JavaScript from zero: pass the object

Once the drink is an object, functions should take the drink. They read the fields they need. Adding a field later does not mean threading a new argument through every call. price returns the label, the same text the method built.

Pass the object the function reads#

price does not close over a global item. It receives the object and reads name and cents. The caller chooses which object. That is the whole contract: an object with those properties. There is no class requirement. A misspelled property is undefined, and the label would show that, so a caller that cares should check. This program passes a latte and prints latte $4.50.

1. lattelabellatte $4.50

function price(item) {
  const frac = String(item.cents % 100).padStart(2, "0");
  return item.name + " $" + Math.floor(item.cents / 100) + "." + frac;
}

const latte = { name: "latte", cents: 450 };
console.log(price(latte));

latte $4.50

One argument, named fields#

price takes item and returns the label. The print is latte $4.50. A caller who passes the cents and the name in the wrong order cannot, because there is nothing to swap. There is one argument.

price(null) throws when it reads .cents. If a missing item is normal, check item == null first and return a value you chose, the way the Java lesson printed missing. If a missing item is a bug, the throw is the right signal. It points at price.

Methods and functions can share the work#

latte.label() is the method on the class. price(latte) is a function that takes the object. Use a method when the behavior belongs to the drink and uses this. Use a function when the same step should work for any object with name and cents, including a plain object that never went through new. Both are ordinary tools. Pick the one that keeps the call obvious.