May 26, 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!

About the Author

Jeff Sheets profile.

Jeff Sheets

VP - Technology

Jeff has developed Java, Groovy, Grails, and Javascript web apps for industries as varied as Defense, Energy, Weather, Insurance, and Telecom. He is a co-organizer of the Omaha Java Users Group. Jeff has worked on Grails projects since the Grails 1.3.x days, and has experience with production Groovy code as well as Spock tests and Gradle builds. His latest focus has been on AngularJS and Spring Boot applications using JHipster. Jeff also enjoys volunteering at local CoderDojo events to teach programming to our next generation.

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