Mocking database calls in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output when mocking a database call with Jest in Express?

Consider this Express route handler that uses a mocked database call. What will be the response sent to the client?

Express

const express = require('express');
const app = express();

// Mock database module
const db = {
  getUser: jest.fn(() => Promise.resolve({ id: 1, name: 'Alice' }))
};

app.get('/user', async (req, res) => {
  const user = await db.getUser();
  res.json(user);
});

// Simulate a request to /user
(async () => {
  const req = { method: 'GET', url: '/user' };
  const res = { json: (data) => console.log(JSON.stringify(data)) };
  app.handle(req, res, () => {});
})();

A TypeError: db.getUser is not a function

B undefined

C {"id":1,"name":"Alice"}

D SyntaxError

💡 Hint

Think about what the mocked function returns and how the route sends the response.

The mocked db.getUser returns a resolved promise with the user object. The route awaits this and sends it as JSON, so the output is the user object.

📝 Syntax

intermediate

Which option correctly mocks a database call returning a rejected promise in Jest?

You want to mock a database call that fails with an error. Which code correctly mocks this behavior?

Express

const db = {
  getUser: jest.fn()
};

// Your mocking code here

A db.getUser.mockResolvedValue(new Error('DB error'));

B db.getUser.mockRejectedValue('DB error');

C db.getUser.mockReturnValue(new Error('DB error'));

D db.getUser.mockReturnValue(Promise.reject(new Error('DB error')));

💡 Hint

Use the Jest method designed for rejected promises.

mockRejectedValue correctly mocks a promise that rejects with the given error. mockReturnValue with Promise.reject returns a rejected promise but is less idiomatic. mockResolvedValue returns a resolved promise, not rejected. mockReturnValue with an error object returns the error object directly, not a promise.

🔧 Debug

advanced

Why does this mocked database call not update the state in Express?

Given this Express route and mock, why does the response always show the initial user name instead of the updated one?

Express

const db = {
  user: { id: 1, name: 'Alice' },
  getUser: jest.fn(() => Promise.resolve(db.user)),
  updateUser: jest.fn((newName) => { db.user.name = newName; return Promise.resolve(db.user); })
};

app.post('/user', async (req, res) => {
  await db.updateUser('Bob');
  const user = await db.getUser();
  res.json(user);
});

A The mock functions share the same user object, so the update should reflect; no issue here.

B The updateUser mock does not actually change the user object because of closure scope.

C The getUser mock returns a new object copy, so changes to db.user are not seen.

D The asynchronous calls are not awaited properly, causing stale data.

💡 Hint

Look at how the user object is shared between mocks.

Both mocks use the same db.user object reference. updateUser changes its name property, so getUser returns the updated object. The response will show the updated name 'Bob'.

❓ lifecycle

advanced

What happens if you forget to reset mocks between tests in Express with Jest?

You have multiple tests mocking the same database call. What is the likely effect of not resetting mocks between tests?

A Mocks retain call history and implementations, causing tests to interfere and produce false positives or negatives.

B Mocks automatically reset after each test, so no effect occurs.

C Tests will fail immediately with a Jest error about mock state.

D Mocks become undefined and cause runtime errors.

💡 Hint

Think about how Jest tracks mock calls and implementations.

Without resetting mocks, call counts and mock implementations persist across tests. This can cause tests to pass or fail incorrectly due to leftover state.

🧠 Conceptual

expert

Which option best describes the benefit of mocking database calls in Express unit tests?

Why do developers mock database calls when testing Express route handlers?

A To replace Express middleware with database logic for integration testing.

B To ensure the database schema is tested thoroughly during route handler tests.

C To increase test execution time by adding complexity with mocks.

D To isolate route logic from database dependencies, making tests faster and more reliable by avoiding real database access.

💡 Hint

Think about what makes unit tests fast and stable.

Mocking database calls isolates the route handler logic from external dependencies. This makes tests faster, more predictable, and easier to run without needing a real database.