99 Problems, #1: last in list.js

/*
Write a function last that accepts a list and returns the last element in the list.

If the list is empty:

In languages that have a built-in option or result type (like OCaml or Haskell), return an empty option

In languages that do not have an empty option, just return null
*/
const last = xs => xs.length===0?null:xs[xs.length-1]