Dropdownlist controls ListBox content

I. I am working on a project to prevent page refreshing and use dropdownlist to control the content of ListBox.

The Code is as follows:

1. Function deletemethod ()
2. {
3. VaR objselect = Document. getelementbyid ('listbox1 ');
4. If (objselect. selectedindex! =-1)
5. {
6. Objselect. Remove (objselect. selectedindex );
7. }
8. If (document. getelementbyid ('listbox1'). Options. Length <3)
9. {
10. Document. getelementbyid ('slt _ person'). Disabled = false;
11. }
12. }
13. Function checkstate ()
14. {
15. If (document. getelementbyid ('listbox1'). Options. Length = 0)
16. {
17. Document. All. Remove. Disabled = true;
18. }
19. Else
20. {
21. Document. All. Remove. Disabled = false;
22. }
23. If (document. getelementbyid ('listbox1'). Options. Length = 3)
24. {
25. Document. getelementbyid ('slt _ person'). Disabled = true;
26. }
27. }
28. Function additem ()
29. {
30. VaR text = Document. All. slt_person.options [document. All. slt_person.selectedindex]. text;
31. VaR values = Document. All. slt_person.options [document. All. slt_person.selectedindex]. value;
32. VaR list = Document. getelementbyid ('listbox1 ');
33. If (values! = '-1 '&&! Isexist (values, list ))
34. {
35. List. Options. Add (New Option (text, values, values, 0 ));
36. }
37. }
38. Function isexist (SS, LL)
39. {
40. VaR length = Document. getelementbyid ('listbox1'). Options. length;
41. For (VAR I = 0; I <length; I ++)
42. {
43. If (LL. Options [I]. value = SS)
44. Return true
45. }
46. Return false;
47. }

2. listbox1 binds the database content, and then binds the script to the event on the corresponding control.