Using the Count property
var fdoc = new ActiveXObject("FileDocument.FileDocument");
var msel = fdoc.CreateEmptySelection();
alert(msel.Count); // displays "0"
msel.AddRange(100, 20); // selection: [100..120)
alert(msel.Count); // displays "1"
msel.AddRange(150, 20); // selection: [100..120) U [150..170)
alert(msel.Count); // displays "2"
msel.AddRange(80, 80); // selection: [80..170)
alert(msel.Count); // displays "1"