jquery - Add value in span using JavaScript -

i want add 500 using onclick function in span 1000 means onclick @ check box sapn value 1500

js

function add() {     if (document.form1.checkbox1.checked == true) {         // code here sum 500  in 1000        } } 

html

<form name="form1" style="color: green ; font-size: 150%" action="#">     <input type="checkbox" name="checkbox1" onclick="add()" />500     <br />     <span>1000         </span>             </form>                 </body> 

if don't want use jquery, the following can used.

function add(element) {    var form = document.getelementsbyname("form1")[0];    var val = form.getelementsbytagname('span')[0].innerhtml;    if (element.checked == true) {      form.getelementsbytagname('span')[0].innerhtml = parseint(val) + parseint(element.value);    } else {      form.getelementsbytagname('span')[0].innerhtml = parseint(val) - parseint(element.value);    }  }

<form name="form1" style="color: green ; font-size: 150%" action="#">    <input type="checkbox" name="checkbox1" onclick="add(this)" value="500" />500    <br />    <span>1000</span>  </form>