Mocking database calls in Express - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the mocking library for testing database calls.

Express

const [1] = require('sinon');

Drag options to blanks, or click blank then click option'

Ajest

Bexpress

Cmongoose

Dsinon

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'express' instead of a mocking library.

Confusing 'mongoose' (database ORM) with mocking tools.

The sinon library is commonly used to mock functions and database calls in Express apps.

2fill in blank

medium

Complete the code to stub the database method findUser to return a fake user.

Express

const stub = sinon.[1](db, 'findUser').returns({ id: 1, name: 'Test User' });

Drag options to blanks, or click blank then click option'

Astub

Bmock

Cspy

Dfake

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'mock' which is different from 'stub' in Sinon.

Using 'spy' which only observes calls but does not replace behavior.

The stub method replaces the original function with a fake one that returns specified data.

3fill in blank

hard

Fix the error in restoring the stub after the test to avoid side effects.

Express

stub.[1]();

Drag options to blanks, or click blank then click option'

Areset

Bclear

Crestore

DresetHistory

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'reset' which only resets call history but does not restore original function.

Using 'clear' which is not a Sinon method.

The restore method returns the original function to its normal state after stubbing.

4fill in blank

hard

Fill both blanks to create a mock Express route that uses the stubbed database call and sends a JSON response.

Express

app.get('/user', async (req, res) => {
  const user = await db.[1]();
  res.[2](user);
});

Drag options to blanks, or click blank then click option'

AfindUser

Bsend

Cjson

DgetUser

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'send' instead of 'json' which may send data but not set JSON content type.

Using wrong database method name like 'getUser'.

The route calls the stubbed findUser method and sends the user data as JSON with res.json().

5fill in blank

hard

Fill all three blanks to write a test that stubs the database call, calls the route handler, and asserts the response.

Express

const stub = sinon.[1](db, 'findUser').returns({ id: 2, name: 'Mocked' });

await request(app).get('/user').expect(200).expect(res => {
  if (res.body.name !== [2]) throw new Error('Wrong user');
});
stub.[3]();

Drag options to blanks, or click blank then click option'

Astub

B'Mocked'

Crestore

Dmock

Attempts:

3 left

💡 Hint

Common Mistakes

Not restoring the stub after the test.

Checking for the wrong user name string.

Using 'mock' instead of 'stub' for replacing the function.

The test stubs findUser, expects the response to have the mocked name, and restores the stub after.

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