Inside an object is it possible for a method to use the return value of another method?

Question

In the test code below a factory function creates an object. Inside the new object are 2 methods, totalCost & withShipping. Is there a pattern I can use that will allow withShipping to use the return value of totalCost? As configured it throws an error. Thanks so much for any help!

"use strict"

function factoryTest(x) {
    let returnTest = {
        numberOfEngines: x.numberOfEngines,
        costPerEngine: x.costPerEngine,
        totalCost: function() {
            return x.numberOfEngines * x.costPerEngine;
        },
        withShipping: function() {
            return x.totalCost() * 2;
        }

    }
    return returnTest;
}

let aircraft = factoryTest({numberOfEngines: 2, costPerEngine: 40000});

console.log(aircraft.totalCost());
console.log(aircraft.withShipping());

Answer 1

The easiest method would be to use this to access the current instantiation:

"use strict"

function factoryTest(x) {
    let returnTest = {
        numberOfEngines: x.numberOfEngines,
        costPerEngine: x.costPerEngine,
        totalCost: function() {
            return x.numberOfEngines * x.costPerEngine;
        },
        withShipping: function() {
            return this.totalCost() * 2;
        }

    }
    return returnTest;
}

let aircraft = factoryTest({numberOfEngines: 2, costPerEngine: 40000});

console.log(aircraft.totalCost());
console.log(aircraft.withShipping());

Another workable method, since you're using the factory function pattern, would be to define the totalCost function and everything else outside of returnTest and then call it:

"use strict"

function factoryTest({
  numberOfEngines,
  costPerEngine
}) {
  const totalCost = () => numberOfEngines * costPerEngine;
  return {
    numberOfEngines,
    costPerEngine,
    totalCost,
    withShipping: () => totalCost() * 2,
  };
}

const aircraft = factoryTest({
  numberOfEngines: 2,
  costPerEngine: 40000
});

console.log(aircraft.totalCost());
console.log(aircraft.withShipping());