用js截取字符串

截取字符串abcdefg的efg
abcdefg
    <script type="text/javascript">
  var mytext=document.getElementByIdx_x_x("text");
  var myvalue=mytext.innerHTML;
  var jiequ=myvalue.substring(myvalue.length-3,myvalue.length);
  alert(jiequ)
</script>