My Experience Running Jest In A Real Angular CLI Project On Windows

Jest is an alternative test runner by Facebook. It’s popular in React world. I was wondering how it’ll be like in an Angular CLI app. I knew a new semi-empty app won’t be a good enough test, so, I checked it in a real private project I’m working on, so see if it has any benefits for the project and the team.

While I cannot share the project codes, I can still share how the experiment went, as I talked about it yesterday at the ng-sydney Angular usergroup April gathering, also, as I logged it in a related Angular CLI github issue by a friend ng-sydney attendee.


For the record, I tried Jest with instructions from @thymikee‘s post (which uses jest-preset-angular), and it worked just fine.

Let me share my findings:

Result Summary

  • It works!
  • It took some small changes.
  • It seemed slower in my case.
  • It caught more errors than Webpack/Karma/Jasmine, which I was very grateful for.
  • The experience is quite different though that I won’t make it the default my team yet.

Required Changes

  • I started with all the steps in the post above.

  • For some libraries I had to follow this github guidance, which made the jest property in my package.json look like:

    The transformIgnorePatterns is the most interesting here. I used \\ because I’m on Windows, and I used ^<rootDir>\\ to try to speed things, although it didn’t make a big difference. There’s also "allowJs": true in "compilerOptions" in src/tsconfig.spec.json.

  • I also had to change the tests. The post mentions all the changes needed (mostly only changing jasmine. to expect. in a few places).

    But there’s another change I had to do, which is removing any expectationFailOutput, which is when you give a custom error message for when a matcher fails to match. Seems Jest does not support that.


  • Jest seems to run tests in more isolation than the current Webpack/Karma/Jasmine combo, which showed me some errors in my tests that somehow didn’t show before.

  • Jest is reporting which tests are taking too long, the slowness warnings were useful smells to identify not-greatly-written tests.

  • The watch mode is very nice, even though it seems to work off git changes not file watching, which can be confusing at first when you see slightly more files than expected, but it’s still very useful.


  • As mentioned in Required Changes above, now I cannot use custom error messages in my test matchers (that said, it’s true that default matcher errors are very beautiful and obvious, but still).

  • At the moment, it seems the jest-preset-angular initialization code needs optimization. It doesn’t call polyfills.ts and instead calls its own set, even though it’s optimized for Angular CLI (for example, it uses src/tsconfig.spec.json).

    More importantly, it imports the entire rxjs library, which might hide errors when you forget to import some operators.

  • It’s surprisingly slower than the Webpack/Karma/Jasmine combo!

    For my 196 tests, the Angular CLI v1.0.0 default test runner (with Angular 4) takes ~ 55 seconds, while Jest takes ~100 seconds.

    Update: That conclusion might not be accurate. The time I quoted for each test runner is what the test runner reported. This might for example not include Webpack compilation and browser opening in Karma and might be total time in Jest. If so, then the total time would be the same.
    Thanks @hugoccampos for bringing this to my attention.

  • No browser tests debugging apparently (It might be my ignorance here). A bit harder maybe for those starting testing.

    The Angular CLI does a great job at making testing easy for those not used to it.

  • When Jest itself fails to run, for example if I put a badly formatted regex for transformIgnorePatterns shown above, or mess up something in my src/tsconfig.spec.json file, the errors it shows are very cryptic and tell you nothing that can lead to the real issue (unlike matching errors in tests, which are very nice).

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  • Thank you so much for the post. I read the blog post by Michal Pierzchala as well and was interested in following this approach for my angular tests as well. I had hoped for greater speed, more isolation(as you speak about) as well as abstracting away the browsers to allow for the tests to run more easily in a cloud CI system. I started trying to incorporate his additions but am stuck and am getting cryptic messages as well. If you have anytime that you could look and see if there is something obvious that I have configured incorrectly, I would greatly appreciate any help(

    Thanks again for the wonderful and informative post!!!