var fields = {
    'name': 'Name',
    'email': 'E-mail address',
    'phone': 'Phone number',
    'enquiry': 'Enquiry'
};

var check_empty_fields = ['name', 'email', 'enquiry'];

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

function check_empty(id, name)
{
    if ($(id).value != '')
    {
        return true;
    }
    else
    {
        return 'Please fill field "' + name + '"';
    }
}

function check_mail(mail) 
{
    var check = /^[a-z][\w\-\.]*@[\w\-\.]+\.[a-z]{2,3}/i;
    if (check.test(mail)) 
    {
        return true;
    }
    else 
    {
        return false;
    }
}

function check()
{
    var i, res;

    for (i in check_empty_fields)
    {
        res = check_empty(check_empty_fields[i], fields[check_empty_fields[i]]);
        if (res !== true)
        {
            $(check_empty_fields[i]).focus();
            alert(res);
            return false;
        }
    }

    if (!check_mail($('email').value)) 
    {
        alert("The email address you provided is invalid! Please correct it.");
        $('email').focus();
        return false;
    }

    return true;
}