How do I read only the first value inside a Stream in order to execute a function once?

I have a Readable Stream from which I get ids 1,2,3,4 etc and for each id, I get numbers like 10,11,12 etc. Numbers or values could be anything. A sample workflow would be as follows​IDValue11001872154210913862478As seen above, when I start streaming, the first value of ID 1 is 100 and first value of ID 2 is 154, I want to call a function ONCE with these values but how do I capture them. Some direction would be appreciated

Submitted February 23, 2019 at 10:20AM by mypirateapp