composition.js

// https://medium.com/@genildocs/mastering-object-oriented-programming-in-javascript-from-zero-to-hero-c718c3182eba

// Mixin for logging functionality
const LoggerMixin = {
  log(message) {
    console.log(`[${this.constructor.name}] ${message}`);
  },

  logError(error) {
    console.error(`[${this.constructor.name}] ERROR: ${error}`);
  }
};

// Mixin for validation
const ValidatorMixin = {
  validate(data, rules) {
    for (const [field, rule] of Object.entries(rules)) {
      if (!rule(data[field])) {
        return { valid: false, field };
      }
    }
    return { valid: true };
  }
};

class UserService {
  constructor() {
    // Apply mixins
    Object.assign(this, LoggerMixin, ValidatorMixin);
  }

composition2.js

let sayHiMixin = {
  sayHi() {
    alert(`Hello ${this.name}`);
  }
}
 
 
let sayByeMixin = {
  sayBye() {
    alert(`Bye ${this.name}`);
  }
}
 
 
class User {
  constructor(name) {
    this.name = name;
  }
}
 
// copy the methods
Object.assign(User.prototype, sayHiMixin);
Object.assign(User.prototype, sayByeMixin);
 
 
const user = new User("Dude")
 
user.sayHi()
user.sayBye()

composition3.js

// Instead of deep inheritance hierarchies
class DatabaseLogger extends Logger extends BaseService {}

// Use composition
class UserService {
  constructor(logger, database) {
    this.logger = logger;
    this.database = database;
  }

  createUser(data) {
    this.logger.info("Creating user");
    return this.database.save(data);
  }
}