Each way of pricing shipping lives in its own interchangeable object: Checkout does not know which one it is using, only that it has a cost().
See it run
With and without the pattern
Which pattern is it?
Code smells
Switch the shipping and compute the total: in the trace, Checkout calls cost() on a different object, without a single if.
- Shipping
- StandardShipping
The code that runs
class StandardShipping {
cost(order) {
return order.weight <= 2 ? 4.99 : 4.99 + (order.weight - 2) * 1.5;
}
}
class ExpressShipping {
cost(order) {
return 9.99 + order.weight * 2;
}
}
class StorePickup {
cost(order) {
return 0;
}
}
class Order {
constructor(weight) {
this.weight = weight;
}
}
class Checkout {
constructor(shipping) {
this.shipping = shipping;
}
setShipping(shipping) {
this.shipping = shipping;
}
total(order, subtotal) {
return subtotal + this.shipping.cost(order);
}
}
function setup() {
const standard = new StandardShipping();
return {
order: new Order(3),
standard,
express: new ExpressShipping(),
pickup: new StorePickup(),
checkout: new Checkout(standard),
};
}
const actions = {
total: (w) => w.checkout.total(w.order, 30),
useStandard: (w) => w.checkout.setShipping(w.standard),
useExpress: (w) => w.checkout.setShipping(w.express),
usePickup: (w) => w.checkout.setShipping(w.pickup),
};
Sequence diagram · at setup
Step 5 of 5 you → Checkout: «new» Checkout(StandardShipping)