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If anyone on mixpanel is reading this I would love to know how do they maintain mocks? We've been burnt so many times with outdated mocks in my experience that I've always tried to supplement our front-end tests with Selenium as they run against a real server.
OP here. In our front-end tests, I haven't really noticed cases where the mocks get out of date that don't cause the test cases using the mock to fail, at which point we'd update the mock data. We also have end-to-end Selenium tests as supplement to our front-end tests which helps with catching format mismatch between front-end and backend (granted they're not that comprehensive at the moment).
Cypress looks interesting as an all-in-one solution for test runner + paid remote executor service. Similar builtins to WCT (Mocha, Chai). WCT was attractive for us as an open-source runner in that it isn't tied into any company's product; while it comes with good out-of-the-box support for running on Sauce Labs via standard tools (sauce-connect), there are also ways to use it with Browserstack and has a plugin system. Feels like a more generic pluggable building block for testing and less locked in to a particular vendor.
Our experience is that Cypress is the best tool out there in terms of better integration with CI tools even though it's not stable yet. We also connect to our cross-origin staging servers because it feels more e2e to us.