devise_error_messages!' do
+    subject { helper.devise_error_messages! }
+
+    # create a mock of the Devise Controller resource method
+    before do
+      class MockResource
+        def errors
+          MockError.new
+        end
+      end
+      def helper.resource
+        MockResource.new
+      end
+    end
+
+    context 'When the controller.resource contains no errors' do
+      class MockError
+        def empty?
+          true
+        end
+      end
+      example { expect(subject).to be_nil }
+    end
+
+    context 'When the controller.resource contains an error' do
+      class MockError
+        def empty?
+          false
+        end
+
+        def full_messages
+          [{ msg: 'an error message' }]
+        end
+      end
+
+      example { expect(subject).to be_nil }
+
+      example do
+        subject
+        expect(helper).to set_flash[:devise_error].now
+      end
+    end
+  end
+end