Apply function of two arguments cumulatively to the items of sequence, from left to right, so as to reduce the sequence to a single value. For example:

reduce([1, 2, 3, 4, 5], (x, y) => x + y, 0)

calculates

(((((0+1)+2)+3)+4)+5)

The left argument, x, is the accumulated value and the right argument, y, is the update value from the sequence.

Difference between reduce() and reduce\_(): reduce() requires an explicit initializer, whereas reduce_() will automatically use the first item in the given iterable as the initializer. When using reduce(), the initializer value is placed before the items of the sequence in the calculation, and serves as a default when the sequence is empty. When using reduce_(), and the given iterable is empty, then no default value can be derived and undefined will be returned.