var Nom = ['Justin','Michel','Pascal'];
alert(Nom) // alerts: "Justin,Michel,Pascal,Philipe"
That happens because the variable tbl gets a reference value, not a primitive value. That assignment var tbl=Nom set a reference, thus from now on, all the changes made in the reference (tbl) will be reflected in the main object (Nom)
DFORMS might have thought that on using var tbl=Nom he can create another, independent, array, but he was wrong. From this point of view, a reference of the object is that object.