* Calls the specified callback function for all the elements in an array. The return value of
* the callback function is the accumulated result, and is provided as an argument in the next
* call to the callback function.
* @param callbackfn A function that accepts up to four arguments. The reduce method calls the
* callbackfn function one time for each element in the array.
* @param initialValue If initialValue is specified, it is used as the initial value to start
* the accumulation. The first call to the callbackfn function provides this value as an argument
* instead of an array value.