function yay(array) {
  return new Proxy(array, {
    get (target, property, receiver) {
      if (property < 0) {
        property = target.length + Number(property)
      }
      return Reflect.get(target, property, receiver)
    }
  })
}

// Demo use:

const arr = yay(['foo', 'bar', 'baz'])
arr[-1] // => 'baz'