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

Repository Pattern for Data Access in Next.js

📖 Scenario: You are building a simple Next.js app that fetches user data. To keep your code clean and organized, you will use the repository pattern. This means you will create a separate module to handle data fetching, so your components stay simple and focused on showing the data.

🎯 Goal: Build a repository module that fetches user data from a mock API and use it in a Next.js page component.

📋 What You'll Learn

Create a repository module with a function to fetch user data

Add a configuration variable for the API endpoint URL

Implement the data fetching logic inside the repository function

Use the repository function inside a Next.js page component to display user names

💡 Why This Matters

🌍 Real World

The repository pattern helps keep your data fetching code organized and reusable, making your Next.js apps easier to maintain and test.

💼 Career

Many companies use the repository pattern to separate concerns in their codebase. Knowing this pattern is useful for frontend and full-stack developer roles.

1

Create the initial user data array

Create a constant called mockUsers that is an array of objects. Each object should have id and name properties with these exact values: { id: 1, name: 'Alice' }, { id: 2, name: 'Bob' }, and { id: 3, name: 'Charlie' }.

NextJS

// Create the mockUsers array with 3 user objects
// Your code here

Hint

Use const mockUsers = [ ... ] to create the array with the exact user objects.

2

Add API endpoint configuration

Create a constant called API_URL and set it to the string 'https://example.com/api/users'.

NextJS

export const mockUsers = [
  { id: 1, name: 'Alice' },
  { id: 2, name: 'Bob' },
  { id: 3, name: 'Charlie' }
];

// Create the API_URL constant with the exact URL string
// Your code here

Hint

Use const API_URL = 'https://example.com/api/users' to store the API endpoint.

3

Implement the repository fetch function

Create an async function called fetchUsers that returns the mockUsers array. Use return mockUsers; inside the function body.

NextJS

export const mockUsers = [
  { id: 1, name: 'Alice' },
  { id: 2, name: 'Bob' },
  { id: 3, name: 'Charlie' }
];

export const API_URL = 'https://example.com/api/users';

// Create async function fetchUsers that returns mockUsers
// Your code here

Hint

Define async function fetchUsers() and inside it return mockUsers;.

4

Use the repository in a Next.js page component

Create a default exported async function component called UsersPage. Inside it, call fetchUsers() and store the result in a constant called users. Return a JSX <main> element containing an unordered list <ul>. Map over users to create list items <li> showing each user's name. Use user.id as the key for each list item.

NextJS

export const mockUsers = [
  { id: 1, name: 'Alice' },
  { id: 2, name: 'Bob' },
  { id: 3, name: 'Charlie' }
];

export const API_URL = 'https://example.com/api/users';

export async function fetchUsers() {
  return mockUsers;
}

// Create default async function UsersPage
// Call fetchUsers and store in users
// Return JSX main with ul and li for each user name
// Your code here

Hint

Define export default async function UsersPage(), call await fetchUsers(), and return JSX with a list of user names.

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