- var calendar = document.getElementById('calendar');
- var userdate = document.getElementById('date_' + date_box).value;
+ var calendar;
+ var userdate;
+
+ calendar = document.getElementById('calendar');
+ if(calendar == undefined) {
+ window.alert("Can not find element 'calendar'");
+ return;
+ }
+
+ userdate = document.getElementById('date_' + date_box);
+
+ if(userdate == undefined) {
+ window.alert("Can not find element 'date_'" + date_box);
+ return;
+ }
+
+ userdate = userdate.value;