与slice()和substr()方法不同的是，substring() 不接受负的参数。

<script type="text/javascript" >
  var str = "19990929"
  console.log(str.substr(3,4))   //90929
  var str = "19990929"
  console.log(str.substring(3, 4))   //90929
  var str = "19990929"
  console.log(str.substr(-3, 2))   //92
< /script>