A clean way to test rejected promise

To test the the exception in a rejected promise could be a little bit painful.

And Mocha is Promise-friendly, which means it fails the test if exception is not caught.

So as a result, here is a simple code example to explain how it is being done:

it 'should throw session-expired exception'
  new Session().generateId().bindUserFromRedis()
  .then ->
    null
  .catch (ex) ->
    ex
  .then (ex) ->
    expect(ex).to.be.instanceof(ResponseError)
              .and.that.have.property('errorName', 'session-expired')

---

Update: A Chai Plugin can mitigate the pain

it('should throw session-expired exception', () => {
  return expect(new Session().generateId().bindUserFromRedis())
           .to.evetually.rejected
              .and.that.have.property('errorName', 'session-expired')
})

Update 2: With async, it can be converted into normal test

it('should throw session-expired exception', () => {
  return expect(async () => await new Session().generateId().bindUserFromRedis())
          .to.throw
             .and.that.have.property('errorName', 'session-expired')
})