Functional programming in Js - Reduce

reduce.js

Array.prototype.reduce = function(combiner, initialValue) {
  var counter, accumulatedValue;

  // If the array to be reduced is empty, then do nothing
  if (this.length === 0) {
    return this; //this refers to the Array being reduced
  } else {
    /*
     If the user did not pass an initialValue, that is if the number of arguments passed to the reduce function is just 1, 
     then use the first item from the array as the accumulatedValue to begin with
    */
    if (arguments.length === 1) {
      counter = 1; // this makes the second element of the array the currentValue in the combiner function
      accumulatedValue = this[0]; // this makes the first element the accumulatedValue in the combiner function
    } else if (arguments.length === 2) {
      counter = 0; // this makes the first element the currentValue, because the user has provided an initialValue to begin with as the accumulatedValue
      accumulatedValue = initialValue;
    } else {
      throw "Invalid arguments passed to reduce";
    }

    /*
      Loop through the array, reading the current value and the result of the previous computation back into the combiner function,
      until we have exhausted the entire array
    */
    while (counter < this.length) {
      accumulatedValue = combiner(accumulatedValue, this[counter]);
      counter++;

    }

    // return the final value accumulated, after exhausting the list
    return [accumulatedValue];

  }



};

console.log([1, 2, 3].reduce(function(acValue, currValue) {
  return acValue + currValue;
}, 10));