const arr = [1, 2, 3, 4]

const handler = {
  get(target, prop, receiver) {
    // try to convert property to a number
    const n = +prop
    // if it's a number less than zero, return the correct item from the array
    if (!isNaN(n) && n<0) {
      return Reflect.get(target, target.length + n, receiver)
    // otherwise continue as normal with the property
    } else {
      return Reflect.get(...arguments)
    }
  }
}

const myArr = new Proxy(arr, handler)

console.log(myArr[0])  // 1
console.log(myArr[2])  // 3
console.log(myArr[-1])  // 4
console.log(myArr[-3])  // 2