Make arrays negative indexable using a Proxy

negativeIndexArray.js

function makeArrayNegativeIndexable(arr) {
  if (!Array.isArray(arr)) return arr

  return new Proxy(arr, {
    get(target, prop) {
      return target[getIndex(target.length, prop)]
    },
    set(target, prop, val) {
      let index = getIndex(target.length, prop)

      target[index] = val
      return val
    }
  })
}

function getIndex(arrLen, prop) {
  // calling parseInt on a symbol throws hence this is first
  if (typeof prop === 'symbol') return prop

  let index = Number.parseInt(prop, 10)
  
  return !Number.isNaN(index) && index == prop && index < 0 ? arrLen + index : prop
}

const x = [1, 2]
const y = makeArrayNegativeIndexable(x)

console.assert(x[0] === y[-2])
console.assert(x[1] === y[-1])

y[2] = 'test'
y[-2] = false

console.assert(x[2] === y[2])
console.assert(x[1] === y[-2])