Service layer pattern in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What does this Express service layer return?

Consider this service function called by a route handler. What will be the resolved value when calling getUserById(2)?

Express

const users = [{id: 1, name: 'Alice'}, {id: 2, name: 'Bob'}];

function getUserById(id) {
  return new Promise((resolve) => {
    const user = users.find(u => u.id === id);
    resolve(user ? user.name : null);
  });
}

A "Bob"

B "{ id: 2, name: 'Bob' }"

C null

D undefined

💡 Hint

Look at what the promise resolves with: the user's name or the whole user object.

The service function finds the user object by id and resolves the promise with the user's name property if found, otherwise null. For id 2, the user is {id: 2, name: 'Bob'}, so it resolves with 'Bob'.

❓ state_output

intermediate

What is the final state of req.user after middleware and service call?

Given this Express middleware and service call, what will req.user contain after next()?

Express

const users = [{id: 1, name: 'Alice'}, {id: 2, name: 'Bob'}];

function userService(id) {
  return users.find(u => u.id === id) || null;
}

function authMiddleware(req, res, next) {
  req.user = userService(1);
  next();
}

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

B null

C undefined

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

💡 Hint

Check what userService returns for id 1.

The middleware calls userService with id 1, which returns the user object {id: 1, name: 'Alice'}. This is assigned to req.user before calling next().

📝 Syntax

advanced

Which option correctly implements an async service function in Express?

You want to create a service function that fetches user data asynchronously. Which option is syntactically correct and returns a promise resolving to the user object?

Express

const users = [{id: 1, name: 'Alice'}, {id: 2, name: 'Bob'}];

A async function getUser(id) { return users.find(u => u.id === id); }

B function getUser(id) { return new Promise(resolve => resolve(users.find(u => u.id === id))); }

C async function getUser(id) { const user = users.find(u => u.id === id); return user; }

D function getUser(id) { return await users.find(u => u.id === id); }

💡 Hint

Remember that Array.find is synchronous and does not return a promise.

Option B correctly returns a Promise that resolves with the user object. Option B is async but returns a synchronous value, which is allowed but may confuse. Option B uses await outside async function causing syntax error. Option B awaits a synchronous function, which is allowed but unnecessary.

🔧 Debug

advanced

Why does this Express service call cause an unhandled promise rejection?

Examine the code below. Why does calling getUserById(3) cause an unhandled promise rejection?

Express

const users = [{id: 1, name: 'Alice'}, {id: 2, name: 'Bob'}];

function getUserById(id) {
  return new Promise((resolve, reject) => {
    const user = users.find(u => u.id === id);
    if (!user) reject('User not found');
    else resolve(user);
  });
}

getUserById(3).then(user => console.log(user));

A Because the promise rejects but there is no catch handler to handle the rejection.

B Because users.find throws an error when user is not found.

C Because resolve is called twice causing a conflict.

D Because the function does not return a promise.

💡 Hint

Check how the promise rejection is handled after calling getUserById.

The promise rejects with 'User not found' when no user is found. The caller uses then() but no catch() to handle rejection, causing unhandled promise rejection error.

🧠 Conceptual

expert

What is the main benefit of using a service layer in Express apps?

Why do developers use a service layer between Express route handlers and data access logic?

A To replace middleware functions with simpler code.

B To speed up HTTP requests by caching all responses automatically.

C To separate business logic from routing logic, making code easier to maintain and test.

D To directly connect route handlers to the database without abstraction.

💡 Hint

Think about code organization and responsibilities.

The service layer separates business logic from routing, improving maintainability and testability. It does not automatically cache or replace middleware or bypass abstraction.