JSFiddle - React, Tailwind, and code Playground

HTML

<script>
function formatReal( int )
{
        var tmp = int+'';
        var neg = false;
        if(tmp.indexOf("-") == 0)
        {
            neg = true;
            tmp = tmp.replace("-","");
        }
        
        if(tmp.length == 1) tmp = "0"+tmp
    
        tmp = tmp.replace(/([0-9]{2})$/g, ",$1");
        if( tmp.length > 6)
            tmp = tmp.replace(/([0-9]{3}),([0-9]{2}$)/g, ".$1,$2");
        
        if( tmp.length > 9)
            tmp = tmp.replace(/([0-9]{3}).([0-9]{3}),([0-9]{2}$)/g,".$1.$2,$3");
    
        if( tmp.length > 12)
            tmp = tmp.replace(/([0-9]{3}).([0-9]{3}).([0-9]{3}),([0-9]{2}$)/g,".$1.$2.$3,$4");
        
        if(tmp.indexOf(".") == 0) tmp = tmp.replace(".","");
        if(tmp.indexOf(",") == 0) tmp = tmp.replace(",","0,");
    
    document.getElementById('local').innerHTML = neg ? '-'+tmp : tmp;
}
</script>

<input type="text" name="teste" id="teste" onkeyup="return formatReal(this.value)" /><Br>
Valor Transformado: <div id="local"></div>
</div>