// A generic function for checking to see if an input element
// looks like an email address
function checkEmail( elem ) {
    // Make sure that something was entered and that it looks like
    // a valid email address
    return !elem.value || /^[a-z0-9_+.-]+\@([a-z0-9-]+\.)+[a-z0-9]{2,4}$/i.test( elem.value );
}

// Get an input element to check
var elem = document.getElementById("email");

// Check to see if the field is valid, or not
if ( ! checkEmail( elem ) ) {
    alert( "Field is not an email address." );
}