Rspec + Capybara : How to click on alert box

I have gone through the post of capybara + click on alert box but nothing seems to be work. Following is my scenario:

Scenario: When I click update button An alert box appears which contains "OK" and "Cancel" button. Click on "Ok" then new form appears.

1. I am writing request specs i.e. using rspec and capybara. Main problem is to click on the alert box: Following is my code:

    context "update" do
      before(:all) do
        Capybara.current_driver = :selenium
      end
      after(:all) do
        Capybara.use_default_driver
      end
   
      it "update user to trainer" do
        click_button('Search')
        sleep 3 
        page.evaluate_script('data-confirm = function() { return true; }')
        page.click('OK')      
        click_button('Upgrade')
      end
    end
   

   But executing script gives me following error:

   Failure/Error: page.evaluate_script('data-confirm = function() { return true; }') Selenium::WebDriver::Error::UnexpectedJavascriptError: invalid assignment left-hand side # ./spec/requests/user_upgrades_spec.rb:30

2. For the second example i.e.

   page.driver.browser.switch_to.alert.accept

   My code :

    context "update" do
      before(:all) do
        Capybara.current_driver = :selenium
      end
      after(:all) do
        Capybara.use_default_driver
      end
      it "update user to trainer" do
        click_button('Search')
        sleep 3   
        click_button('Upgrade') 
        page.driver.browser.switch_to.alert.accept
      end
    end 
   

I get error:

Failure/Error: page.driver.browser.switch_to.alert.accept Selenium::WebDriver::Error::UnhandledError:

Please let me know how to proceed further