Before a project framework is the.net 3.5, modified and adjusted for. NET after version 4.5, a front form validation JS method existing in debugging pop-up message is the following problems:

Error: unable to get undefined or null reference attribute "value",

 
The function checkmail () {
Var txtmail=document. GetElementById (" txtmail "); 
Var txtmailTips=document. GetElementById (" txtmailTips "); 

If (txtmail value! {
="")Var isreg=txtmail. Value. The match (/^ \ w/- \ w. + * @ ([A Za - z0-9] [9] - A - Za - z0 - + \.) + [A Za - z] {2, 14} $/); 
If (isreg==null) {
TxtmailTips. The innerText='please enter the correct email address; 
return false; 
} 
The else {
TxtmailTips. The innerText=' '; 
return true; 
} 
} 
The else {
TxtmailTips. The innerText=' '; 
return true; 
} 
} 
</script> 

Should be how to modify?