Javascript Array reduce(testFunction,initialValue)

// Reduce always returns an array with one item


Array.prototype.reduce = function(testFunction,initialValue){
 var accumulatedValue,counter;
 if(this.length == 0){
  return this;/* w  w w .java 2 s.co  m*/
 }else{
  if(arguments.length == 1){
   counter = 1;
   accumulatedValue = this[0];
  }else if(arguments.length >= 2){
   counter = 0;
   accumulatedValue = initialValue;
  }else{
   throw 'Invalid arguments.'
  }

  while(counter < this.length){
   accumulatedValue = testFunction(accumulatedValue,this[counter]);
   counter++;
  }
  return [accumulatedValue];
 }

}