//Remove Spaces on both sides of the input string
function trim(s) {  
   var count = s.length;  
   var st    = 0;       // start  
   var end   = count-1; // end  

   if (s == "") return s;  
   while (st < count) {  
     if (s.charAt(st) == " ")  
       st ++;  
     else 
       break;  
   }  
   while (end > st) {  
     if (s.charAt(end) == " ")  
       end --;  
     else 
       break;  
   }  
   return s.substring(st,end + 1);  
 }

Suppose the form looked like this:

<form action="testnew.html" name="form1">  
  username: <input type="text" name="name">  
  password: <input type="password" name="pwd">   <br>  
  <input type="submit" value=" submit " onclick="isEmpty()">  
</form>

To determine whether the input is null, the function can be defined as follows:

function isEmpty(){  
    //Form1 is the name property & NBSP; in the form;  
    var _form = document.form1;  

    if(trim(_form.name.value)==""){  
        alert(" The username cannot be empty !");          
        return false;  
    }  
    if(trim(_form.pwd.value)==""){  
        alert(" The password cannot be empty !");         
        return false;  
    } 
    return true;
}