有沒有給定日期，得到星期幾的函數？

<script>
var month_names=new Array(12)
month_names[0]="Jan"
month_names[1]="Feb"
month_names[2]="Mar"
month_names[3]="Apr"
month_names[4]="May"
month_names[5]="Jun"
month_names[6]="July"
month_names[7]="Aug"
month_names[8]="Sept"
month_names[9]="Oct"
month_names[10]="Nove"
month_names[11]="Dec"var day_names=new Array
day_names[0]="Sun"
day_names[1]="Mon"
day_names[2]="Tues"
day_names[3]="Wedn"
day_names[4]="Thur"
day_names[5]="Fri"
day_names[6]="Sat"var user_year=prompt("enter your year:","2003")
var user_month=prompt("enter your month","")
var user_day=prompt("enter your day","")var user_date=new Date()
user_date.setFullYear(user_year)
user_date.setMonth(user_month-1)
user_date.setDate(user_day)var day_name=day_names[user_date.getDay()]
var month_name=month_names[user_date.getMonth()]
alert("the date you entered was:"+month_name+""+user_date+","+user_year+"\n"+"the day of the week is:"+day_name)
 
</script>