Mocking database calls in Express - Mini Project: Build & Apply

Mocking database calls in Express

📖 Scenario: You are building a simple Express server that fetches user data from a database. To test your server without a real database, you will mock the database calls.

🎯 Goal: Create an Express server with a mocked database call that returns user data. You will set up the data, configure a mock function, implement the route using the mock, and complete the server setup.

📋 What You'll Learn

Create a mock user data array called users with exact user objects

Create a mock function called getUsersFromDb that returns the users array

Create an Express route /users that uses getUsersFromDb to send user data as JSON

Complete the Express server setup with app.listen on port 3000

💡 Why This Matters

🌍 Real World

Mocking database calls helps developers test server routes without needing a real database connection. This speeds up development and testing.

💼 Career

Backend developers often mock database calls to write unit tests and develop APIs before the database is ready or to isolate code during testing.

1

DATA SETUP: Create mock user data

Create a constant array called users with these exact objects: { id: 1, name: 'Alice' } and { id: 2, name: 'Bob' }.

Express

// Create the mock user data array called users
// Your code here

Hint

Use const users = [ ... ] with two objects inside the array.

2

CONFIGURATION: Create mock database function

Create a function called getUsersFromDb that returns the users array.

Express

const users = [
  { id: 1, name: 'Alice' },
  { id: 2, name: 'Bob' }
];

// Create the mock function getUsersFromDb
// Your code here

Hint

Define a function named getUsersFromDb that returns the users array.

3

CORE LOGIC: Create Express route using mock function

Import Express, create an app with express(), and add a GET route /users that calls getUsersFromDb() and sends the result as JSON with res.json().

Express

const users = [
  { id: 1, name: 'Alice' },
  { id: 2, name: 'Bob' }
];

function getUsersFromDb() {
  return users;
}

// Import express, create app, and add GET /users route
// Your code here

Hint

Use require('express') to import Express, create app, and define the GET route with app.get.

4

COMPLETION: Start the Express server

Add app.listen to start the server on port 3000.

Express

const express = require('express');
const users = [
  { id: 1, name: 'Alice' },
  { id: 2, name: 'Bob' }
];

function getUsersFromDb() {
  return users;
}

const app = express();

app.get('/users', (req, res) => {
  const data = getUsersFromDb();
  res.json(data);
});

// Start the server on port 3000
// Your code here

Hint

Use app.listen(3000) to start the server on port 3000.

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