Check Alert Message with Poltergeist or Capybara Webkit

Use accept_alert to get the alert message.

• Poltergeist ignores alerts/confirms by default(source)
• Poltergeist has had support for Capybaras modal api since September 2015( source)

That means you can same syntax for Poltergeist or capybara-webkit.

expect(accept_alert).to eq("Javascript alert message")

# or with block syntax

# will fail if alert message doesn't match
accept_alert("Javascript alert message") do 
  # next expectation
end

The first one expect(accept_alert).to reads a bit weird, but that's the return value of accept_alert.