What are JavaScript Design Patterns?

Design patterns are reusable solutions to common programming problems.

1. Module Pattern:

const Module = (function() {
  let privateVar = 0;
  
  return {
    increment() {
      privateVar++;
    },
    getCount() {
      return privateVar;
    }
  };
})();

2. Singleton Pattern:

class Singleton {
  constructor() {
    if (Singleton.instance) {
      return Singleton.instance;
    }
    Singleton.instance = this;
  }
}

3. Observer Pattern:

class EventEmitter {
  constructor() {
    this.events = {};
  }
  
  on(event, callback) {
    if (!this.events[event]) {
      this.events[event] = [];
    }
    this.events[event].push(callback);
  }
  
  emit(event, data) {
    if (this.events[event]) {
      this.events[event].forEach(callback => callback(data));
    }
  }
}

4. Factory Pattern:

class CarFactory {
  createCar(type) {
    switch(type) {
      case "sedan": return new Sedan();
      case "suv": return new SUV();
    }
  }
}