SOURCE

let arrs = [1,2,3]
// let arr = new Proxy(arrs,{
//     get(target, p, receiver) {
//         // console.log(p)
//         if(parseInt(p)===-1){
//             return target[target.length-1]
//         }
//     }
// })

Object.defineProperty(arrs,'-1',{
  get(){
      return arrs[arrs.length-1]
  }
})

console.log(arrs[-1])