Join an array with proper English

oxford.js

function oxford (arr, last) {
  last = last || ', and '
  return arr.reduce(function (a, b, i, arr) {
    return (
      !arr[i + 1] ? a + b :
      !arr[i + 2] ? a + b + last :
      a + b + ', '
    )
  }, '')
}