var url = 'https://xzdx.xyz/';

var cut = url.substring(url.lastIndexOf(".")+1);

console.log(cut);

打印出来的是 xyz

lastIndexOf 用法：https://www.runoob.com/jsref/jsref-lastindexof-array.html

substring 用法：https://www.runoob.com/jsref/jsref-substring.html