implement `window.fetch` capturing/mocking

[terinjokes]

Many developers are migrating to window.fetch from XHRs due to the simplified async interface it exposes. I happen to be one of those developers.

Cypress should support window.fetch with similar abilities to those of XHRs: I should be able to query the request and response, wait on them, and mock with data and fixtures.

Cypress team edit

This feature will work seamlessly once Full network stubbing is implemented.

Workarounds

• use experimental fetch polyfill available in v4.9.0

or do it yourself:

• delete window.fetch before visiting the page to force your application to polyfill it on top of XHR. If the application does not include a polyfill, you can load one from your tests.
• stub window.fetch directly in JavaScript

See these solutions in Stubbing window.fetch example recipe

[brian-mann]

Per the 0.17.0 release at the very least window.fetch will actually work properly now.

It's on our radar to adding stubbing to these as well. We're a lot closer to being able to do it now.

[PhilippSpo]

I would love to see this feature. I am very excited about stubbing. It will definitely solve a lot of problems when doing these tests. window.fetch is very widely used by now 👍

[thomas-huston-zocdoc]

My team would love this too! window.fetch is very commonly used in React applications now, so I'm sure other teams would appreciate this as well.

[nwehrle]

Same here. We use fetch for it's simplicity in our react application. Until yet we use our own proxy that mocks the ajax requests, but your XHR approuch is much more readable.

[nwehrle]

Maybe you could offer a way to use fetch outside the service-worker's context first and in a second phase you could add fetch in service-workers?

[brian-mann]

There is now an example of using cy.stub to control window.fetch. It's not the same API as cy.route but it will work today!

We have a bunch of examples of it here: https://github.com/cypress-io/cypress-example-recipes#controlling-behavior-with-spies-stubs-and-clocks

[stigkj]

As not all browsers support window.fetch yet, I guess most use some form of polyfill for this. Then one could do something like this:

cy
  .visit('<some url>', {
    onBeforeLoad: (win) => {
      win.fetch = null
    }
  })

After doing this, the polyfill will just use XHR instead of fetch in the test, and one can use the normal Cypress's capturing/mocking tools. What do you think of this, @brian-mann?

[nwehrle]

Thank you for the alternative approach. Will try that.
What is the state of the real implementation? https://github.com/cypress-io/cypress/projects/20

[mvandebunt]

@stigkj thx for the workaround/solution.
the solution in https://github.com/cypress-io/cypress-example-recipes#controlling-behavior-with-spies-stubs-and-clocks was working for several projects for me.
Now I need to test an app using redux-saga with yield call(POST, { url: '/api/login/users', options }); and there it is not working, your solution is working!

[mukaibot]

@stigkj thanks for the work around, like @mvandebunt this has worked for me in Chromium 56 :)

I find this a bit easier than the spy approach. I'm looking forward to Cypress getting better support for window.fetch :)