/*
  Array.prototype.reduce()
  The reduce() method executes a reducer function (callback) on each element of the array, resulting in a single output value.
  
  Syntax:
    * the reducer function takes in four four arguments, namely Accumulator, curren value, current index, & source array.
    * in absence of an initial value, the first value of the array is used as the starting point for reduce function
  
  Sample signatures:
  
  reduce(reducerFn)
  reduce(reducerFn, initialValue)
  
  reduce(function reducerFn(accumulator, currentValue) { ... })
  reduce(function reducerFn(accumulator, currentValue, index) { ... })
  reduce(function reducerFn(accumulator, currentValue, index, array){ ... })
  reduce(function reducerFn(accumulator, currentValue, index, array) { ... }, initialValue) 
*/

// Custom Polyfill
Array.prototype.customReduce = function(callback, initialValue) {
  var accumulator = intialValue === undefined ? undefined : intialValue;
  
  for (var i = 0; i < this.length; i++) {
    if (accumulator !== undefined)
        accumulator = callback.call(undefined, accumulator, this[i], i, this);
    else
        accumulator = this[i];
  }
  
  return accumulator;
}

// Proper Polyfill with error handling
Array.prototype.betterReduce = function(callback /*, intialVlaue */) {
  if (this == null)
    throw new TypeError('betterReduce called on null or not defined');
  
  if (typeof callback !== 'function')
    throw new TypeError('callback is not a function ');
  
  var Obj = this;
  var len = this.length;
  var counter = 0;
  var value;
  
  if (arguments.length >= 2)
    value = arguments[1];
  
  while (counter < len) {
    value = callback(value, this[counter], counter, this);
  }
  
  return value;
}