TypeScript support for custom commands in Cypress - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a custom Cypress command with TypeScript

What will be the output in the Cypress test runner console after running this test code with the custom command?

Cypress

declare namespace Cypress {
  interface Chainable {
    login(email: string, password: string): Chainable<string>;
  }
}

Cypress.Commands.add('login', (email, password) => {
  cy.get('#email').type(email);
  cy.get('#password').type(password);
  cy.get('#submit').click();
  return cy.url();
});

describe('Login Test', () => {
  it('should navigate to dashboard after login', () => {
    cy.login('[e-mail]', 'pass123').then(url => {
      cy.log(url);
    });
  });
});

A Logs the URL string of the page after clicking submit, e.g., 'https://app.example.com/dashboard'

B Logs undefined because the custom command does not return a value

C Throws a TypeScript error due to missing return type in the custom command

D Logs the Cypress chainable object instead of the URL string

💡 Hint

Check what the custom command returns and how the .then() callback receives it.

The custom command returns cy.url(), which yields the current URL string. The .then() callback receives this string and logs it. The TypeScript declaration ensures the command is recognized with correct parameters and return type.

❓ assertion

intermediate

Correct TypeScript assertion for a custom command result

Given a custom command that returns a user object, which assertion correctly checks the user's email property in TypeScript?

Cypress

declare namespace Cypress {
  interface Chainable {
    getUser(): Chainable<{ email: string; id: number }>;
  }
}

Cypress.Commands.add('getUser', () => {
  return cy.wrap({ email: '[e-mail]', id: 42 });
});

cy.getUser().should( /* Which assertion? */ );

A user => expect(user.email).toBe('[e-mail]')

B user => expect(user).to.have.property('email', '[e-mail]')

C user => expect(user.email).to.equal('[e-mail]')

D user => expect(user['email']).equals('[e-mail]')

💡 Hint

Use the correct Chai assertion syntax supported by Cypress.

Option B uses the correct Chai assertion syntax with .to.have.property() and .equal() to check the email property value. Option B is close but .to.equal() is not used as a function inside should(). Option B uses Jest syntax (.toBe) which is invalid here. Option B uses .equals() without .to, which is incorrect.

🔧 Debug

advanced

Debugging TypeScript error in custom command declaration

Why does this TypeScript error occur when adding a custom command? Error: Property 'fillForm' does not exist on type 'Chainable'.

Cypress

Cypress.Commands.add('fillForm', (name: string, age: number) => {
  cy.get('#name').type(name);
  cy.get('#age').type(age.toString());
});

// Usage
cy.fillForm('Alice', 30);

// TypeScript declaration missing

A Because the custom command 'fillForm' was added but not declared in Cypress.Chainable interface

B Because the command implementation has a syntax error in the arrow function

C Because the parameters types are incorrect and cause a type mismatch

D Because Cypress.Commands.add cannot add commands with multiple parameters

💡 Hint

Check if the TypeScript interface was extended to include the new command.

TypeScript requires extending the Cypress.Chainable interface to declare new custom commands. Without this declaration, TypeScript does not recognize 'fillForm' as a valid method on cy.

🧠 Conceptual

advanced

Understanding TypeScript declaration merging for Cypress commands

Which statement best describes how TypeScript supports adding custom commands in Cypress?

A You create a new interface with the same name as Cypress.Chainable and export it to override the original.

B You add JSDoc comments above the command implementation to inform TypeScript about the new command.

C You extend the Cypress.Chainable interface inside a declare namespace block to add method signatures for your commands.

D You must modify the Cypress source code to add new commands to the Chainable interface.

💡 Hint

Think about how TypeScript merges declarations from multiple files.

TypeScript supports declaration merging, so you extend existing interfaces by redeclaring them inside a declare namespace block. This way, your custom commands become recognized as part of Cypress.Chainable.

❓ framework

expert

TypeScript typing for custom command returning a typed promise

You want to create a custom Cypress command 'fetchUser' that returns a Promise resolving to a User object with properties 'id' (number) and 'name' (string). How should you declare it in TypeScript to ensure proper typing and chaining?

A

declare namespace Cypress {
  interface Chainable {
    fetchUser(): Chainable<Promise<{ id: number; name: string }>>;
  }
}

B

declare namespace Cypress {
  interface Chainable {
    fetchUser(): Promise<{ id: number; name: string }>;
  }
}

C

declare namespace Cypress {
  interface Chainable {
    fetchUser(): Chainable<void>;
  }
}

D

declare namespace Cypress {
  interface Chainable {
    fetchUser(): Chainable<{ id: number; name: string }>;
  }
}

💡 Hint

Remember Cypress commands always return Chainable wrapping the yielded value, not a raw Promise.

Cypress commands return Chainable wrapping the yielded value, not a raw Promise. Declaring fetchUser() as returning Chainable<{ id: number; name: string }> correctly types the command's yield. Option D wraps a Promise inside Chainable which is incorrect. Option D returns a raw Promise, breaking Cypress chaining. Option D loses the typed return value.