Consider the following DHTML:

 function isNotEmpty(elem) {
  var str = elem.value;
  if(str == null || str.length == 0)
   return false;
  else
   return true;
 }
 
 // THIS FUNCTION PERFORMS VALIDATION BASED ON A SET OF CUSTOM HTML ATTRIBUTES
 function validate(form) {
  var attrVal, attrReg, attrEq, attrFail, strTemp;

  for (var i = 0; i < form.length; i++) {
   attrVal = form[i].getAttribute("validate");
  
   switch (attrVal) {
    case 'required' :
    if (!isNotEmpty(form[i])) {
     attrFail = form[i].getAttribute("failure");
    
     if (attrFail)
      alert(attrFail);
     else    
      alert('You must complete all required form fields.');
     form[i].focus();
     return false;
    }
    break;
   
    case 'regex' :
    attrReg = form[i].getAttribute("regex");
    if (attrReg != null && attrReg.length != 0) {
     var regex = new RegExp(attrReg);
     strTemp = form[i].value;
     if (!strTemp.match(regex)) {
      attrFail = form[i].getAttribute("failure");

      if (attrFail)
       alert(attrFail);
      else
       alert('Invalid data format at field "' + form[i].name + '".');
      form[i].focus();
      return false;
     }
    }
    break;
   
    case 'equals' :
    attrEq = form[i].getAttribute("equals");
    var objEq = document.getElementById(attrEq);
    if (objEq) {
     if (form[i].value != objEq.value) {
      attrFail = form[i].getAttribute("failure");
     
      if (attrFail)
       alert(attrFail);
      else
       alert('Form fields do not match');
     
      form[i].focus();
      return false;
     }
    }
    break;
   }
  
  }
 
  return true;
 }

Examine the function above; the most notable part of the function is the switch statement that tests the value of the element's 'validate" attribute. The function supports required field validation, field equality validation, and regular expression validation. The isNotEmpty function is just a wrapper function to make sure a field is not empty. To enable the form to use validation, we will use the following code for the form's onsubmit handler:

<form onsubmit="return validate(this);" ></form>

The code in the validate function provides functionality to validate information in form fields. Using this DHTML, to make a form field required, we would use the following markup:

<input validate="required" />

Obviously you would have more attributes but this example shows how to use the DHTML above to make a form field required. The next page will discuss slightly more complex uses of the function.