Tuesday, May 11, 2021

Mock Intl and Date globals in Jest

In Javascript land, mocking the browser global objects can be a bit of a pain for tests. Searching StackOverflow gives plenty of complicated answers. Some suggesting using 3rd party mock libraries. Some that overwrite the global object itself.... But Jest already has this capability built-in and it isn't so bad:

So let's say you have a method that gets the user's timezone or the timezone offset. (the timezone offset is used sometimes since IE11 doesn't support easily reading the timezone, but I digress)

Now to test this, we'll need to mock out both the Intl and Date Javascript globals. We can do this using Jest's spyOn method to temporarily replace the global method with our own implementation. Notice that we setup the spy in the beforeEach and reset everything in the afterEach. The setup works something like this:

But that's it! No need to import an extra library. This is all supplied directly in Jest itself!

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