// This example is from the book _JavaScript: The Definitive Guide_.
// Written by David Flanagan.  Copyright (c) 1996 O'Reilly & Associates.
// This example is provided WITHOUT WARRANTY either expressed or implied.
// You may study, use, modify, and distribute it for any purpose.

// A utility function that returns true if a string contains only 
// whitespace characters.
function isblank(s)
{
    for(var i = 0; i < s.length; i++) {
        var c = s.charAt(i);
        if ((c != ' ') && (c != '\n') && (c != '\t')) return false;
    }
    return true;
}

// This is the function that performs form verification.  It will be
// invoked from the onSubmit() event handler.  The handler should return
// whatever value this function returns.
function verify(f)
{
    var msg;
    var empty_fields = "";
    var errors = "";
    // Loop through the elements of the form, looking for all text and
    // textarea elements that have a "required" property defined.
    // Then, check for fields that are empty and make a list of them.
    // Also, if any of these elements have a "min" or a "max" property
    // defined, then verify that they are numbers and that they are in the
    // right range. Put together error messages for fields that are wrong.
    for(var i = 0; i < f.length; i++) {
        var e = f.elements[i];
        if (((e.type == "text") || (e.type == "textarea"))
            && e.required) {
            // first check if the field is empty
            if ((e.value == null) || (e.value == "") 
                || isblank(e.value)) {
                empty_fields += "\n          " + e.label;
                continue;
            }

            // Now check for fields that are supposed to be numeric.
            if (e.numeric || (e.min != null) || (e.max != null)) { 
                var v = parseFloat(e.value);
                if (isNaN(v) || 
                    ((e.min != null) && (v < e.min)) || 
                    ((e.max != null) && (v > e.max))) {
                    errors += "The field " + e.label +
                              " must be a number";
                    if (e.min != null) 
                        errors += " that is greater than " + e.min;
                    if (e.max != null && e.min != null) 
                        errors += " and less than " + e.max;
                    else if (e.max != null)
                        errors += " that is less than " + e.max;
                    errors += ".\n";
                }
            }
        }
    }

    // Now, if there were any errors, then display the messages, and
    // return true to prevent the form from being submitted.  Otherwise
    // return false
    if (!empty_fields && !errors) return true;
    msg  = "The form was not submitted because of these error(s).\n";
    msg += "Please correct these error(s) and re-submit.\n";

    if (empty_fields) {
        msg += "The following required field(s) are empty:" 
                + empty_fields + "\n";
        if (errors) msg += "\n";
    }
    msg += errors;
    alert(msg);
    return false;
}