JSFiddle - React, Tailwind, and code Playground

HTML

<button id="btn"> Try Again </button>

JavaScript

function compress(str){

var cmpStr='',used=[];
str = str.split("");

for (index in str){
var x = str[index];
if ( x != ' ' && used.indexOf(x) == -1 ){
		var counter = 0;
		for (var i=0;i<=str.length;i++){
		if (str[i] == x){
			counter++;
			}
		}
		cmpStr+= x + counter;
		counter=0;
		used.push(x);
		}
	}
	return cmpStr;
}

function ask(){
var str = prompt("Enter String to Compress ..");
alert(compress(str));
}

btn.addEventListener("click",ask,false);

ask();