js_alert.rb

require 'selenium-webdriver'
require 'rspec-expectations'
include RSpec::Matchers

driver = Selenium::WebDriver.for :firefox
driver.get 'http://the-internet.herokuapp.com/javascript_alerts'

driver.find_elements(css: 'button')[1].click
# to click the second button, [0] would click the first, and [2] (or [-1]) would click the last one

popup = driver.switch_to.alert
popup.accept

result = driver.find_element(id: 'result').text
expect(result).to eq('You clicked: Ok')

driver.quit