Mocking database calls in Express - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is mocking in the context of database calls?

Mocking means creating fake versions of database calls to test code without using a real database. It helps check if the code works correctly and quickly.

beginner

Why do we mock database calls in Express apps?

We mock database calls to avoid slow or unreliable real database access during tests. It makes tests faster and safer by isolating the code from the database.

intermediate

Which tool can you use to mock functions in Express tests?

You can use libraries like Sinon.js or Jest to create mocks and spies for database functions in Express tests.

intermediate

How does mocking help when testing error handling in database calls?

Mocking lets you simulate errors from the database, so you can check if your code handles those errors properly without needing a real database failure.

intermediate

What is a stub in mocking database calls?

A stub is a fake function that replaces a real database call and returns preset data or errors to control test scenarios.

What is the main benefit of mocking database calls in Express tests?

A Connects to multiple databases

B Improves database performance

C Automatically fixes database errors

D Faster and isolated tests without real database

Mocking helps tests run faster and isolates code from the real database.

Which library is commonly used to mock functions in JavaScript tests?

A Express

B React

C Sinon.js

D MongoDB

Sinon.js is a popular library for mocking and spying on functions.

What does a stub do in mocking?

A Replaces a function with preset responses

B Deletes the database

C Runs the real database query

D Creates a new database

A stub replaces a real function to return controlled data for tests.

How can mocking help test error handling?

A By ignoring errors

B By simulating database errors

C By fixing errors automatically

D By connecting to a backup database

Mocking can simulate errors so you can check how your code reacts.

When should you avoid mocking database calls?

A When you want to test real database integration

B When writing unit tests

C When testing error handling

D When running fast tests

Mocking is not used when you want to test actual database behavior.

Explain how mocking database calls improves testing in Express applications.

Think about speed and control in tests.

Describe the difference between a mock and a stub when testing database calls.

Focus on purpose and behavior tracking.

Practice

1. What is the main purpose of mocking database calls in Express applications?

easy

A. To test code without needing a real database connection

B. To speed up the database queries in production

C. To permanently replace the database with fake data

D. To make the database store data faster

Solution

1. Step 1: Understand mocking concept

   Mocking means creating fake versions of parts of your app, like database calls, to test without real dependencies.

2. Step 2: Identify the purpose in testing

   Mocking database calls allows testing code logic without needing a real database, making tests faster and more reliable.

3. Final Answer:

   To test code without needing a real database connection -> Option A

4. Quick Check:

   Mocking = Test without real DB [OK]

Hint: Mocking means faking DB calls to test code fast [OK]

Common Mistakes:

• Thinking mocking speeds up production DB
• Confusing mocking with permanent DB replacement
• Assuming mocking changes real data storage

2. Which of the following is the correct way to mock a database call returning a promise in Express?

easy

A. const mockDbCall = Promise.resolve({ id: 1, name: 'Test' });

B. const mockDbCall = () => { return { id: 1, name: 'Test' }; };

C. const mockDbCall = () => new Promise({ id: 1, name: 'Test' });

D. const mockDbCall = () => Promise.resolve({ id: 1, name: 'Test' });

Solution

1. Step 1: Understand promise returning function

   A mock database call should return a promise that resolves to data, so Promise.resolve is correct.

2. Step 2: Check each option

   const mockDbCall = Promise.resolve({ id: 1, name: 'Test' }); assigns a resolved promise directly, making mockDbCall a promise object not a callable function. const mockDbCall = () => { return { id: 1, name: 'Test' }; }; returns plain object, not a promise. const mockDbCall = () => Promise.resolve({ id: 1, name: 'Test' }); returns a function returning a resolved promise, correct. const mockDbCall = () => new Promise({ id: 1, name: 'Test' }); incorrectly uses new Promise without executor function.

3. Final Answer:

   const mockDbCall = () => Promise.resolve({ id: 1, name: 'Test' }); -> Option D

4. Quick Check:

   Promise.resolve returns a resolved promise [OK]

Hint: Use Promise.resolve to return mock data as a promise [OK]

Common Mistakes:

• Returning plain object instead of a promise
• Using new Promise without executor function
• Assigning a promise directly instead of a function returning it

3. Given the mock function below, what will getUser().then(console.log) output?

const getUser = () => Promise.resolve({ id: 5, name: 'Alice' });

medium

A. { id: 5, name: 'Alice' }

B. Promise { { id: 5, name: 'Alice' } }

C. undefined

D. Error: getUser is not a function

Solution

1. Step 1: Understand the mock function

   getUser returns a promise resolved with an object { id: 5, name: 'Alice' }.

2. Step 2: Analyze the .then(console.log) call

   The .then method receives the resolved value and logs it, so the output is the object itself.

3. Final Answer:

   { id: 5, name: 'Alice' } -> Option A

4. Quick Check:

   Promise resolves to object logged [OK]

Hint: Promise.then logs resolved value, not the promise itself [OK]

Common Mistakes:

• Thinking console.log prints the promise object
• Expecting undefined because of missing await
• Assuming getUser is undefined or error

4. Identify the error in this mock database call function:

function fetchData() {
  return new Promise(data => {
    resolve({ id: 1, value: 'mock' });
  });
}

medium

A. The function should be async instead of returning a promise

B. The promise executor function parameter should be (resolve, reject), not (data)

C. The data object is missing a required field

D. The function should not return a promise

Solution

1. Step 1: Review Promise constructor usage

   The Promise constructor expects an executor function with two parameters: resolve and reject.

2. Step 2: Identify misuse of parameter

   The code declares executor parameter as 'data' but calls resolve(...), where resolve is undefined. It should declare (resolve, reject) and call resolve(value).

3. Final Answer:

   The promise executor function parameter should be (resolve, reject), not (data) -> Option B

4. Quick Check:

   Promise executor = (resolve, reject) [OK]

Hint: Promise executor needs (resolve, reject) parameters [OK]

Common Mistakes:

• Using wrong parameter name in Promise executor
• Not returning a promise for async behavior
• Confusing async function with Promise constructor

5. You want to mock a database call in Express that sometimes returns data and sometimes throws an error to test error handling. Which mock function correctly simulates this behavior?

hard

A. const mockDb = () => Promise.resolve({ id: 1 });

B. const mockDb = () => { if (Math.random() > 0.5) return { id: 1 }; else throw new Error('DB error'); };

C. const mockDb = () => Math.random() > 0.5 ? Promise.resolve({ id: 1 }) : Promise.reject(new Error('DB error'));

D. const mockDb = () => new Promise((resolve) => resolve({ id: 1 }));

Solution

1. Step 1: Understand requirement for success and failure

   The mock must sometimes resolve with data and sometimes reject with an error to test both cases.

2. Step 2: Analyze each option

   const mockDb = () => Math.random() > 0.5 ? Promise.resolve({ id: 1 }) : Promise.reject(new Error('DB error')); returns a promise that resolves or rejects based on random chance, correctly simulating success or failure.
   const mockDb = () => { if (Math.random() > 0.5) return { id: 1 }; else throw new Error('DB error'); }; throws error synchronously, not returning a promise rejection.
   const mockDb = () => Promise.resolve({ id: 1 }); always resolves, no error simulation.
   const mockDb = () => new Promise((resolve) => resolve({ id: 1 })); always resolves, no error simulation.

3. Final Answer:

   const mockDb = () => Math.random() > 0.5 ? Promise.resolve({ id: 1 }) : Promise.reject(new Error('DB error')); -> Option C

4. Quick Check:

   Random resolve or reject simulates success/error [OK]

Hint: Use Promise.resolve or Promise.reject to simulate success or error [OK]

Common Mistakes:

• Throwing error instead of rejecting promise
• Always resolving without error case
• Not returning a promise for async simulation