High cohesion and low coupling

Cohesion defines the degree to which class members are related to each other. Ideally, all fields within a class should be used by each method.
We then say that the class is maximally cohesive. In practice, this, however, is not always possible, nor even advisable. You should however prefer cohesion to be high.

Coupling refers to how related or dependent are two classes toward each other. Classes are said to be low coupled if changes in one of them don’t affect the other one.

Good software design has high cohesion and low coupling.

Bad:

class UserManager {
  // Bad: each private variable is used by one or another group of methods.
  // It makes clear evidence that the class is holding more than a single responsibility.
  // If I need only to create the service to get the transactions for a user,
  // I'm still forced to pass and instance of `emailSender`.
  constructor(private readonly db: Database, private readonly emailSender: EmailSender) {}
  async getUser(id: number): Promise<User> {
    return await db.users.findOne({ id });
  }

  async getTransactions(userId: number): Promise<Transaction[]> {
    return await db.transactions.find({ userId });
  }

  async sendGreeting(): Promise<void> {
    await emailSender.send('Welcome!');
  }

  async sendNotification(text: string): Promise<void> {
    await emailSender.send(text);
  }

  async sendNewsletter(): Promise<void> {
    // ...
  }
}

Good:

class UserService {
  constructor(private readonly db: Database) {}
  async getUser(id: number): Promise<User> {
    return await this.db.users.findOne({ id });
  }

  async getTransactions(userId: number): Promise<Transaction[]> {
    return await this.db.transactions.find({ userId });
  }
}

class UserNotifier {
  constructor(private readonly emailSender: EmailSender) {}
  async sendGreeting(): Promise<void> {
    await this.emailSender.send('Welcome!');
  }

  async sendNotification(text: string): Promise<void> {
    await this.emailSender.send(text);
  }

  async sendNewsletter(): Promise<void> {
    // ...
  }
}