unshift.js

Array.prototype.unShift  = function (){
  let newArray  = [...arguments], i = 0, len  =  this.length , newArrLength=newArray.length;
  for(i; i < len; i++){
    newArray[newArrLength+i] = this[i];
  }
  return newArray;
}

let arr =[1,2];
console.log(arr.unShift(4,5,6));