<SCRIPT LANGUAGE="JavaScript1.1">
// A utility function that returns true if a string contains only 
// whitespace characters.
function isblank(s)
{
    for(var i =index.html 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 don't have an "optional" 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 =index.html 0; i < f.length; i++) {
        var e = f.elements[i];
        if (((e.type == "text") || (e.type == "textarea")) && !e.optional) {
            // first check if the field is empty
            if ((e.value == null) || (e.value == "") || isblank(e.value)) {
                empty_fields += "\n          " + e.name;
                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.name + " 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 false to prevent the form from being submitted. Otherwise
    // return true.
    if (!empty_fields && !errors) return true;
    msg  = "______________________________________________________\n\n"
    msg += "The form was not submitted because of the following error(s).\n";
    msg += "Please correct these error(s) and re-submit.\n";
    msg += "______________________________________________________\n\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;
}
</SCRIPT>
<!------------------------------------------------------------------------
     Here's a sample form to test our verification with. Note that we
     call verify() from the onSubmit() event handler, and return whatever
     value it returns. Also note that we use the onSubmit() handler as
     an opportunity to set properties on the form objects that verify()
     will use in the verification process. 
-------------------------------------------------------------------------->
<FORM onSubmit="
    this.firstname.optional =index.html true;
    this.phonenumber.optional = true;
    this.zip.min = 0;
    this.zip.max = 99999;
    return verify(this);
">
First name: <INPUT TYPE=text NAME="firstname">
Last name: <INPUT TYPE=text NAME="lastname"><BR>
Address:<BR><TEXTAREA NAME="address" ROWS=4 COLS=40></TEXTAREA><BR>
Zip Code: <INPUT TYPE=text NAME="zip"><BR>
Phone Number: <INPUT TYPE=text NAME="phonenumber"><BR>
<INPUT TYPE=submit>
</FORM>