Javascript to get number of entries in a value prompt?

[aks12]

Using Cognos 8.4, what is the javascript syntax to return the number of entries in a value prompt? I'm not a javascript coder and have been struggling with this. I've tried variations of the code below with no luck.

var theSpan = document.getElementById("fromDate");
var theSelect = theSpan.getElementsByTagName("select");
theSpanLen =theSpan.length;

canSubmitPrompt();
document.write("Length test value of mylist is: " + theSpanLen + "<br/>");