JSFiddle - React, Tailwind, and code Playground

HTML

<input type=text id=elem>

JavaScript

document.getElementById("elem").onblur =function (){
        var val = this.value.replace(/\,/g,"").split("").reverse();
        if( val.length <= 3) return;
         var newval = [];
        for(var i = 0; i< val.length; i++){
            if( i == 3 || (i > 3 && (i - 3)%2 == 0 ) ){
                newval.push( ","); 
            }
            
            newval.push( val[i]);       
        }
    this.value = newval.reverse().join("");
    };