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Cypress cy.intercept Problems

A few common cy.intercept gotchas and how to avoid them

Note: I am using code from testing-workshop-cypress to demonstrate these cy.intercept gotchas.

The intercept was registered too late

The problem

Let's say the application is making GET /todos call to load its data. We might write a test like this to spy on this call:

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describe('intercept', () => {
  it('is registered too late', () => {
    cy.visit('/')
    cy.intercept('/todos').as('todos')
    cy.wait('@todos')
  })
})

It looks reasonable, it even shows the call in the Command Log - but does NOT pass.

The intercept times out waiting

You can see the XHR call in the command log - and it looks like it should work, right? But if you open the "Routes" section, notice that our intercept was never counted.

The intercept was never counted

The root cause

The application is making the call to load the data right on startup:

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created() {
  this.$store.dispatch('loadTodos')
}

Thus, it makes the call right at the end of cy.visit command. By the time cy.intercept runs, the call is already in progress, and thus not intercepted. Cypress shows XHR calls by default in its Command Log, thus it has nothing to do with our intercept. I always thought NOT showing when cy.intercept happens in the Command Log was a user experience failure.

The solution

Make sure the network intercept is registered before the application makes the call.

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it('is registered too late', () => {
  cy.intercept('/todos').as('todos')
  cy.visit('/')
  cy.wait('@todos')
})

In our case, we need to register the intercept before visiting the page. Once the page is loaded, the application fetches the todo items, and everything is working as expected.

The intercept works correctly

The bonus solution

You can overwrite Cypress commands and log a message to the Command Log. Unfortunately overwriting cy.intercept has a bug with aliases #9580, and thus we cannot always show when the intercept is registered. We can still do it case by case.

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it('is registered too late', () => {
  cy.visit('/')
  cy.log('adding /todos intercept')
  cy.intercept('/todos').as('todos')
  cy.wait('@todos')
})

The above test clearly shows that the intercept is registered too late.

The log message shows the intercept is registered AFTER the application makes the call

Update: the alias bug #9580 has been fixed and released in Cypress v6.2.0

cy.wait uses the intercept

The problem

If you first wait on the intercept and then separately try to cy.get it to validate - well, the cy.get always resolves with null.

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it('is taken by the wait', () => {
  cy.intercept('/todos').as('todos')
  cy.visit('/')
  cy.wait('@todos')
  cy.get('@todos').should('not.be.null')
})

cy.get always yields null after cy.wait

This is different behavior from cy.route where we could wait on the interception, and the get it separately.

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it('is taken by the wait (unlike cy.route)', () => {
  cy.server()
  cy.route('/todos').as('todos')
  cy.visit('/')
  cy.wait('@todos')
  cy.get('@todos').should('not.be.null')
})

cy.wait plus cy.get works for cy.route

The workaround

Honestly, I feel this is a bug #9306 that we should fix shortly. But for now you can validate the interception immediately using the value yielded by cy.wait

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it('should use wait value', () => {
  cy.intercept('/todos').as('todos')
  cy.visit('/')
  cy.wait('@todos').should('include.all.keys', ['request', 'response'])
})

You can validate the interception yielded by cy.wait

If you want to use multiple assertions over the interception, use the .should(cb) or .then(cb) assertions. The .then(cb) makes more sense here, since the interception value would never change after completing.

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it('verifies the waited interception via .then(cb)', () => {
  cy.intercept('/todos').as('todos')
  cy.visit('/')
  cy.wait('@todos').then(interception => {
    expect(interception).to.be.an('object')
    expect(interception.request.url).to.match(/\/todos$/)
  })
})

Update: the bug #9306 has been fixed and released in Cypress v6.2.0

Cached response

The problem

Let's inspect the interception object yielded by the cy.wait command.

The interception does not the response because the server replied that the data was not modified

We cannot validate the data sent by the server, because there is no data in the response. Instead the server tells the browser that the data loaded previously is still valid and has not been modified. Hmm, but we are not the browser - the cy.intercept runs in the proxy outside the browser. Thus we have nothing to test.

We can check the caching by using the following test:

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it('does not have response', () => {
  cy.intercept('/todos').as('todos')
  cy.visit('/')
  cy.wait('@todos')
    .its('response')
    .should('deep.include', {
      statusCode: 304,
      statusMessage: 'Not Modified',
      body: ''
    })
})

The response is cached

This is where it gets a little tricky - since this caching depends on the DevTools setting. If for example, you set the browser DevTools Network tab to disable caching you get the test that passes when the DevTools is closed, and fails when the DevTools is open.

Caching makes the test pass or fail

The solution

The server will always return the actual data if the server cannot determine what the client has already. The server determines the data "cache key" in this case by looking at the if-none-match request header sent by the web application.

The request headers sent by the client

We need to delete this header on the outgoing request. Let's do it using our cy.intercept. Now we can extend the test to validate the response.

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it('always gets the new data', () => {
  cy.intercept('/todos', req => {
    delete req.headers['if-none-match']
  }).as('todos')
  cy.visit('/')
  cy.wait('@todos')
    .its('response')
    .should('deep.include', {
      statusCode: 200,
      statusMessage: 'OK'
    })
    .and('have.property', 'body') // yields the "response.body"
    .then(body => {
      // since we do not know the number of items
      // just check if it is an array
      expect(body).to.be.an('array')
    })
})

The client always receives fresh copy of the server data

Multiple matchers

The problem

The command cy.intercept can match requests using a substring, a minimatch, or a regular expression. By default, it intercepts requests matching any HTTP method. Thus when you define several intercepts, it is easy to get into the situation when multiple intercepts apply. In that case the first cy.wait(alias) "uses up" the intercept's response.

Consider the following test that adds a new todo.

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beforeEach(resetDatabase)

beforeEach(() => {
  cy.visit('/')
})

it('enters 1 todo', () => {
  cy.get('.new-todo').type('Write a test{enter}')
})

We probably want to make sure the new todo sent to the server has the title "Write a test". Let's set up an intercept and validate the sent data.

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beforeEach(resetDatabase)

beforeEach(() => {
  cy.visit('/')
})

it('enters 1 todo', () => {
  cy.intercept('POST', 'todos').as('post')
  cy.get('.new-todo').type('Write a test{enter}')
  cy.wait('@post')
})

If we inspect the value yielded by cy.wait('@post') it shows the object sent in the request

The new item sent to the server

Great, let's validate the new item. Since we do not control the random ID generation (of course we can control it, see the Cypress testing workshop how), we can just validate the title and completed properties.

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beforeEach(() => {
  cy.visit('/')
})

it('enters 1 todo', () => {
  cy.intercept('POST', 'todos').as('post')
  cy.get('.new-todo').type('Write a test{enter}')
  cy.wait('@post')
    .its('request.body')
    .should('deep.include', {
      title: 'Write a test',
      completed: false
    })
})

Validating the item sent to the server

Great, everything works, what's the problem?

Imagine someone else comes along and changes the cy.visit and adds a wait for todo items to load - just like we did before. They do this to guarantee the application has loaded before adding new items.

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beforeEach(resetDatabase)

beforeEach(() => {
  cy.intercept('/todos').as('todos')
  cy.visit('/')
  cy.wait('@todos')
})

it('enters 1 todo', () => {
  cy.intercept('POST', '/todos').as('post')
  cy.get('.new-todo').type('Write a test{enter}')
  cy.wait('@post')
    .its('request.body')
    .should('deep.include', {
      title: 'Write a test',
      completed: false
    })
})

So far so good. The test passes.

Waiting on the todos and validating the sent item

Notice a curious thing in the Command Log though - the XHR request to post the new item to the server has todos(2) badge.

The network request matches two intercepts

The POST /todos network call the application has made matches two intercepts, but the Command Log only prints the first intercept's alias.

In our test both intercepts only spied on the request. Now someone comes along and asks why do we need to reset the database before each test using resetDatabase utility method. It would be so simple to stub the initial GET /todos call and return an empty list of items.

The changed test is below - it simply uses cy.intercept('/todos', []).as('todos').

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beforeEach(() => {
  cy.intercept('/todos', []).as('todos')
  cy.visit('/')
  cy.wait('@todos')
})

it('enters 1 todo', () => {
  cy.intercept('POST', '/todos').as('post')
  cy.get('.new-todo').type('Write a test{enter}')
  cy.wait('@post')
    .its('request.body')
    .should('deep.include', {
      title: 'Write a test',
      completed: false
    })
})

The test fails.

The test now fails

The root cause

If there are multiple matching interceptors, the first intercept that stubs the request stops any further processing. Thus our POST /todos network request gets intercepted by overly broad first * /todos intercept.

The solution

Use specific intercepts and make sure the stubs are not "breaking" any listeners defined afterwards. Change the first intercept to only work with GET /todos call.

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cy.intercept('GET', '/todos', []).as('todos') // stub
...
cy.intercept('POST', '/todos').as('post') // spy

The test using specific intercepts works

Update: I have opened issue #9588 to provide better labels to the intercepted requests.

No overwriting interceptors

Let's imagine the most common scenario - every test starts with zero items. We stab the GET /todos request and visit the site in a beforeEach hook.

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beforeEach(() => {
  cy.intercept('GET', '/todos', []) // start with zero todos
  cy.visit('/')
})

it('adds a todo', () => {
  cy.get('.new-todo').type('write test{enter}')
  cy.get('.todo').should('have.length', 1)
})

it('completes todo', () => {
  cy.get('.new-todo').type('write test{enter}')
  cy.get('.todo')
    .should('have.length', 1)
    .first()
    .find('.toggle')
    .click()
  cy.contains('.todo', 'write test').should('have.class', 'completed')
})

Several tests that start with zero items using network stub

Now you want to confirm the application displays the initial list of todos correctly. Hmm, you would like to stub the initial GET /todos call in the new test:

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beforeEach(() => {
  cy.intercept('GET', '/todos', []) // start with zero todos
  cy.visit('/')
})

it('adds a todo', () => {
  ...
})

it('completes todo', () => {
  ...
})

it('shows the initial todos', () => {
  // hmm overwrite the intercept?
  cy.intercept('GET', '/todos', { fixture: 'two-items.json' })
  cy.visit('/')
  cy.get('.todo').should('have.length', 2)
})

Attempting to overwrite the intercept fails

Unfortunately the test fails. The first intercept cy.intercept('GET', '/todos', []) still executes, our the second intercept never has a chance.

We have a tracker issue #9302 to solve this problem somehow. It is complex - changing an intercept from stub to spy, or from spy to stub is tricky.

The solution

Separate the suites of tests to avoid using the beforeEach hook that sets up the unwanted intercept.

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context('start with zero todos', () => {
  beforeEach(() => {
    cy.intercept('GET', '/todos', [])
    cy.visit('/')
  })

  it('adds a todo', () => {
    cy.get('.new-todo').type('write test{enter}')
    cy.get('.todo').should('have.length', 1)
  })

  it('completes todo', () => {
    cy.get('.new-todo').type('write test{enter}')
    cy.get('.todo')
      .should('have.length', 1)
      .first()
      .find('.toggle')
      .click()
    cy.contains('.todo', 'write test').should('have.class', 'completed')
  })
})

context('start with two items', () => {
  it('shows the initial todos', () => {
    // hmm overwrite the intercept?
    cy.intercept('GET', '/todos', { fixture: 'two-items.json' })
    cy.visit('/')
    cy.get('.todo').should('have.length', 2)
  })
})

We have completely separated the suite of tests that should all start with zero items from the suite of tests that should start with two items. Now the intercepts never "compete". After all - this is just JavaScript and you can compose the test commands and the hooks in any way you want.

Separate suites of tests to avoid competing intercepts

Avoid Cypress commands inside the interceptor

You cannot use Cypress commands inside the interceptor callback, since it breaks the deterministic command chaining. The following test fails

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it('tries to use cy.writeFile', () => {
  cy.visit('/')
  cy.intercept('POST', '/todos', req => {
    console.log('POST /todo', req)
    cy.writeFile('posted.json', JSON.stringify(req.body, null, 2))
  })

  cy.get('.new-todo').type('an example{enter}')
})

Calling cy.writeFile inside the interceptor

The root cause

The interceptor is triggered by the application's code, while other Cypress commands are already running. Cypress has no idea how to chain the cy.writeFile command - should it be after the currently running command? Or later? This leads to non-deterministic and unpredictable tests. Thus this is disallowed.

The solution

Instead save the body of the request in a variable, wait for the network call to happen, and then write the data to a file.

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it('saves it later', () => {
  let body

  cy.visit('/')
  cy.intercept('POST', '/todos', req => {
    console.log('POST /todo', req)
    body = req.body
  }).as('post')

  cy.get('.new-todo')
    .type('an example{enter}')
    .wait('@post')
    .then(() => {
      // this callback executes AFTER the "cy.wait" command above
      // thus by now the "body" variable has been set and we can
      // write the contents to the file
      cy.writeFile('posted.json', JSON.stringify(body, null, 2))
    })
})

Watch the video below for details

Sending different responses

Imagine we stub the loading items from the server. Initially the server returns two items. Then the user adds another item (we stub the POST request too), and when the page reloads, we should receive three items. How would we return different responses for the same GET /todos call? At first we could try to overwrite the intercept

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it('returns list with more items on page reload', () => {
  // we start with 2 items in the list
  cy.intercept('GET', '/todos', twoItems)
  cy.visit('/')
  cy.get('.todo').should('have.length', 2)

  // now we add the third item
  const item = {
    title: 'Third item',
    completed: false,
    id: 101
  }
  // the server replies with the posted item
  cy.intercept('POST', '/todos', item).as('post')
  cy.get('.new-todo').type(item.title + '{enter}')
  cy.wait('@post')

  // when the page reloads we expect the server to send 3 items
  const threeItems = Cypress._.cloneDeep(twoItems).concat(item)
  cy.intercept('GET', '/todos', threeItems)
  cy.reload()
  cy.get('.todo').should('have.length', 3)
})

But of course this does not work - because as I have shown in the previous section, you cannot overwrite an intercept (yet).

The first intercept still returns the two items

The root cause

The first interceptor responds to both GET /todos calls. The cy.intercept('GET', '/todos', threeItems) intercept was never called, as I highlighted in the screenshot.

The solution

Use JavaScript code to return different responses from a single GET /todos intercept. For example, you can place all prepared responses into an array, and then use Array.prototype.shift to return and remove the first item.

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it('returns list with more items on page reload', () => {
  const item = {
    title: 'Third item',
    completed: false,
    id: 101
  }
  // we start with 2 items in the list
  // when the page reloads we expect the server to send 3 items
  const threeItems = Cypress._.cloneDeep(twoItems).concat(item)
  const replies = [twoItems, threeItems]

  // return a different response from the same intercept
  cy.intercept('GET', '/todos', req => req.reply(replies.shift()))
  cy.visit('/')
  cy.get('.todo').should('have.length', 2)

  // the server replies with the posted item
  cy.intercept('POST', '/todos', item).as('post')
  cy.get('.new-todo').type(item.title + '{enter}')
  cy.wait('@post')

  cy.reload()
  cy.get('.todo').should('have.length', 3)
})

The same intercept returning different responses

No overwriting interceptors (again)

Let's get back to overwriting the interceptors again. We want to overwrite the GET /todos response from a particular test.

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context('overwrite interceptors', () => {
  beforeEach(() => {
    cy.intercept('GET', '/todos', []).as('todos')
  })

  it('adds a todo', () => {
    cy.visit('/')
    ...
  })

  it('completes todo', () => {
    cy.visit('/')
    ...
  })

  // DOES NOT WORK
  it('shows the initial todos', () => {
    // overwrite the previous response with the new one
    cy.intercept('GET', '/todos', { fixture: 'two-items.json' })
    cy.visit('/')
    cy.get('.todo').should('have.length', 2)
  })
})

The last test "shows the initial todos" fails, because its intercept is never executed. See the previous section No overwriting interceptors for the details and a solution.

The solution

We cannot replace the intercept again, but we can extend the logic in setting the intercept to use JavaScript code to implement the overwrite. Let's create a new Cypress command cy.http that will stub the network call with a response, but that response will come from an object of intercepts. Any test that wants to return something else should just replace the value in that object.

Tip: I will store the object with intercepts in Cypress.config() object and will reset it before every test.

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beforeEach(function resetIntercepts() {
  Cypress.config('intercepts', {})
})

Cypress.Commands.add('http', (alias, method, url, response) => {
  const key = `${alias}-${method}-${url}`
  const intercepts = Cypress.config('intercepts')

  if (key in intercepts) {
    intercepts[key] = response
  } else {
    intercepts[key] = response
    cy.intercept(method, url, req => {
      return req.reply(intercepts[key])
    }).as(alias)
  }
})

The main logic is inside the cy.intercept call

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cy.intercept(method, url, req => {
  return req.reply(intercepts[key])
}).as(alias)

Notice the value is not hard-coded. Here is how we use cy.http from our tests

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context('overwrite interceptors', () => {
  beforeEach(function resetIntercepts() {
    Cypress.config('intercepts', {})
  })

  Cypress.Commands.add('http', (alias, method, url, response) => {
    const key = `${alias}-${method}-${url}`
    const intercepts = Cypress.config('intercepts')

    if (key in intercepts) {
      intercepts[key] = response
    } else {
      intercepts[key] = response
      cy.intercept(method, url, req => {
        return req.reply(intercepts[key])
      }).as(alias)
    }
  })

  beforeEach(() => {
    cy.http('todos', 'GET', '/todos', [])
  })

  it('adds a todo', () => {
    cy.visit('/')
    cy.get('.new-todo').type('write test{enter}')
    cy.get('.todo').should('have.length', 1)
  })

  it('completes todo', () => {
    cy.visit('/')
    cy.get('.new-todo').type('write test{enter}')
    cy.get('.todo')
      .should('have.length', 1)
      .first()
      .find('.toggle')
      .click()
    cy.contains('.todo', 'write test').should('have.class', 'completed')
  })

  it('shows the initial todos', () => {
    // overwrite the previous response with the new one
    cy.http('todos', 'GET', '/todos', { fixture: 'two-items.json' })
    cy.visit('/')
    cy.get('.todo').should('have.length', 2)
  })
})

The tests work

Changing the response in the same intercept

Notice we still have to call the cy.http before the cy.visit, thus cy.visit cannot be placed in the beforeEach hook. Still, overwriting is very convenient for tests that expect different responses, like adding an item.

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it('adds a todo to the initial ones', () => {
  // the application starts with two items
  cy.http('todos', 'GET', '/todos', { fixture: 'two-items.json' })
  cy.visit('/')
  cy.get('.todo').should('have.length', 2)
  cy.get('.new-todo').type('third item{enter}')

  // now the server should return 3 items
  cy.http('todos', 'GET', '/todos', { fixture: 'three-items.json' })
  cy.reload()
  cy.get('.todo').should('have.length', 3)
})

Tip: add a log message to our cy.http command for better experience.

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Cypress.Commands.add('http', (alias, method, url, response) => {
  const key = `${alias}-${method}-${url}`
  cy.log(`HTTP ${key}`)
  ...
})

Changing the response in the same intercept in the same test

Single use intercept

This is similar to overwriting interceptors. Imagine you want to stub a network call once, but then allow other calls to go through to the server. Just intercepting with a static response would not work, as that intercept would work forever.

Here is the test that does not work.

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context('single use intercept', () => {
  beforeEach(() => {
    // let's reset the server to always have 2 todos
    resetDatabaseTo('two-items.json')
  })

  it('stubs the first load (does not work)', () => {
    // this test wants to have no todos at first
    cy.intercept('GET', '/todos', []).as('todos')
    cy.visit('/')
    cy.wait('@todos')
    cy.get('.todo-list li').should('have.length', 0)

    cy.log('adding an item')
    cy.get('.new-todo').type('new todo{enter}')
    cy.contains('li.todo', 'new todo').should('be.visible')

    // reload and expect to see the new todo again
    cy.reload()
    cy.contains('li.todo', 'new todo').should('be.visible')
  })
})

The item disappears on reload

Notice how the new item appears in the UI, we also see it being sent to the server using POST /todos call. Yet, the intercept "todos" again returns the empty list of todos on page reload.

The solution

We want to intercept only once. We do not need to even write a custom command, a simple function would do:

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/**
* Intercept the first matching request and send the response object.
* Do nothing on the second and other requests.
* @param {string} method HTTP method to intercept
* @param {string} url URL to intercept
* @param {any} response Response to send back on the first request
*/
const interceptOnce = (method, url, response) => {
  // I am using "count" to show how easy you can implement
  // different responses for different interceptors
  let count = 0
  return cy.intercept(method, url, req => {
    count += 1
    if (count < 2) {
      req.reply(response)
    } else {
      // do nothing
    }
  })
  }

Let's see the test

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beforeEach(() => {
  // let's reset the server to always have 2 todos
  resetDatabaseTo('two-items.json')
})

it('stubs the first load and does nothing after that', () => {
  // this test wants to have no todos at first
  interceptOnce('GET', '/todos', []).as('todos')
  cy.visit('/')
  cy.wait('@todos')
  cy.get('.todo-list li').should('have.length', 0)

  cy.log('adding an item')
  cy.get('.new-todo').type('new todo{enter}')
  cy.contains('li.todo', 'new todo').should('be.visible')

  // reload and expect to see the new todo again
  cy.reload()
  cy.contains('li.todo', 'new todo').should('be.visible')
  // since we reset the database with 2 todos, plus entered a new todo
  // thus the total number of items should be 3
  cy.get('.todo-list li').should('have.length', 3)
})

Nice! And since the intercept continues spying on the requests, even if it does not stub them, we can get the last call and confirm other details.

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// Tip: you can still spy on "todos" intercept
// for example let's validate the server response has the new item
// at index 2 and it has the title and completed properties
cy.get('@todos')
  .its('response.body')
  .should('have.length', 3)
  .its('2')
  .should('include', {
    title: 'new todo',
    completed: false
  })

The intercept once in action

Get number of times an intercept was matched

Sometimes you want to know / assert the number of times an intercept was matched. You can see this information in the "Routes" table in the Command Log, how can you confirm it from the test?

Note: for now, using cy.state('routes') is an implementation detail, and might change in the future, but you could do the following:

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const getAliasCount = (alias) => {
  // implementation details, use at your own risk
  const testRoutes = cy.state('routes')
  const aliasRoute = Cypress._.find(testRoutes, { alias })

  if (!aliasRoute) {
    return
  }

  return Cypress._.keys(aliasRoute.requests || {}).length
}

it('confirms the number of times an intercept was called', () => {
  cy.intercept('/users?_limit=3').as('users3')
  cy.intercept('/users?_limit=5').as('users5')

  cy.get('#load-users').click().click()
  cy.wait('@users3')

  // to avoid clicking too quickly, add small pauses
  cy.get('#load-five-users').click()
  .wait(20).click()
  .wait(20).click()
  .wait(20).click()

  cy.wait('@users5')
  .then(() => {
    const users3 = getAliasCount('users3')
    const users5 = getAliasCount('users5')

    cy.wrap({
      users3,
      users5,
    }).should('deep.equal', {
      users3: 2,
      users5: 4,
    })
  })
})

Asserting the number of times an intercept was used

Set an alias dynamically

In case of GraphQL, all requests go through a single endpoint. Thus it is important for the intercept to inspect the request first, before deciding what to do. A convenient way to deal with such requests is to set the alias dynamically. For example, let's inspect all GET /users requests, and set an alias for the request that limits the number of users to 5:

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it('can set an alias depending on the request', () => {
  cy.visit('/')
  cy.intercept('GET', '/users', (req) => {
    if (req.url.endsWith('/users?_limit=5')) {
      req.alias = "load5"
    }
  })

  cy.get('#load-users').click()
  cy.get('#load-five-users').click()
  cy.wait('@load5') // the second request created this alias dynamically
    .its('response.body').should('have.length', 5)
})

The test runs, and an alias is created. The test can wait on that alias:

Waiting for dynamically created alias

More info