var $ = function (id) {
 return document.getElementById(id);}
 

var fields = [];
 
// Define field objects
fields["email"] = {};
fields["firstname"] = {};
fields["lastname"] = {};
fields["zip"] = {};
 
// Default field messages
fields["email"].message = "Must be a valid email address.";
fields["zip"].message = "Use 5 digit ZIP code.";
 
// Field errors
fields["email"].required = new Error("Email is required.");
fields["email"].invalid = new Error("Email is not valid.");
fields["first_name"].required = new Error("First name is required.");
fields["last_name"].required = new Error("Last name is required.");
fields["zip"].required = new Error("ZIP Code is required.");
fields["zip"].invalid = new Error("ZIP Code is not valid.");

 
// Field validation functions
fields["email"].validate = function () {
 
    var email = $("email").value;
    if (email.length == 0) throw this.required;
    var parts = email.split("@");
    if (parts.length != 2 ) throw this.invalid;
    if (parts[0].length > 64) throw this.invalid;
    if (parts[1].length > 255) throw this.invalid;
     
    var local_part = /^([-\w!#$%&'*+-/=?^`{
|
}~]+)|("[^"]*")$/;
    var local_dots = /(^\.)|(^[^"].*\.\..*[^"]$)|(\.$)/;
    if ( !parts[0].match(local_part) ) throw this.invalid;
    if (  parts[0].match(local_dots) ) throw this.invalid;
     
    var host_pattern =
        "(([a-zA-Z0-9])\\.|([a-zA-Z0-9][-a-zA-Z0-9]{
0,62
}[a-zA-Z0-9])\\.)+";
    var tld_pattern = "[a-zA-Z0-9]{
2,6
}";
    var domain = new RegExp("^" + host_pattern + tld_pattern + "$");
    if ( !parts[1].match(domain) ) throw this.invalid;
 
}
 
fields["firstname"].validate = function () {
 
    var first_name = $("first_name").value;
    if ( first_name.length == 0 ) throw this.required;
 
}
 
fields["lastname"].validate = function () {
 
    var last_name = $("last_name").value;
    if ( last_name.length == 0 ) throw this.required;
 
}
 
fields["zip"].validate = function () {
 
    var zip = $("zip").value;
    if ( zip.length == 0 ) throw this.required;
     
    var zip_pattern = /^[\d]{
5
}(-[\d]{
4
})?$/;
    if ( !zip.match(zip_pattern) ) throw this.invalid;
 
}

window.onload = function(){
	$("button").onclick = fields["zip"].validate;
}