MobX plugin for unexpected.js

Assertions on effects triggered by actions

Let's start with a simple MobX store as example:

class Store {

  @observable data = null;
  @observable error = null;

  @action.bound fetchData() {
    // ...
  }

  @computed get loaded() {
    return this.data || this.error;
  }
}

export default new Store();

Now let's assert that after invoking fetchData the data property eventually gets set and error remains null:

import expect from 'unexpected';
import unexpectedMobX from 'unexpected-mobx';

expect.use(unexpectedMobX);

expect(store.fetchData,
  'when observing',
  () => store.loaded && store,
  'to satisfy',
  {
    data: { foo: 42 },
    error: null
  }
)

The pattern looks like this:

<function> when observing <function> <assertion>

The first parameter is a function that invokes an action. If you use @action.bound as in the example above, you can pass the store's action method directly.

The second parameter is a reactive function that is invoked via mobx.when. Once it returns a truthy value, this value will be passed on to the nested assertion.