JavaScript判断用户是否对表单进行了修改

这段JS代码可以判断出用户是否对表单内容进行了修改，如果修改了表单，并退出浏览器，则会提醒用户是否要保存表单的内容，非常有用的代码，75271.com提供。

/**
 * Determines if a form is dirty by comparing the current value of each element
 * with its default value.
 * http://www.75271.com
 * @param {Form} form the form to be checked.
 * @return {Boolean} <code>true</code> if the form is dirty, <code>false</code>
 *                   otherwise.
 */
function formIsDirty(form) {
  for (var i = 0; i < form.elements.length; i++) {
    var element = form.elements[i];
    var type = element.type;
    if (type == "checkbox" || type == "radio") {
      if (element.checked != element.defaultChecked) {
        return true;
      }
    }
    else if (type == "hidden" || type == "password" ||
             type == "text" || type == "textarea") {
      if (element.value != element.defaultValue) {
        return true;
      }
    }
    else if (type == "select-one" || type == "select-multiple") {
      for (var j = 0; j < element.options.length; j++) {
        if (element.options[j].selected !=
            element.options[j].defaultSelected) {
          return true;
        }
      }
    }
  }
  return false;
}

使用示例：当退出浏览器是，如果用户修改了表单，则提醒用户是否要保存

window.onbeforeunload = function(e) {
  e = e || window.event;  
  if (formIsDirty(document.forms["someForm"])) {
    // For IE and Firefox
    if (e) {
      e.returnValue = "You have unsaved changes.";
    }
    // For Safari
    return "You have unsaved changes.";
  }
};

下面是一个完整的范例代码

Click on below button. Now change some values in form and click the button again.
<form name="fooForm">
    <input type="text" name="t"><br>
    <input type="text" name="2" value="default"><br>
    <select name="some">
        <option value="fooo" selected="">foo</option>
        <option value="bar">bar</option>
    </select><br>
</form>
    <button onclick="alert(formIsDirty(document.fooForm))">Click to check if Form is Dirty</button>
<br>
<script>
// http://www.75271.com
function formIsDirty(form) {
  for (var i = 0; i < form.elements.length; i++) {
    var element = form.elements[i];
    var type = element.type;
    if (type == "checkbox" || type == "radio") {
      if (element.checked != element.defaultChecked) {
        return true;
      }
    }
    else if (type == "hidden" || type == "password" ||
             type == "text" || type == "textarea") {
      if (element.value != element.defaultValue) {
        return true;
      }
    }
    else if (type == "select-one" || type == "select-multiple") {
      for (var j = 0; j < element.options.length; j++) {
        if (element.options[j].selected !=
            element.options[j].defaultSelected) {

          return true;
        }
      }
    }
  }
  return false;
}
</script>