Add transform operators

[andres-kovalev]

buffer(size, stream) creates new stream where all source items grouped into sub groups

buffer(2, [1,2,3,4,5,6]) => [[1,2], [3,4], [5,6]]
buffer(3, [1,2,3,4,5,6]) => [[1,2,3], [4,5,6]]

bufferBy(predicate, stream) similar to previous one, but groups by predicate result

bufferBy(i => i%2===1, [1,2,3,4,5,6]) => [[1],[2],[3],[4],[5],[6]]
bufferBy(i < 3, [1,2,3,4,5,6]) => [[1,2],[3,4,5,6]]
bufferBy(i%3>0, [1,2,3,4,5,6]) => [[1,2],[3],[4,5],[6]]

pluck(field, stream) extracts field value from each item

pluck('a')
// equivalent to
map(({ a }) => a)

scan(accFn, stream) accumulates value of each item to produce new stream (like history of reduce() operation)

const add = (a,b) => a + b;
scan(add, [1,2,3,4,5,6]) => [1,3,6,10,15,21]

flatten(stream) flattens initial stream

flatten([[1,2,3],[4,5,6]]) => [1,2,3,4,5,6];