Every array knows how many items it has, through its length:
const foods = ["burger", "omelet", "pancakes", "soup"];
console.log(foods.length);It prints
4
Since indexes start at 0, the last item is always at index length - 1. Or, just like strings, you can use .at(-1):
const foods = ["burger", "omelet", "pancakes", "soup"];
console.log(foods[foods.length - 1]);
console.log(foods.at(-1));
console.log(foods.at(-2));It prints
soup soup pancakes