[javascript] Remove Last Comma from a string

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;
}

Remove last comma. Working example

function truncateText() {_x000D_
  var str= document.getElementById('input').value;_x000D_
  str = str.replace(/,\s*$/, "");_x000D_
  console.log(str);_x000D_
}

<input id="input" value="address line one,"/>_x000D_
<button onclick="truncateText()">Truncate</button>

First, one should check if the last character is a comma. If it exists, remove it.

if (str.indexOf(',', this.length - ','.length) !== -1) {
    str = str.substring(0, str.length - 1);
}

NOTE str.indexOf(',', this.length - ','.length) can be simplified to str.indexOf(',', this.length - 1)

you can remove last comma from a string by using slice() method, find the below example:

var strVal = $.trim($('.txtValue').val());
var lastChar = strVal.slice(-1);
if (lastChar == ',') {
    strVal = strVal.slice(0, -1);
}

Here is an Example

function myFunction() {_x000D_
 var strVal = $.trim($('.txtValue').text());_x000D_
 var lastChar = strVal.slice(-1);_x000D_
 if (lastChar == ',') { // check last character is string_x000D_
  strVal = strVal.slice(0, -1); // trim last character_x000D_
  $("#demo").text(strVal);_x000D_
 }_x000D_
}

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>_x000D_
_x000D_
_x000D_
<p class="txtValue">Striing with Commma,</p>_x000D_
_x000D_
<button onclick="myFunction()">Try it</button>_x000D_
_x000D_
<p id="demo"></p>

long shot here

var sentence="I got,. commas, here,";
var pattern=/,/g;
var currentIndex;
while (pattern.test(sentence)==true)  {    
  currentIndex=pattern.lastIndex;
 }
if(currentIndex==sentence.trim().length)
alert(sentence.substring(0,currentIndex-1));
else
 alert(sentence);

function removeLastComma(str) {
   return str.replace(/,(\s+)?$/, '');   
}

The greatly upvoted answer removes not only the final comma, but also any spaces that follow. But removing those following spaces was not what was part of the original problem. So:

let str = 'abc,def,ghi, ';
let str2 = str.replace(/,(?=\s*$)/, '');
alert("'" + str2 + "'");
'abc,def,ghi '

https://jsfiddle.net/dc8moa3k/

In case its useful or a better way:

str = str.replace(/(\s*,?\s*)*$/, "");

It will replace all following combination end of the string:

1. ,<no space>
2. ,<spaces> 
3. ,  ,  , ,   ,
4. <spaces>
5. <spaces>,
6. <spaces>,<spaces>

you can remove last comma:

var sentence = "I got,. commas, here,";
sentence = sentence.replace(/(.+),$/, '$1');
console.log(sentence);

The problem is that you remove the last comma in the string, not the comma if it's the last thing in the string. So you should put an if to check if the last char is ',' and change it if it is.

EDIT: Is it really that confusing?

'This, is a random string'

Your code finds the last comma from the string and stores only 'This, ' because, the last comma is after 'This' not at the end of the string.