Why Repository pattern for data access in Express? - Purpose & Use Cases

The Big Idea

What if changing your database didn't mean rewriting half your app?

The Scenario

Imagine building an app where you write raw database queries everywhere in your Express routes. Every time you want to change how data is fetched or saved, you must hunt through all your files to update those queries.

The Problem

Scattering database code all over makes your app hard to maintain and test. If the database changes, you risk breaking many parts. It's slow to add features or fix bugs because you repeat similar code everywhere.

The Solution

The Repository pattern creates a single place to handle all data access. Your routes just ask the repository for data, so you can change database details inside the repository without touching the rest of your app.

Before vs After

✗ Before

app.get('/users', (req, res) => { db.query('SELECT * FROM users', (err, results) => { res.send(results); }); });

✓ After

class UserRepository { getAll() { return db.query('SELECT * FROM users'); } } const userRepository = new UserRepository(); app.get('/users', async (req, res) => { const users = await userRepository.getAll(); res.send(users); });

What It Enables

This pattern lets you swap databases, add caching, or change queries easily without rewriting your whole app.

Real Life Example

When your app grows and you switch from a simple JSON file to a real database, the Repository pattern lets you make that change smoothly without breaking your routes.

Key Takeaways

Manual data access scattered in routes is hard to maintain.

Repository pattern centralizes data logic for easier updates.

It improves app flexibility, testing, and scalability.

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