How to change const values while testing with Jest - node.js

Question

Hello I'm working on writing a test for my node.js API and I'm running into an issue. I'm validating if an email exists inside of my code using "const = emailCount". If it does exists it returns an error JSON. If it does NOT exist it returns a success JSON. However I'm not sure how to mock the internal constant I declared in my code.
Here is the code:
async function registerUser(req, res) {
// Request
const email = req.body.email;
const password = req.body.password;
const firstName = req.body.firstName;
const lastName = req.body.lastName;
const inviteCode = req.body.inviteCode;
let errMessage = [];
if (!firstName) {
errMessage.push("first Name Required")
}
if (!lastName) {
errMessage.push("last Name Required")
}
if (!inviteCode) {
errMessage.push("inviteCode Required")
}
if (!email) {
errMessage.push("email Required")
}
if (!password) {
errMessage.push("password Required")
}
if (errMessage.length > 0) {
res.json({ code: "422", message: errMessage })
}
const accessToken = jwt.sign({
email: email,
firstName: firstName,
lastName: lastName
}, config.jwtSecret);
const emailCount = await db.doesEmailExists(email)
if (emailCount.doesEmailExists > 0) {
res.json({ Errors: "Account already exists" })
} else {
db.createUser({
username: email,
hashedPassword: password,
firstName: firstName,
lastName: lastName,
}).then(data => {
res.json({
id: data.insertId,
firstName: firstName,
lastName: lastName,
token: accessToken,
role: 'user'
})
}).catch(err => res.json({ Error: err }))
}
}
Here is my test code
test('POST /user/register', async () => {
//use super test to send post method with json payload of newUser
const res = await agent.post('/user/register').send(newUser);
expect(res.statusCode).toEqual(200)
expect(res.body).toHaveProperty('Errors') || expect(res.body).toHaveProperty('token');
})
Ultimately I want to change the value of emailCount within my test if possible to test for different responses if there is a user and if there is NOT a user.

Answer 1

You should not mock your code, but rather your dependencies and db is exactly that.
For example you can write your test scenario like this:
const db = require('./path/to/db.js');
// auto-create mock
jest.mock('./path/to/db.js')
describe('POST /user/register', () => {
describe('when email Exists'), () => {
// defining the "res" object here
// will allow you to execute the request one
// and separate the expectations in different
// test cases, which will provide better visibility
// on what exactly have failed (in the future)
let res;
beforeAll(async () => {
db.doesEmailExists.mockResolvedValue({
doesEmailExists: 789
});
res = await agent.post('/user/register').send(newUser);
});
it('should probably return something more than 200', () => {
expect(res.statusCode).toBeGreaterThanOrEqual(200)
});
it('should return Error in response Body', () => {
expect(res.body).toHaveProperty('Errors')
});
});
describe('when email DOES NOT Exists'), () => {
let res;
beforeAll(async () => {
db.doesEmailExists.mockResolvedValue({
doesEmailExists: 0
});
res = await agent.post('/user/register').send(newUser);
});
it('should probably return statusCode 200', () => {
expect(res.statusCode).toEqual(200)
});
it('should return token', () => {
expect(res.body).toHaveProperty('token')
});
});
});
Note: you'll also need to mock the return value of db.createUser as the auto-mock will generate a jest.fn() which returns undefined