i need to throw error in java script

Question

i need output like this [email protected]

 var eml=document.getElementById('ctl00_ContentPlaceHolder3_textemail').value;

    if(eml=="")

      {

                 alert("Email id can not be blank");

                

                return false;

      }

     var emailPat = /^(\".*\"|[A-Za-z]\w*)@(\[\d{1,3}(\.\d{1,3}){3}]|[A-Za-z]\w*(\.[A-Za-z]\w*)+)$/;

     var emailid=document.getElementById('ctl00_ContentPlaceHolder3_textemail').value;

     var matchArray = emailid.match(emailPat);

     if (matchArray == null)

    {

               alert("Your email address seems incorrect. Please try again.");

               

               return false;

    }