JSFiddle - React, Tailwind, and code Playground

HTML

<body>
    <br>x:
    <input type="text" id="txt1" name="text1">
    <br>y:
    <input type="text" id="txt2" name="text2">
    <br>    
    <button onclick="add()">Add</button>
    <p id="demo"></p>
    <script>
      function add() {
      document.getElementById("demo").innerHTML = "";
        var x = document.getElementById("txt1").value;
        var y = document.getElementById("txt2").value;
        var len;
        var lenx = x.length;
				var leny = y.length;
        var x1,y1,rem,div=0;
				if(lenx>leny) len = lenx; else len = leny;
    
        for(var i=0;i<len;i++){
        	if(i>=lenx) x1  = 0;
          else x1 = parseInt(x[lenx-i-1]);
          if(i>=leny) y1 = 0;
          else y1 = parseInt(y[leny-i-1]);
          rem = (x1+y1+div)%10;
					div = Math.floor((x1 + y1+div)/10);
        	document.getElementById("demo").innerHTML = rem + document.getElementById("demo").innerHTML;
        }
        if(div>0){
            document.getElementById("demo").innerHTML = div + document.getElementById("demo").innerHTML;
       }
      }
    </script>
  </body>