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

The Big Idea

What if you could change your entire data source by updating just one file?

The Scenario

Imagine building a Next.js app where you fetch data directly from the database in many places across your code.

Each time you want to change how data is fetched or stored, you must update many files.

The Problem

Manually accessing data everywhere makes your code messy and hard to maintain.

If the database changes, you risk breaking many parts of your app.

It's like having many keys for different doors instead of one master key.

The Solution

The Repository pattern creates a single place to handle all data access.

Your app talks only to this repository, so changes happen in one spot.

This keeps your code clean, easier to test, and safer to update.

Before vs After

✗ Before

const user = await db.query('SELECT * FROM users WHERE id = ?', [id]);

✓ After

const user = await userRepository.getUserById(id);

What It Enables

It enables you to swap databases or change data logic without touching your whole app.

Real Life Example

Think of a library where all books are organized in one place, so you don't have to search every shelf yourself.

Key Takeaways

Manual data access scatters code and causes maintenance headaches.

Repository pattern centralizes data logic for cleaner, safer code.

It makes your app flexible and easier to update or test.

Practice

1. What is the main purpose of using the Repository pattern in a Next.js application?

easy

A. To style components with CSS modules

B. To organize data access in one place for cleaner and reusable code

C. To handle routing between pages

D. To manage component state with hooks

Solution

1. Step 1: Understand the role of the Repository pattern

   The Repository pattern centralizes data access logic, separating it from UI code.

2. Step 2: Identify benefits in Next.js context

   This separation makes code cleaner, reusable, and easier to maintain.

3. Final Answer:

   To organize data access in one place for cleaner and reusable code -> Option B

4. Quick Check:

   Repository pattern = centralized data access [OK]

Hint: Repository pattern centralizes data access, not UI or styling [OK]

Common Mistakes:

• Confusing repository with UI state management
• Thinking repository handles routing
• Mixing repository with styling concerns

2. Which of the following is the correct way to define a simple repository class in Next.js using TypeScript?

easy

A. class UserRepository { async getAll() { return fetch('/api/users').then(res => res.json()) } }

B. function UserRepository() { return fetch('/api/users') }

C. const UserRepository = () => fetch('/api/users').json()

D. class UserRepository { getAll() { return fetch('/api/users').json() } }

Solution

1. Step 1: Check class syntax and async usage

   class UserRepository { async getAll() { return fetch('/api/users').then(res => res.json()) } } correctly defines a class with an async method returning parsed JSON from fetch.

2. Step 2: Identify errors in other options

   function UserRepository() { return fetch('/api/users') } is a function, not a class. const UserRepository = () => fetch('/api/users').json() incorrectly calls .json() directly on fetch result without awaiting. class UserRepository { getAll() { return fetch('/api/users').json() } } misses async and awaits, causing runtime error.

3. Final Answer:

   class UserRepository { async getAll() { return fetch('/api/users').then(res => res.json()) } } -> Option A

4. Quick Check:

   Async class method with fetch and json parsing = class UserRepository { async getAll() { return fetch('/api/users').then(res => res.json()) } } [OK]

Hint: Repository methods fetching data must be async and parse JSON [OK]

Common Mistakes:

• Forgetting async keyword on methods
• Calling .json() without awaiting fetch response
• Using function instead of class for repository

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

class UserRepository {
  async getAll() {
    return [{ id: 1, name: 'Alice' }, { id: 2, name: 'Bob' }]
  }
}

const repo = new UserRepository()
const users = await repo.getAll()
console.log(users)

medium

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

B. Promise { <pending> }

C. undefined

D. Error: await is only valid in async function

Solution

1. Step 1: Analyze the getAll method

   getAll returns a resolved array of user objects immediately (no fetch or delay).

2. Step 2: Understand await usage

   Await unwraps the Promise returned by async getAll, so users is the array itself.

3. Final Answer:

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

4. Quick Check:

   Await unwraps async return value = array [OK]

Hint: Await unwraps async method result to actual data [OK]

Common Mistakes:

• Thinking users is a Promise object
• Forgetting await causes Promise logged
• Assuming undefined without return

4. Identify the error in this Next.js repository code snippet:

class ProductRepository {
  getAll() {
    const response = fetch('/api/products')
    return response.json()
  }
}

medium

A. json() method does not exist on response

B. fetch URL is incorrect

C. Missing async keyword and await for fetch and json calls

D. Class name should be lowercase

Solution

1. Step 1: Check fetch usage

   fetch returns a Promise, so response is a Promise, not the actual response object.

2. Step 2: Identify missing async/await

   To get JSON data, method must be async and await fetch and response.json() calls.

3. Final Answer:

   Missing async keyword and await for fetch and json calls -> Option C

4. Quick Check:

   Async fetch requires await to get response data [OK]

Hint: Always use async/await with fetch and response.json() [OK]

Common Mistakes:

• Not marking method async
• Not awaiting fetch or json()
• Assuming fetch returns data synchronously

5. You want to create a repository in Next.js that fetches user data and caches it to avoid repeated API calls. Which approach correctly applies the repository pattern with caching?

hard

A. class UserRepository {
async getAll() {
return fetch('/api/users').then(res => res.json())
}
}

B. function getAllUsers() {
return fetch('/api/users').then(res => res.json())
}

C. const cache = []
async function getAllUsers() {
if (cache.length) return cache
const res = await fetch('/api/users')
cache.push(...await res.json())
return cache
}

D. class UserRepository {
cache = null
async getAll() {
if (this.cache) return this.cache
const res = await fetch('/api/users')
this.cache = await res.json()
return this.cache
}
}

Solution

1. Step 1: Understand caching in repository context

   Caching should be encapsulated inside the repository class to keep data access logic together.

2. Step 2: Evaluate options for caching implementation

   class UserRepository { cache = null async getAll() { if (this.cache) return this.cache const res = await fetch('/api/users') this.cache = await res.json() return this.cache } } uses a class property to store cache and returns it if available, avoiding repeated fetch calls.

3. Step 3: Compare other options

   Options A and B do not implement caching. const cache = [] async function getAllUsers() { if (cache.length) return cache const res = await fetch('/api/users') cache.push(...await res.json()) return cache } uses an external cache variable, breaking encapsulation.

4. Final Answer:

   class UserRepository { cache = null async getAll() { if (this.cache) return this.cache const res = await fetch('/api/users') this.cache = await res.json() return this.cache } } -> Option D

5. Quick Check:

   Repository with internal cache property = class UserRepository { cache = null async getAll() { if (this.cache) return this.cache const res = await fetch('/api/users') this.cache = await res.json() return this.cache } } [OK]

Hint: Cache inside repository class to keep data logic together [OK]

Common Mistakes:

• Caching outside repository breaks encapsulation
• Not checking cache before fetching
• Using functions instead of class for caching