Using Mocha to test ClojureScript

As a bit of an experiment, last month I had a look at how to use Mocha to write CLJS tests, including exercising reagent components via JSDom and testing library. The end result was (opinion warning!) surprisingly pleasant.

(describe "[foo.components.views/article ...]"
  (it "asserts click behaviour"
    (let [!count (atom 0)
          mounted (render-el [fcv/article {:on-click #(swap! !count inc)
                                           :title "Title"}])
          button (by-test-id mounted "a1")]
      (click button)
      (click button)
      (assert (= @!count 2))))
  (it "contains the title"
    (let [title (str (random-uuid))
          mounted (render-el [fcv/article {:title title}])
          h1 (by-test-id mounted "maintitle")]
      (assert h1 "Title Exists")))

  (it "asserts against ratom updates"
    (let [mounted (render-el [fcv/article {:title "THING"}])
          button (by-test-id mounted "a2")]
      (assert (.getByText mounted "THING"))
      (click button)
      ;; Returns a promise to a query, or throws failing the test
      (.findByText mounted "clicked"))))

The technique is to make a shadow-cljs build outputting a bundle from the tests, which Mocha is then told to watch and run when it changes. Macros wrap expressions with the globals mocha injects, describe, it, etc, and those are executed and reported as you would expect. They are incredibly dumb macros, which are purely to reduce boilerplate.

The output, using the spec reporter, looks like:

> test
> NODE_ENV=test mocha --exit --reporter spec 'target/*_test.js'

    ✔ is true
    - has a pending test
    ✔ wraps functions
    ✔ catch assertion failure

  Promises are handled automatically
    ✔ happy path resolution
    ✔ happy path rejection
    ✔ should warn about timeouts without any interruption

  using promesa
    ✔ changes state (80ms)

  [foo.components.views/article ...]
    ✔ asserts click behaviour
    ✔ contains the title
    ✔ asserts against ratom updates

  10 passing (152ms)
  1 pending

In previous life as a JS dev, Mocha served well - it's fast, simple and flexible. I'd be curious what a CLJS native solution that had similar goals would look like...

Other things to do would be to use linkedom instead of JSDom, look into a better assertion library than assert etc.

GitHub repo

Tags: mocha testing cljs

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