var fieldToFocus;
function computeForm(form) {
  error = false;


if (error == false && isBlank(form.friendName,"Friend Name"))
  { 
      error = true;                                       
  }

if (error == false && !(validEmail(form.friendEmail)))
  {
	error = true;
  }

  if (error)
     return false;
    else
     return true;
}


<!-- To check if the input is blank-->
	function isBlank(field,msg)
	{
		if (field.value == "") {
    			alert ("You Must Enter "+msg);
    			field.focus();
    			field.select();
    			return true ;
 		} 
		else 
		{
   	 		return false ;
 		}
	}

<!-- To check if the input is blank For Password-->
	function isBlank2(field,msg)
	{
		if (field.value == "") {
    			alert ("You Must Veryfy The "+msg);
    			field.focus();
    			field.select();
    			return true ;
 		} 
		else 
		{
   	 		return false ;
 		}
	}


<!--To check for valid Email address-->
function validEmail(eAddr) 
{ 
   var ndxAt = ndxDot = 0;

   ndxAt  = eAddr.value.indexOf("@");
   ndxDot = eAddr.value.indexOf(".");

   if ( (ndxDot < 0) || (ndxAt < 0) || (ndxDot < ndxAt) || ((ndxDot - 3) <= ndxAt) || (eAddr.length < (ndxDot + 4)) )
   {
      alert("Your email is invalid");  
      eAddr.focus();
      eAddr.select();
      return false;  
    }
    else
    {
      return true;
    }                      
} 


<!-- To check if value is selected from select list-->
     function isSelected(field,msg)
     {
		if (field.selectedIndex == 0) {
    			alert ("Please Select a "+msg);
    			field.focus();
    			return true ;
 		} 
		else 
		{
   	 		return false ;
 		}
     }

<!--To check if the input has only numbers-->
     function isNotaNumber(field,msg)
     {
		if (isNaN(field.value)) {
    			alert ("You Must Enter Valid"+msg+". Numbers Only");
    			field.focus();
    			field.select();
    			return true ;
 		} 
		else 
		{
   	 		return false ;
 		}
     }


<!--To check the retype password -->
	function verify(field1,field2)
	{
		if(field1.value==field2.value)
			{
				return false ;
			}
		else
			{
				alert("Both Passwords are not the same, please try again!")
				field1.focus();
	 			field1.select();
				return true ;
			}
	}