Props and event testing in Cypress - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Cypress test for a button click event?

Consider this Cypress test code that checks if a button click triggers an event handler:

cy.get('button#submit').click().then(() => {
  cy.get('@clickHandler').should('have.been.calledOnce')
})

Assuming the alias @clickHandler is correctly set for the click event, what will be the test result?

Cypress

cy.get('button#submit').click().then(() => {
  cy.get('@clickHandler').should('have.been.calledOnce')
})

A Test fails because the click event handler was never called

B Test passes because the click event handler was called exactly once

C Test fails with a syntax error due to incorrect alias usage

D Test passes but the event handler was called multiple times

💡 Hint

Think about what should('have.been.calledOnce') checks after the click.

The test checks that the click event handler alias @clickHandler was called exactly once after clicking the button. If the alias is set correctly, the test passes.

❓ locator

intermediate

Which locator is best to select a React component by its prop for testing?

You want to select a React component in Cypress by a custom prop data-test-id="login-button". Which locator is best practice?

A cy.get('button[data-test-id="login-button"]')

B cy.get('button#login-button')

C cy.get('button.login-button')

D cy.get('button').contains('Login')

💡 Hint

Use attributes designed for testing to avoid brittle selectors.

Using a custom attribute like data-test-id is best practice because it is stable and does not depend on styling or text content.

❓ assertion

advanced

Which assertion correctly verifies a prop value in a Vue component using Cypress?

You want to check that a Vue component has a prop visible set to true. Which assertion is correct?

Cypress

cy.get('my-component').should( /* assertion here */ )

A have.attr('visible', 'true')

B have.class('visible')

C have.prop('visible', true)

D contain('visible: true')

💡 Hint

Props are JavaScript properties, not HTML attributes or classes.

The have.prop assertion checks the JavaScript property visible on the component instance, which is the correct way to verify a prop value.

🔧 Debug

advanced

Why does this Cypress test fail to detect the emitted event?

Given this test code:

cy.get('child-component').invoke('trigger', 'customEvent')
cy.get('@eventSpy').should('have.been.called')

The test fails because @eventSpy is never called. What is the likely cause?

A The event spy was not attached to the component before triggering the event

B The event name <code>customEvent</code> is misspelled in the trigger

C The component does not emit events in Cypress tests

D The trigger method cannot simulate custom events

💡 Hint

Check when the spy is attached relative to the event trigger.

If the spy is attached after the event is triggered, it will never detect the event. The spy must be set up before triggering the event.

❓ framework

expert

In Cypress, how do you best test a component's prop change triggers an event?

You want to test that changing a prop isActive on a component triggers an event statusChanged. Which approach is best?

A Use <code>cy.get</code> to find the component, then call <code>.trigger('statusChanged')</code> manually

B Mount the component and assert the event was emitted without changing the prop

C Mount the component, update the prop directly in the DOM, then check for the event

D Mount the component with <code>isActive</code>, attach an event spy, then update the prop using <code>cy.wrap</code> and check the spy

💡 Hint

Think about how to simulate prop changes and listen for events in component tests.

Mounting the component with initial props, attaching an event spy, then updating the prop programmatically allows testing if the event is emitted on prop change.

Practice

1. What is the main purpose of props testing in Cypress component tests?

easy

A. To verify that components receive and display data correctly

B. To check if the component's CSS styles are applied

C. To test if the component renders without errors

D. To measure the performance of component rendering

Solution

1. Step 1: Understand what props are

   Props are data passed to components to control their behavior or display.

2. Step 2: Identify the goal of props testing

   Props testing ensures the component uses and shows this data correctly.

3. Final Answer:

   To verify that components receive and display data correctly -> Option A

4. Quick Check:

   Props testing = verify data usage [OK]

Hint: Props testing checks data passed to components [OK]

Common Mistakes:

• Confusing props testing with style or performance testing
• Thinking props testing checks event handlers
• Assuming props testing is about rendering errors

2. Which Cypress command is used to spy on an event handler function passed as a prop?

easy

A. cy.spy()

B. cy.stub()

C. cy.mount()

D. cy.visit()

Solution

1. Step 1: Recall Cypress commands for spying

   cy.spy() creates a spy to watch function calls without changing behavior.

2. Step 2: Match command to event handler spying

   Spying on event handlers uses cy.spy() to check if they were called.

3. Final Answer:

   cy.spy() -> Option A

4. Quick Check:

   Spy event handler = cy.spy() [OK]

Hint: Use cy.spy() to watch function calls [OK]

Common Mistakes:

• Using cy.stub() which replaces function behavior
• Confusing cy.mount() with spying
• Using cy.visit() which is for page navigation

3. Given this Cypress test snippet:

const onClick = cy.spy();
cy.mount();
cy.get('button').click();
expect(onClick).to.have.been.calledOnce();

What will be the test result?

medium

A. Test passes because the click triggers onClick once

B. Test passes but onClick is called multiple times

C. Test fails because onClick is never called

D. Test fails due to syntax error in expect statement

Solution

1. Step 1: Analyze the spy and mount setup

   onClick is a spy passed as prop; Button is mounted with it.

2. Step 2: Check the assertion syntax

   The assertion uses calledOnce() which is incorrect syntax; it should be calledOnce without parentheses.

3. Final Answer:

   Test fails due to syntax error in expect statement -> Option D

4. Quick Check:

   calledOnce() is invalid syntax [OK]

Hint: Use calledOnce without () in assertions [OK]

Common Mistakes:

• Adding parentheses after calledOnce
• Assuming spy is not called when it is
• Ignoring syntax errors in assertions

4. You wrote this Cypress test to check a button click event:

const onClick = cy.spy();
cy.mount();
cy.get('button').click();
expect(onClick).to.have.been.called();

The test fails. What is the likely problem?

medium

A. The spy was not created correctly with cy.spy()

B. The event handler prop name is wrong

C. The assertion incorrectly uses parentheses after called

D. The button selector is incorrect

Solution

1. Step 1: Check spy creation and usage

   Spy is created correctly with cy.spy() and passed as prop.

2. Step 2: Review assertion syntax

   called is a property, not a function, so no parentheses should be used; parentheses cause failure.

3. Final Answer:

   The assertion incorrectly uses parentheses after called -> Option C

4. Quick Check:

   called is property, no () [OK]

Hint: Don't add () after called in assertions [OK]

Common Mistakes:

• Using called() instead of called
• Incorrect selector causing no click
• Wrong prop name like onClickHandler

5. You want to test a component that receives a disabled prop and an onSubmit event handler prop. Which Cypress test approach correctly verifies that clicking the submit button calls onSubmit only when disabled is false?

hard

A. Mount component with disabled=true and spy on onSubmit; click button and assert spy called once; then mount with disabled=false and assert spy not called

B. Mount component with disabled=false and spy on onSubmit; click button and assert spy called once; then mount with disabled=true and assert spy not called on click

C. Mount component once with disabled=false and spy on onSubmit; click button twice and assert spy called twice regardless of disabled prop

D. Mount component with disabled=false without spy; click button and check UI changes only

Solution

1. Step 1: Test enabled state with spy

   Mount with disabled=false, spy on onSubmit, click button, expect spy called once.

2. Step 2: Test disabled state with spy

   Mount with disabled=true, click button, expect spy not called to confirm no event triggered.

3. Final Answer:

   Mount component with disabled=false and spy on onSubmit; click button and assert spy called once; then mount with disabled=true and assert spy not called on click -> Option B

4. Quick Check:

   Test both states with spy to confirm event firing [OK]

Hint: Test enabled and disabled states separately with spy [OK]

Common Mistakes:

• Testing only one disabled state
• Not using spy to check event calls
• Assuming event fires regardless of disabled prop