How to test Cmd output from methods?

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1

Hi all,

I tried asking this on the Beginners thread in Reddit but I got no answer :slight_smile:

I’m trying to build an Elm app and when building tests I find an issue: how can I test a method returns a Cmd I expect it to return?

Example: my init method returns (model, command). I want to add a test that ensures we trigger a specific command on init. But, at least with elm-test, I cannot use Expect.equal (Cmd msg).

How can I achieve this?

Thanks!

2

Hi there!

First, sorry to hear that the Reddit thread wasn’t helpful. I don’t spend much time on Reddit myself, but the Elm Slack organization is very active. If you’re interested in more real-time feedback, I’d suggest asking stuff in there.

You’re right that testing Cmds can be tricky, depending on what exactly you want to test. There are a couple options though:

  • Try to test your data before you turn it into a command. I like to think of the Cmd as the final layer before I transfer control back to the Elm runtime. So I try to avoid testing Cmds directly; instead, I test the functions used in creating those values. If I have a function that like getFoo : Foo -> Cmd Msg, I might split that into two functions to make testing easier. The first would be getFooData : Foo -> FooData, which I would expose for testing, and the second would be sendFooData : FooData -> Cmd Msg, which would be very simple and untested. I’ve found a lot of success with this approach to splitting “logic” and effects.
  • Try elm-testable. This works for a subset of Elm Cmds, and it requires you to change your imports. I’ve not used it myself, but it is recommended often in the #testing channel on Slack.

Hope one of these works out for you—or maybe others will chime in with different ideas!

3

Nothing to be sorry about! Thanks, I’ll check the options you mentioned :slight_smile:

4

Worth noting that after Elm 0.19 comes out, I have plans to add Cmd testing to elm-test directly. (The planned API depends on changes in 0.19.) So this should get easier in the future!