strategy.js

class Payment {
  constructor() {
    this.paymentMethod = ''
  }
  
  setPaymentMethod(paymentMethod) {
    this.paymentMethod = paymentMethod
  }
  
  pay(amount) {
    this.paymentMethod.pay(amount)
  }
}

class CashPayment {
  constructor() {}
  pay(amount) {
    // make payment
  }
}

class CreditPayment {
  constructor() {}
  pay(amount) {
    // make payment
  }
}

const payment = new Payment()
const cash = new CashPayment()
const credit = new CreditPayment()

payment.setPaymentMethod(credit)
payment.pay() // pay with credit

payment.setPaymentMethod(cash)
payment.pay() // pay with cash

Personally, Payment would rather be an interface than an actual class because Payment doesn't hold any logic. I imagine there is a scenario that an user is able to set his payment option as Credit

User  {
   constructor() {
       this.payment = new CashPayment(); // default
   }


   setPaymentMethod(payment) {
      this.payment = payment;
   }
}