Send Receive data between server and client without postback

as per my flow on every click i have a postback  without which i cannot store data to client or server

i have 2 ways of data storing one is client side in javascript variables in form of array(which can be lost on postback)

the data to be stored are of differnt types some are numeric , some single characters some sepearted with comma

other way is on each post back save the data in a session varibale on server & then access

currently i have second way

considering both the ways and in accrodance with the current flow which is best for maintenance & access speed

reasking as i am notyet cleared abt the things