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Stubs, Spies, and Clocks


What you'll learn

  • Which libraries Cypress includes to provide typical testing functionality
  • How to use stubs for asserting that code was called but preventing it from executing
  • How to use spies for asserting that code was called without interfering with its execution
  • How to control time for deterministically testing code that is time-dependent
  • How Cypress improves and extends the included libraries


Cypress comes built in with the ability to stub and spy with cy.stub(), cy.spy() or modify your application's time with cy.clock() - which lets you manipulate Date, setTimeout, clearTimeout, setInterval, or clearInterval.

These commands are useful when writing both unit tests and integration tests.

Libraries and Tools

Cypress automatically bundles and wraps these libraries:

NameWhat it does
sinonprovides the cy.stub() and cy.spy() APIs
lolexprovides the cy.clock() and cy.tick() APIs
sinon-chaiadds chai assertions for stubs and spies

You can refer to each of these libraries' documentation for more examples and explanations.

Common Scenarios


A stub is a way to modify a function and delegate control over its behavior to you (the programmer).

A stub is most commonly used in a unit test but is still useful during some integration/e2e tests.

// create a standalone stub (generally for use in unit test)

// replace obj.method() with a stubbed function
cy.stub(obj, 'method')

// force obj.method() to return "foo"
cy.stub(obj, 'method').returns('foo')

// force obj.method() when called with "bar" argument to return "foo"
cy.stub(obj, 'method').withArgs('bar').returns('foo')

// force obj.method() to return a promise which resolves to "foo"
cy.stub(obj, 'method').resolves('foo')

// force obj.method() to return a promise rejected with an error
cy.stub(obj, 'method').rejects(new Error('foo'))

You generally stub a function when it has side effects you are trying to control.

Common Scenarios:

  • You have a function that accepts a callback, and want to invoke the callback.
  • Your function returns a Promise, and you want to automatically resolve or reject it.
  • You have a function that wraps window.location and don't want your application to be navigated.
  • You're trying to test your application's "failure path" by forcing things to fail.
  • You're trying to test your application's "happy path" by forcing things to pass.
  • You want to "trick" your application into thinking it's logged in or logged out.
  • You're using oauth and want to stub login methods.


A spy gives you the ability to "spy" on a function, by letting you capture and then assert that the function was called with the right arguments, or that the function was called a certain number of times, or even what the return value was or what context the function was called with.

A spy does not modify the behavior of the function - it is left perfectly intact. A spy is most useful when you are testing the contract between multiple functions and you don't care about the side effects the real function may create (if any).

cy.spy(obj, 'method')


There are situations when it is useful to control your application's date and time in order to override its behavior or avoid slow tests.

With cy.clock() you can control:

  • Date
  • setTimeout
  • setInterval

Common Scenarios

Control setInterval
  • You're polling something in your application with setInterval and want to control that.
  • You have throttled or debounced functions which you want to control.

Once you've enabled cy.clock() you can control time by ticking it ahead by milliseconds.

cy.get('#search').type('Acme Company')

You can call cy.clock() prior to visiting your application and we will automatically bind it to the application on the next cy.visit(). The same concept applies to mounting a component with cy.mount(). We bind before any timers from your code can be invoked.

Restore the clock

You can restore the clock and allow your application to resume normally without manipulating native global functions related to time. This is automatically called between tests.

cy.get('#search').type('Acme Company')
// more test code here

// restore the clock
cy.clock().then((clock) => {
// more test code here

You could also restore by using .invoke() to invoke the restore function.



Once you have a stub or a spy in hand, you can then create assertions about them.

const user = {
getName: (arg) => {
return arg

updateEmail: (arg) => {
return arg

fail: () => {
throw new Error('fail whale')

// force user.getName() to return "Jane"
cy.stub(user, 'getName').returns('Jane Lane')

// spy on updateEmail but do not change its behavior
cy.spy(user, 'updateEmail')

// spy on fail but do not change its behavior
cy.spy(user, 'fail')

// invoke getName
const name = user.getName(123)

// invoke updateEmail
const email = user.updateEmail('[email protected]')

try {
// invoke fail
} catch (e) {}

expect(name).to.eq('Jane Lane') // true
expect(user.getName) // true
expect(user.getName) // true
expect(user.getName) // true
expect(user.getName) // true

expect(email).to.eq('[email protected]') // true
expect(user.updateEmail)'[email protected]') // true
expect(user.updateEmail).to.have.returned('[email protected]') // true

expect('Error') // true

Integration and Extensions

Beyond integrating these tools together, we have also extended and improved collaboration between these tools.

Some examples:

  • We replaced Sinon's argument stringifier for a much less noisy, more performant, custom version.
  • We improved the sinon-chai assertion output by changing what is displayed during a passing vs. failing test.
  • We added aliasing support to stub and spy APIs.
  • We automatically restore and tear down stub, spy, and clock between tests.

We also integrated all of these APIs directly into the Command Log, so you can visually see what's happening in your application.

We visually indicate when:

  • A stub is called
  • A spy is called
  • A clock is ticked

When you use aliasing with the .as() command, we also correlate those aliases with the calls. This works identically to aliasing cy.intercept().

When stubs are created by calling the method .withArgs(...) we also visually link these together.

When you click on a stub or spy, we also output remarkably helpful debugging information.

For instance we automatically display:

  • The call count (and the total number of calls)
  • The arguments, without transforming them (they are the real arguments)
  • The return value of the function
  • The context the function was invoked with

See also