Example: overloaded controller

Here is some code that demonstrates what not to do. It's an example of a RESTful API controller class. In it, only the createUser method is shown, but given the class name AppController, it's rightful to assume that this controller would contain all of the methods for the app's REST api.

// AppController.ts

export class AppController {
  createUser (req: express.Request, res: express.Response): Promise<void> {
    // Get values from request
    const { username, email, password } = req.body;

    // Validate request values
    const isUsernameValid = TextUtils.isAtLeast(3, username) 
      && TextUtils.isAtMost(30, username);
    const isEmailValid = TextUtils.isValidEmail(email)
    const isPasswordValid = TextUtils.isAtLeast(3, password) 
      && TextUtils.isAtMost(25, password);

    // Check if username has already been taken
    const existingUserByUserName = await this.userRepo

    if (existingUserByUserName) {
      return res.status(409).json({ 
        message: "Username already taken" 

    // Check if email already exists
    const existingUserByEmail = await this.userRepo
    if (existingUserByEmail) {
      return res.status(409).json({ 
        message: "User already exists. Try logging in." 

    // Otherwise, create the user
    const user = await this.userRepo.create({

    // Send email verification email
    await this.emailService.sendVerificationEmaill(user.email);

    // Add email to mailing list
    await this.mailingList.add(user.email);
    // Do more stuff...

Without going on a tangent, this file has the potential to become very large.

In a request, there are…

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