JSFiddle - React, Tailwind, and code Playground

JavaScript

var str="hello all, this is derp. thank you all to answer my query.";
function sentenceCase(strval){
    
 var newstrs = strval.split(".");
    var finalstr="";
    //alert(strval);
    for(var i=0;i<newstrs.length;i++)
    	if(i == 0)
      	finalstr=finalstr+"."+ newstrs[i].substr(0,1).toUpperCase()+newstrs[i].substr(1);
        else
        finalstr=finalstr+"."+ newstrs[i].substr(0,2).toUpperCase()+newstrs[i].substr(2);
    return finalstr.substr(1);
}

alert(sentenceCase(str));