Javascript Array reduceRight(iteratee, initialValue)

Array.prototype.reduceRight = function(iteratee, initialValue) {
 if (typeof iteratee !== "function") {
  throw new TypeError(iteratee + ' is not a function')
 }
 let arr = this;// w ww.  j  av  a  2 s  . co  m
 let result;
 let index;
 if (initialValue != void 0) {
  result = initialValue;
  index = arr.length - 1;
 } else {
  result = arr[arr.length - 1];
  index = arr.length - 2;
 }
 for (let i = index; i >= 0; i--) {
  result = iteratee(result, arr[i]);
 }
 return result;
}