gametest API: can't get id item by ItemStack component

follow your newest document at this time on learn.minecraft.com, i can get the name of the item by ItemStack component, example: minecraft:skull. But i can't specify the id of item.

minecraft:skull with id 0 is skeleton skull, id 1 is wither skeleton skull. I can't see difference between them by ItemStack component

the first picture: from ItemStack
the second picture: from ItemType
all them return string (the name of item, not id of item (number))