#mySelect.options
回傳選項清單
select#mySelect
option apple
option banana
option grape
console.log(mySelect.options)
//回傳所有options的集合
//型態為HTMLOptionsCollection
#mySelect.selectedIndex
回傳目前被選取到的index
select#mySelect
option apple
option banana
option grape
console.log(mySelect.selectedIndex)
//0