JavaScript检测表单中checkbox被选中的数量

下面的JS代码可以统计表单中有多少个checkbox被选中，在问卷调查中经常会用到

<form name="form3" action="#">

<div>Who are your favorite browncoats?  Please select all that apply to you:

<input type="checkbox" name="Zoe" />Zoe 
<input type="checkbox" name="Mal" />Mal 
<input type="checkbox" name="Inara" />Inara
<input type="checkbox" name="Kaylee" /> Kaylee 
<input type="checkbox" name="River" />River 
<input type="checkbox" name="Simon" /> Simon
</div>
<input type="submit" name="NEXT" value="GO! " />
</form>
<script type="text/javascript" ><!--
//returns how many checkboxes in the form are checked
function howManyChecks(form) {
   var selections = 0;
	for (var i=0; i<form.length; i++) {
          var theElement = form.elements[i];
          if(theElement.type == "checkbox") {
              if(theElement.checked == true) {
                 selections++
              }
          }
      }
   return selections
}
function validate() {
   var num = howManyChecks(document.forms["form3"])
   if( num == 0) {
      alert("Please make a selection.");
   } else {
      alert("Number of favorite characters: "+num)
   }
   return false;
}
document.forms["form3"].elements["NEXT"].onclick=validate;
//-->
</script>