参考链接：https://blog.csdn.net/zhenyu5665/article/details/72829971

var fileName = "www.aaa.com/index.html";

方法1

var fileExtension = fileName.substring(fileName.lastIndexOf('.') + 1);


方法2

var fileExtension =fileName.substring(fileName.length-3,fileName.length).toLowerCase();

方法3

var fileExtension = fileName.split('.').pop().toLowerCase();