https://stackoverflow.com/questions/51801027/how-do-i-mock-a-function-that-is-called-inside-the-function-i-am-testing

sinonTest.js

    import UserConnector from './user'
    import sinon from 'sinon'
    import mongoose from 'mongoose'
    import UserModel from '../../models/user'
    
    describe("User connector", () => {
    
        it("should register user", async () => {
    
            const expectedUser = {
                firstName: "adsfja",
                lastName: "adsfja",
                email: "adsfja@alsda.com",
                password: "password123"
            }
            
            var myStub = sinon
              .stub(UserModel.prototype, 'save')
              .callsFake(() => Promise.resolve(expectedUser))
            
            const userConnector = new UserConnector();
            // since register is used as async, we should expect it to return a promise
            const user = await userConnector.register(expectedUser)
    
            expect(user).toEqual({
                firstName: "adsfja",
                lastName: "adsfja",
                email: "adsfja@alsda.com"
            })
            myStub.restore() // don't forget to restore stubbed function

        })
    })