I am not sure how to test a nodejs's exports Function. Consider the code below:

exports.create_expense = (req, res, next) ->
  User = database.db_model 'user'
  req.body.parsed_dt = Date.parse(req.body.date)
  req.body.amount = parseInt(req.body.amount)
  User.update {_id: req.api_session.id}, {$push: {expenses: req.body}}, (err, numberAffected, raw) ->
    if err?
      res.send 500
    else 
      res.send 200

User is a mongoose object here. I want to write a test (using mocha) to test this function (where in my test I will call create_expense) but since User.update is async I can't just call create_expense without passing some form of Promise? I know I can use supertest but that also test the route which I dont want to do here. Is there any way to test this any npm here is useful?

On User.update you should call next. And later in test you should call done.

edit:

Sorry for my CoffeeScript in advance.

Call next at the end of the callback to User.update.

Your test should look something like this then:

describe '#create_response', () ->
  response = false

  req.body.date = new Date
  req.body.amount = 123
  req.api_session.id = 'asd'

  res.send = (code) ->
    response = code

  it 'should return 500 on invalid request', (done) ->
    create_expense req, res, () ->
      assert.equal response, 500
      done