从字符串中删除最后一个逗号

使用JavaScript，如何删除最后一个逗号，但前提是逗号是最后一个字符，或者逗号后面只有空格?这是我的代码. 我有一个正在工作的小提琴.但是它有一个错误.

Using JavaScript, how can I remove the last comma, but only if the comma is the last character or if there is only white space after the comma? This is my code. I got a working fiddle. But it has a bug.

var str = 'This, is a test.'; 
alert( removeLastComma(str) ); // should remain unchanged

var str = 'This, is a test,'; 
alert( removeLastComma(str) ); // should remove the last comma

var str = 'This is a test,          '; 
alert( removeLastComma(str) ); // should remove the last comma

function removeLastComma(strng){        
    var n=strng.lastIndexOf(",");
    var a=strng.substring(0,n) 
    return a;
}