Make Own Reducer function in js

MyReducer.js

// Problem: Make your own reducer function in javascript.

//Solution:
// First Add MyReducer function as prototype in array.
Array.prototype.MyReducer = function (fn, init) {
  let current;
  let startIndex = 0;
  // If initial value is not defined assign it to with this first index value.
  if (init) {
    current = init;
  } else {
    // this refer to the array from which MyReducer is called.
    current = this[0];
    // If already take first index value then start loop from index 1.
    startIndex = 1;
  }
  for (let count = startIndex; count < this.length; count++) {
    current = fn(this[count], current);
  }
  return current;
}

//Way of calling MyReducer fucntion.
[1,2,3,4].MyReducer((a,b) => a+b); // Here 1 will be the initial value.
[1,2,3,4].MyReducer((a,b) => a+b, 10); // Here 10 will be the initial value.