Repository pattern for data access in Express - Mini Project: Build & Apply

Repository pattern for data access in Express

📖 Scenario: You are building a simple Express app that manages a list of books. To keep your code clean and organized, you will use the repository pattern to handle data access. This means you will create a separate module that manages the books data, so your main app code stays simple and easy to maintain.

🎯 Goal: Build a repository module that stores books data and provides methods to get all books and add a new book. Then use this repository in an Express route to show all books.

📋 What You'll Learn

Create a books array with initial book objects

Create a BookRepository object with methods to get all books and add a book

Use the repository methods inside an Express route to send the list of books as JSON

Set up a basic Express server listening on port 3000

💡 Why This Matters

🌍 Real World

The repository pattern helps keep your Express app organized by separating data access from route handling. This makes your code easier to maintain and test.

💼 Career

Many backend jobs require clean code organization. Using repository pattern is a common practice to manage data access in Node.js and Express applications.

1

DATA SETUP: Create the initial books data array

Create a variable called books that is an array containing these two book objects exactly: { id: 1, title: 'The Hobbit' } and { id: 2, title: '1984' }.

Express

// Create the books array with two book objects
// Your code here

Hint

Use const books = [ ... ] and include the two objects inside the array.

2

CONFIGURATION: Create the BookRepository object

Create a constant called BookRepository that is an object with two methods: getAll which returns the books array, and add which takes a book parameter and pushes it into the books array.

Express

const books = [
  { id: 1, title: 'The Hobbit' },
  { id: 2, title: '1984' }
];

// Create BookRepository object with getAll and add methods
// Your code here

Hint

Define BookRepository as an object with two functions: getAll and add.

3

CORE LOGIC: Use the repository in an Express route

Create an Express app by requiring express and calling express(). Then add a GET route on '/books' that uses BookRepository.getAll() to get all books and sends them as JSON with res.json().

Express

const books = [
  { id: 1, title: 'The Hobbit' },
  { id: 2, title: '1984' }
];

const BookRepository = {
  getAll() {
    return books;
  },
  add(book) {
    books.push(book);
  }
};

// Create Express app and add GET /books route
// Your code here

Hint

Use require('express') and express() to create the app. Then add a GET route on /books that sends JSON from the repository.

4

COMPLETION: Start the Express server on port 3000

Add a line to start the Express app listening on port 3000 using app.listen(3000).

Express

const books = [
  { id: 1, title: 'The Hobbit' },
  { id: 2, title: '1984' }
];

const BookRepository = {
  getAll() {
    return books;
  },
  add(book) {
    books.push(book);
  }
};

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

app.get('/books', (req, res) => {
  const allBooks = BookRepository.getAll();
  res.json(allBooks);
});

// Start 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 using the Repository pattern in an Express app?

easy

A. To manage user authentication

B. To handle HTTP requests and responses

C. To style the frontend components

D. To separate data access logic from business logic

Solution

1. Step 1: Understand the role of repository pattern

   The repository pattern is designed to isolate data access logic from the rest of the application.

2. Step 2: Identify the correct purpose in Express context

   In Express apps, it helps keep business logic clean by moving database queries into repositories.

3. Final Answer:

   To separate data access logic from business logic -> Option D

4. Quick Check:

   Repository pattern = separate data access [OK]

Hint: Repository pattern means separate data access from app logic [OK]

Common Mistakes:

• Confusing repository with routing or middleware
• Thinking it handles UI or styling
• Mixing it with authentication logic

2. Which of the following is the correct way to define a repository class in Express using ES6 syntax?

easy

A. class UserRepository { getAll() { /* code */ } }

B. var UserRepository = new Object({ getAll: function() {} })

C. const UserRepository = () => { return { getAll() {} } }

D. function UserRepository() { this.getAll = () => {}; }

Solution

1. Step 1: Identify ES6 class syntax

   ES6 classes use the class keyword with methods inside without the function keyword.

2. Step 2: Match the correct syntax

   class UserRepository { getAll() { /* code */ } } correctly defines a class with a method getAll() using ES6 syntax.

3. Final Answer:

   class UserRepository { getAll() { /* code */ } } -> Option A

4. Quick Check:

   ES6 class syntax = class UserRepository { getAll() { /* code */ } } [OK]

Hint: ES6 classes use 'class' keyword and method shorthand [OK]

Common Mistakes:

• Using function keyword inside class methods
• Confusing arrow functions with class methods
• Using old object syntax instead of class

3. Given this repository method, what will console.log(users) output?

class UserRepository {
  constructor() {
    this.users = [{id:1, name:'Anna'}, {id:2, name:'Bob'}];
  }
  getAll() {
    return this.users;
  }
}
const repo = new UserRepository();
const users = repo.getAll();
console.log(users);

medium

A. [{id:1, name:'Anna'}, {id:2, name:'Bob'}]

B. undefined

C. Error: getAll is not a function

D. []

Solution

1. Step 1: Understand the constructor initialization

   The constructor sets this.users to an array of two user objects.

2. Step 2: Check the getAll method return value

   The getAll() method returns this.users, so calling it returns the array.

3. Final Answer:

   [{id:1, name:'Anna'}, {id:2, name:'Bob'}] -> Option A

4. Quick Check:

   getAll returns users array [OK]

Hint: getAll returns stored users array directly [OK]

Common Mistakes:

• Assuming getAll returns undefined or empty
• Confusing method call with property access
• Expecting an error due to missing binding

4. Identify the error in this repository code snippet:

class ProductRepository {
  constructor() {
    this.products = [];
  }
  add(product) {
    this.products.push(product);
  }
  getAll() {
    return products;
  }
}

medium

A. Missing return statement in getAll method

B. Using undefined variable 'products' instead of 'this.products'

C. Constructor does not initialize products array

D. add method does not push product correctly

Solution

1. Step 1: Check variable usage in getAll method

   The method returns products but this variable is not defined in the method scope.

2. Step 2: Correct usage of instance variable

   It should return this.products to access the class property.

3. Final Answer:

   Using undefined variable 'products' instead of 'this.products' -> Option B

4. Quick Check:

   Use this.products to access class property [OK]

Hint: Use 'this.' to access class properties inside methods [OK]

Common Mistakes:

• Forgetting 'this.' prefix for class properties
• Assuming variables are global inside class methods
• Ignoring constructor initialization

5. You want to extend a base repository to add a custom method findByEmail(email) for users. Which approach correctly implements this in Express?

hard

A. const UserRepository = Object.create(BaseRepository);
UserRepository.findByEmail = function(email) {
return this.data.find(u => u.email === email);
};

B. function UserRepository() {
this.findByEmail = (email) => this.data.find(u => u.email === email);
}
UserRepository.prototype = BaseRepository.prototype;

C. class UserRepository extends BaseRepository {
findByEmail(email) {
return this.data.find(u => u.email === email);
}
}

D. class UserRepository {
findByEmail(email) {
return BaseRepository.data.find(u => u.email === email);
}
}

Solution

1. Step 1: Understand class inheritance in ES6

   Extending a base class uses extends keyword and super() if constructor is needed.

2. Step 2: Check method implementation accessing instance data

   class UserRepository extends BaseRepository { findByEmail(email) { return this.data.find(u => u.email === email); } } correctly extends BaseRepository and uses this.data to find user by email.

3. Final Answer:

   class UserRepository extends BaseRepository { findByEmail(email) { return this.data.find(u => u.email === email); } } -> Option C

4. Quick Check:

   Extend base class and add method using this.data [OK]

Hint: Use 'extends' to inherit and add custom methods in subclass [OK]

Common Mistakes:

• Not using 'extends' for inheritance
• Accessing base class data incorrectly
• Overwriting prototype without proper inheritance