Jest shows undefined when error is thrown from my controller

Question

I have a Node controller to test with Jest, my aim is to test if user has already a Stripe subscription:

controllers/userSubscriptionController.js

const userSubscription = async (req, res, next) => {
    const trowErr = true;
    if(throwErr){
       throw new Error("User has already subscribed.");
    }
}
module.exports = {userSubscription}

controllers/__tests__/userSubscriptionController.js

const {userSubscription} = require('../controllers/userSubscriptionController');
const { mockRequest, mockResponse, mockNext } = require("./interceptor");
let req = mockRequest();
let res = mockResponse();
let next = mockNext();
describe("My first test", async () => {
    it("should throws an error", () => {
        const s = await userSubscription(req, res, next)
        expect(s).toThrow()
    })
})

So when launching the test I received:

expect(received).toThrow(), Matcher error: received value must be a function , Received has value: undefined** 

Why received has an undefined value that makes the test fail?

Answer 1

In your test when the userSubscription throws no value is assigned to the const s so it's undefined.

To test your async function you can write the test like this

 describe('using returned promise', () => {
    it("should throws an error", () => {
      return expect(userSubscription(req, res, next)).rejects.toThrow('User has already subscribed')
    })
  })

or like this:

  describe('using await', () => {
    it("should throws an error", async () => {
      await expect(userSubscription(req, res, next)).rejects.toThrow('User has already subscribed')
    })
  })

https://repl.it/repls/AgreeableSlipperyHacks