📌Introduction

The Adapter pattern is used to allow a new use of an existing interface of a class, with a new interface, using the same methods.
We separate the code well (Single responsibility principle).
We dont need to change the existing class (open/closed principle).

⚡️In short

We have:
Target: Interface the client uses.
Adapter: Transforms the request of the client to a specific request.
Adapteee: Class we want to adapt.

💡Intent

Wrap an existing class with a new interface.
Convert an interface into another one.
Let classes work together that couldn't otherwise because of incompatible interfaces.

🔧 Apply when (we want to)

Re-use an existing class, but the interface is not compatible
Combine unrelated classes with an incompatible interface

✅Pro

Single Responsibility Principle: We can separate the different interfaces' logic.
Open/Closed Principle. We can introduce new adapters without breaking the existing code.

⚠️Cons

How much adapting does the adapter do? Complex adaptations could be be hard to write
The overall complexity of the code increases
We need to introduce new interfaces and classes.
Sometimes it’s better to change the class. (Not always possible by the way).

🏆 Great for

Connect to legacy systems
Adapt one interface to another

/** Adapter Design pattern
 *  
 *  Structural Design Pattern
 */

//Target
class LegacyShopper {
  constructor() {
    this.request = (product, quantity, price) => `${price * quantity}$`;
  }
}

// Adapteee
class OnlineShopping {
  constructor() {
    this.authenticate = (username, password) => console.log(`login logic for...${username} // ${password}`);
    this.setProduct = (product) => console.log(`setting product to ${product}`);
    this.setQuantity = (quantity) => console.log(`setting quantity to ${quantity}`);
    this.applyDiscount = (price) => price * 0.9; // online discount
    this.checkout = (quantity, price) => `${(this.applyDiscount(price)) * quantity}$`;
  }
}

// Adapter interface
class Adapter {
  constructor(username, password) {
    const cart = new OnlineShopping();
    cart.authenticate(username, password);

    this.request = (product, quantity, price) => {
      cart.setProduct(product);
      cart.setQuantity(quantity);
      return cart.checkout(quantity, price);
    }
  };
}

// legacy shopping
const cart = new LegacyShopper();
console.log("***Legacy system***");
console.log("Standard cost: " + cart.request("apples", 10, 3));

// OUTPUT
//
//***Legacy system***
//Standard cost: 30$

//////////////////////////////////////////////////////////////

// new shopping with Adapted interface
console.log("\n***New system***");

const adapter = new Adapter("Francesco", "123456"); //login logic simulation
//Same request, using the adapter
console.log("Online cost: " + adapter.request("apples", 10, 3)); //27$

// OUTPUT:
//
// ***New system***
// login logic for...Francesco // 123456
// setting product to apples
// setting quantity to 10
// Online cost: 27$

Github link: https://github.com/FrancescoXX/Design-Pattern-Adapter-Javascript