Observer.js

class Subject {
    constructor() {
        this._observers = []
    }

    subscribe(observer) {
        this._observers.push(observer)
    }

    unsubscribe(observer) {
        this._observers = this._observers.filter(obs => obs !== observer)
    }

    fire(productName) {
        this._observers.forEach(observer => observer.addToWishList(productName))
    }
}

class WishList {
    constructor() {
        this.products = []
    }

    addToWishList(productName) {
        console.log(`Add the product ${productName} to the wishlist`)
        this.products.push(productName)
    }
}


class Mailer {
    constructor(email) {
        this._email = email
    }

    addToWishList(productName) {
        console.log(`Send an email to ${this._email} with the product name ${productName}`)
    }
}

const Sub = new Subject()

const Mail = new Mailer('email@example.com')
const WL = new WishList()

Sub.subscribe(Mail)
Sub.subscribe(WL)


Sub.fire('Terminator')