Getting the size of an array in an object
Javascript arrays have a length property. Use it like this:
st.itemb.length
Arrays have a property .length that returns the number of elements.
var st = { "itema":{}, "itemb": [ {"id":"s01","cd":"c01","dd":"d01"}, {"id":"s02","cd":"c02","dd":"d02"} ] };st.itemb.length // 2