How to pass next as an argument on Mocha test - express

Question

I am trying to test a function that checks if the user enter the email and if so it returns true, otherwise it passes an error argument to the next function and then it returns false. The test when the user passes an email it runs succesfully, but the test when the user does not provide an email fails. The error logs is that next is not a function. How is it possible to pass next as argument?
const crypto = require("crypto");
const bcrypt = require("bcryptjs");
const jwt = require("jsonwebtoken");
const User = require("../model/userModel");
const throwsAnError = require("../utils/throwsAnError");
exports.signup = async (req, res, next) => {
const {email, username, password, confirmPassword} = req.body;
if(!checkIfEmailExists(email, next)) {
return;
}
try{
const user = await User.create({
email: email,
userName: username,
password: password,
confirmPassword: confirmPassword
});
res.status(200).json({
message: "success",
data: user
})
}
catch(e){
next(new throwsAnError("Ο χρήστης δεν μπορεί να δημιουργηθεί", 400, e));
console.log("I'm in");
}
};
function checkIfEmailExists(email, next) {
if(!email) {
next(new throwsAnError("Συμπληρώστε το e-mail", 400));
return false;
}
return true;
}
exports.checkIfEmailExists = checkIfEmailExists;
const expect = require("chai").expect;
const authController = require("../controller/authController");
describe("Testing if email exist", function() {
it("should return true if email exists", function() {
expect(authController.checkIfEmailExists("email#email.com")).to.be.true;
})
it("should return false if email does not exist", function() {
expect(authController.checkIfEmailExists(undefined, next)).to.be.false;
})
});

Answer 1

When you call the function you should pass a mock for next, it can be as simple as passing:
() => {} or any other mock (using jest/sinon/etc), depends on the behavior you want.
So change:
expect(authController.checkIfEmailExists("email#email.com")).to.be.true;
to:
expect(authController.checkIfEmailExists("email#email.com", () => {})).to.be.true;
Further, it looks like you're mixing middleware with app-logic: you didn't show us how checkIfEmailExists() looks like and I don't see any good reason to pass in next.
If it's a middleware it should be called as a middleware (from the route) and not explicitly like it's done here.