How to test an Alert with Cypress

I'm testing a data form using Cypress, but am stuck on a step that displays an alert on the page.

This is the test, but it's not working.

describe('Alert is displayed with warning text', () => {

  it('Assert that the alert is displayed after entering data', () => {
    cy.visit('/')     
    cy.get('input').type('some data').blur()
    cy.on ('window:alert', (text) => {
      cy.wrap(text).should('eq', 'alert text')
    })    
  })
})

How do I test this alert that pops up on the page?

You can modify your code to use expect() instead of should().

The problem is Cypress does not like commands inside event listeners.

You must use a done() callback to avoid a false positive when the alert is not fired.

describe('Alert is displayed with warning text', () => {

  it('Assert that the alert is displayed after entering data', (done) => {
    cy.visit('/')     

    cy.on('window:alert', (text) => {
      expect(text).to.eq('alert text')
      done()                              // waiting for event, fails on timeout    
    })

    cy.get('input').type('some data').blur()
  })
})