Repository pattern for data access in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

Repository Pattern: Data Fetching Behavior

Given the repository pattern implementation below, what will be the output when calling userRepository.getById(2) if the data source contains users with ids 1 and 2?

Express

class UserRepository {
  constructor() {
    this.users = [
      { id: 1, name: 'Alice' },
      { id: 2, name: 'Bob' }
    ];
  }

  getById(id) {
    return this.users.find(user => user.id === id);
  }
}

const userRepository = new UserRepository();
console.log(userRepository.getById(2));

A { id: 2, name: 'Bob' }

B undefined

C { id: 1, name: 'Alice' }

D Throws TypeError

💡 Hint

Look at how the getById method searches the array.

The getById method uses Array.find to locate the user with the matching id. Since user with id 2 exists, it returns that user object.

📝 Syntax

intermediate

Correct Repository Method Syntax

Which option correctly defines a repository method addUser that adds a user object to the internal array users?

Express

class UserRepository {
  constructor() {
    this.users = [];
  }

  // Define addUser method here
}

A addUser(user) => { this.users.push(user); }

B addUser = (user) => { this.users.push(user); }

C addUser(user) { this.users.push(user) }

D addUser(user) { this.users.push(user); }

💡 Hint

Check the correct method syntax inside a class in JavaScript.

In a class, methods are defined without the = or arrow syntax. Option D uses the correct method syntax with parentheses and curly braces.

Option D uses arrow function syntax which is invalid as a class method declaration. Option D has invalid arrow function syntax. Option D is missing a semicolon but that is allowed in JavaScript, so it would also work but is less standard.

🔧 Debug

advanced

Identify the Error in Repository Method

What error will occur when running the following repository method removeUser if called with an id that does not exist in the users array?

Express

class UserRepository {
  constructor() {
    this.users = [{ id: 1, name: 'Alice' }];
  }

  removeUser(id) {
    const index = this.users.findIndex(user => user.id === id);
    this.users.splice(index, 1);
  }
}

const repo = new UserRepository();
repo.removeUser(2);

A Removes user with id 1

B Throws RangeError due to invalid splice index

C Throws TypeError because index is undefined

D Removes no user, no error thrown

💡 Hint

What does findIndex return if no match is found?

findIndex returns -1 if no matching element is found. Calling splice(-1, 1) removes the last element in the array. But since the array has only one user with id 1, calling removeUser(2) will remove the last user incorrectly.

However, in JavaScript, splice(-1, 1) removes the last element, so it does remove user with id 1, which is wrong behavior.

Therefore, the correct answer is that it removes user with id 1, which is option A.

❓ state_output

advanced

State of Repository After Multiple Operations

After running the following code, what is the content of repo.users?

Express

class UserRepository {
  constructor() {
    this.users = [];
  }

  addUser(user) {
    this.users.push(user);
  }

  clearUsers() {
    this.users = [];
  }
}

const repo = new UserRepository();
repo.addUser({ id: 1, name: 'Alice' });
repo.addUser({ id: 2, name: 'Bob' });
repo.clearUsers();
repo.addUser({ id: 3, name: 'Charlie' });

A [{ id: 1, name: 'Alice' }, { id: 2, name: 'Bob' }, { id: 3, name: 'Charlie' }]

B [{ id: 3, name: 'Charlie' }]

C []

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

💡 Hint

Look at what clearUsers does to the users array.

The clearUsers method resets the users array to an empty array. After clearing, only the last added user remains in the array.

🧠 Conceptual

expert

Purpose of Repository Pattern in Express Apps

Which option best describes the main benefit of using the repository pattern for data access in an Express application?

A It replaces the need for middleware in Express routes.

B It automatically caches all database queries to improve performance.

C It separates data access logic from business logic, making code easier to maintain and test.

D It enforces a strict schema on all data models without external libraries.

💡 Hint

Think about how separating concerns helps in coding.

The repository pattern abstracts data access logic from the rest of the app. This separation makes the code cleaner, easier to maintain, and easier to test by isolating database operations.

Options B, C, and D describe unrelated or incorrect benefits.