Cypress handles JavaScript window.alert(), window.confirm(), and window.prompt() differently: alerts are automatically accepted and can be observed with window:alert; confirms are automatically accepted unless a window:confirm handler returns false; prompts are best tested by stubbing window.prompt before application code runs. These tests exercise your app’s behavior, not the browser’s native dialog appearance.
How Cypress handles each browser dialog
| Dialog | Cypress default | How to test it | Can the test control the result? |
|---|---|---|---|
window.alert() |
Automatically accepts it. | Listen for window:alert and assert the message. |
No. The event lets you observe the alert; it does not provide an accept-or-reject choice. |
window.confirm() |
Automatically accepts it. | Listen for window:confirm. |
Yes. Return false from the handler to cancel; otherwise Cypress accepts it. |
window.prompt() |
Use a stub for a controlled response. | Stub window.prompt before app code runs. |
Yes. Return a string to model input, or null to model cancellation. |
These are JavaScript window methods, not Cypress’s separate cy.prompt() command. Cypress documents its alert and confirmation event behavior in the Catalog of Events.
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Test an alert message
Register the event handler before the click that triggers the alert. Cypress accepts the alert automatically, so the test can continue without manual interaction.
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it('asserts the alert message', () => {
const alertStub = cy.stub().as('alert')
cy.on('window:alert', alertStub)
cy.get('[data-cy="show-alert"]').click()
cy.get('@alert').should('have.been.calledWith', 'Saved successfully')
})The assertion verifies the string passed to window.alert(). It does not verify how a native browser dialog looks or whether a user can interact with its visual rendering.
Test both confirmation outcomes
Accept the confirmation
With no cancellation handler, Cypress automatically accepts the confirmation. Trigger the action and assert the accepted result in the application, such as the item disappearing after a successful deletion.
it('deletes an item after confirmation', () => {
cy.get('[data-cy="delete"]').click()
cy.get('[data-cy="item"]').should('not.exist')
})Cancel the confirmation
Return false from the event handler to exercise the cancelled branch. Assert both the message and the resulting application state.
it('keeps an item when deletion is cancelled', () => {
cy.on('window:confirm', (message) => {
expect(message).to.equal('Delete this item?')
return false
})
cy.get('[data-cy="delete"]').click()
cy.get('[data-cy="item"]').should('exist')
})A handler that returns nothing does not cancel the confirmation. Cypress’s event documentation describes returning false as the cancellation mechanism.
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Install the stub before application code
For an end-to-end test, use cy.visit()’s onBeforeLoad callback to replace window.prompt before the page’s app code executes. The stub returns the value the app should receive.
it('uses the entered prompt value', () => {
cy.visit('/', {
onBeforeLoad(win) {
cy.stub(win, 'prompt').as('prompt').returns('Ada Lovelace')
},
})
cy.get('[data-cy="set-name"]').click()
cy.get('[data-cy="greeting"]').should('have.text', 'Hello, Ada Lovelace!')
cy.get('@prompt').should('have.been.calledWith', 'Enter your name:')
})This checks both the prompt’s argument and the user-visible result produced from the returned string.
Model prompt cancellation
Return null to represent a cancelled prompt. Then assert the app’s intended cancellation behavior; for example, that it leaves the name unchanged or displays no greeting.
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it('does not set a name when the prompt is cancelled', () => {
cy.visit('/', {
onBeforeLoad(win) {
cy.stub(win, 'prompt').as('prompt').returns(null)
},
})
cy.get('[data-cy="set-name"]').click()
cy.get('[data-cy="greeting"]').should('not.contain', 'Hello,')
cy.get('@prompt').should('have.been.calledWith', 'Enter your name:')
})Adjust the final assertion to match the app’s actual cancellation contract. For component tests, stub the browser method before mounting the component; onBeforeLoad is the end-to-end page-load technique.
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- Attach
cy.on('window:alert', ...)orcy.on('window:confirm', ...)before the action that opens the dialog, or the event may already have fired. - Use
.as()on a stub when you want to assert it later withcy.get('@name'). - Cypress documents that listeners attached with
cy.on()are removed at the end of the test. A per-test listener avoids leaking behavior into later tests. - Cypress stubs are restored between tests. The cy.stub() documentation covers stubbing and restoration; the bundled Sinon-Chai assertions support assertions such as
calledWith.
Keep beforeunload tests separate
A page’s beforeunload event is not the same as window.prompt(). Do not use the prompt-stub pattern to test navigation warnings. Cypress’s migration guide demonstrates listening to window:before:unload and checking event.returnValue.
Best Value
Troubleshoot failed dialog tests
- The alert assertion never runs: Register the
window:alertlistener before clicking the trigger, and confirm that the app callswindow.alert()in that test path. - A confirmation always takes the accepted branch: Return
falsefrom thewindow:confirmhandler to cancel. An assertion alone does not cancel it. - The prompt returns the real dialog result or does not use the stub: Install the stub in
onBeforeLoadfor an end-to-end visit, before app code starts. If application code captures or calls the original method earlier, a later replacement may be too late. - A prompt cancellation test behaves like empty input: Return
nullfor cancellation, not an empty string. The app may deliberately treat those values differently. - A test passes on the message but misses a bug: Also assert the UI or application state that should result from acceptance, cancellation, or entered prompt text.
Or skip the browser setup
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curl -G "https://api.screenshotneo.com/v1/shot" -d access_key=YOUR_API_KEY --data-urlencode url=https://stripe.com -o shot.webp- Cookie and consent banners, newsletter popups, and chat widgets are removed before capture; each step can be turned off.
- Bot checks and CAPTCHAs, blank pages, timeouts, failed loads, and cache hits are not billed. Responses identify the page verdict and billing status in headers.
- An MCP server provides
take_screenshot,get_page_info, andcapture_pdftools for AI agents and MCP clients. - The free plan includes 1,000 screenshots per month without a card; paid plans start at $5 for 3,000.
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Quick Recap
Coverage checklist
- Assert the alert message and remember that Cypress auto-accepts it.
- Exercise both accepted and cancelled confirmation paths.
- Test prompt input and cancellation with a string and
null. - Check the app’s resulting UI or state, not only the dialog call.
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