Note to self: Remember to assign the destructured array/object an empty array/object when giving destructured function arguments default values.

factory.js

const makeSandwich = ({
  bread = 'wheat', 
  meat = 'turkey', 
  cheese = 'american', 
  toppings = []
} = {}) => ({
  bread,
  meat,
  cheese,
  toppings,
  addToppings (...toppings) {
    this.toppings = [...this.toppings, ...toppings];
    return this;
  },
});

const defaultSandwich = makeSandwich();
console.log(defaultSandwich);
/*
{
  "bread": "wheat",
  "cheese": "american",
  "meat": 'turkey',
  "toppings": [],
  "addToppings": [Function addToppings],
}
*/