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
Capabilities
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
, setInterval
, amongst others.
These commands are useful when writing both unit tests and integration tests.
Libraries and Tools
Cypress automatically bundles and wraps these libraries:
Name | What it does |
---|---|
sinon | provides the cy.stub() and cy.spy() APIs |
lolex | provides the cy.clock() and cy.tick() APIs |
sinon-chai | adds chai assertions for stubs and spies |
You can refer to each of these libraries’ documentation for more examples and explanations.
Common Scenarios
Example test!Check out our example recipe testing spying, stubbing and time
Stubs
A stub is a way to modify a function and delegate control overs 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) cy.stub() // 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.
cy.stub()
Spies
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')
cy.spy()
Clock
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.
cy.clock()
gives you the ability to control:
Date
setTimeout
setInterval
Common Scenarios
- 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.clock() cy.visit('http://localhost:3333') cy.get('#search').type('foobarbaz') cy.tick(1000)
cy.clock()
is special in that it can be called prior to visiting your application, and we will automatically bind it to the application on the next cy.visit()
. We bind before any timers from your application can be invoked. This works identically to cy.server()
+ cy.route()
.
Assertions
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 user.fail() } catch (e) { } expect(name).to.eq('Jane Lane') // true expect(user.getName).to.be.calledOnce // true expect(user.getName).not.to.be.calledTwice // true expect(user.getName).to.be.calledWith(123) expect(user.getName).to.be.calledWithExactly(123) // true expect(user.getName).to.be.calledOn(user) // true expect(email).to.eq('[email protected]') // true expect(user.updateEmail).to.be.calledWith('[email protected]') // true expect(user.updateEmail).to.have.returned('[email protected]') // true expect(user.fail).to.have.thrown('Error') // true
Integration and Extensions
Beyond just 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
andspy
APIs. - We automatically restore and tear down
stub
,spy
, andclock
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 a cy.route()
.
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 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