JSFiddle - React, Tailwind, and code Playground

HTML

<input type = "text" id = "ck" value = "1000.123456789"><button onclick = "format()">
Click
</button>
<br />
<div id = "result">

</div>

JavaScript

function format(){
	document.getElementById("result").innerHTML = formatNumberWithCommasDec(document.getElementById("ck").value);
}
  
 function formatNumberWithCommasDec(d) {
      d += "";
      var c = d.split(".");
      var f = c[1];
      var a = c.length > 1 ? c[0] + '.' : '.', flag = false;
      var g = f.split('').reverse(), y = 1, s = '';
      for (var i = g.length - 1; i >= 0; i--) {
      	flag = false;
    		var e = g[i];
        var h = (y === 3) ? s = s + e + ',' : s = s + e;
        console.log(e);
        if(y === 3){
        	y = 1;
          flag = true;
        } else {
        	y = y + 1;
        }
			}
     if(flag){
     		s = s.substring(0, s.length - 1);
     } else {
     		s = s;
     }
     return a + s;     
  }