Controllers and Services

As an Express app grows, putting all logic inside route handlers becomes messy.

Controllers and services help separate responsibilities.

Controller

A controller handles HTTP concerns:

• read params/query/body,
• call service,
• choose response status,
• send JSON response,
• pass errors to middleware.

Example:

export async function createUserController(req, res, next) {
  try {
    const user = await userService.createUser(req.body)
    res.status(201).json(user)
  } catch (error) {
    next(error)
  }
}

Service

A service handles business logic:

• validation rules beyond shape,
• database operations,
• calculations,
• domain decisions,
• calling other services.

Example:

export async function createUser(input) {
  const existingUser = await User.findOne({ email: input.email })

  if (existingUser) {
    throw new Error('Email already exists')
  }

  return User.create(input)
}

Why this matters

If everything sits inside routes:

• code becomes hard to test,
• logic gets duplicated,
• controllers become huge,
• changing business rules becomes risky.

Simple folder idea

src/
  routes/
  controllers/
  services/
  models/
  middlewares/

Interview answer

Controllers handle HTTP-specific work such as reading request data and sending responses. Services contain business logic and database operations. Separating them makes Express apps easier to test, reuse, and maintain.