ItemInInv

Player.getAmountOfItemInInv = function(id) {
    let count = 0;
    for(let i = 0; i < 36; i++) {
        if(Player.getInventorySlot(i) == id) {
            count += Player.getInventorySlotCount(i);
        }
    }
    return count;
}

//add this on top of your script, then you will be able to use Player.getAmountOfItemInInv(itemId) as a function. :)