check if username is available in registration form

spec/controllers/users/registrations_controller_spec.rb

@@ -0,0 +1,34 @@
+require 'rails_helper'
+
+describe Users::RegistrationsController do
+
+  describe "POST check_username" do
+
+    before(:each) do
+      @request.env["devise.mapping"] = Devise.mappings[:user]
+    end
+
+    context "when username is available" do
+      it "should return true with no error message" do
+        post :check_username, username: "available username"
+
+        data = JSON.parse response.body, symbolize_names: true
+        expect(data[:available]).to be true
+        expect(data[:message]).to eq I18n.t("devise_views.users.registrations.new.username_is_available")
+      end
+    end
+
+    context "when username is not available" do
+      it "should return true with no error message" do
+        user = create(:user)
+        post :check_username, username: user.username
+
+        data = JSON.parse response.body, symbolize_names: true
+        expect(data[:available]).to be false
+        expect(data[:message]).to eq I18n.t("devise_views.users.registrations.new.username_is_not_available")
+      end
+    end
+
+  end
+
+end