SOURCE

function add(a,b) {
  if(typeof a === 'number' && typeof b  === 'number') {
    	var getFix = function (z) {
      var _z = z.toString().match(/(\.(\d*))?$/)[2];
      return _z ? _z : 0;
    }
    var size = Math.max( getFix(a), getFix(b)).toString().length;
    var m = Math.pow(10,size);
    return (a*m + b*m)/m;
  }else {
    return NaN
  }
  
}


document.getElementById("main").innerHTML = `add(0.1, 0.2) ===> ${add(0.1,0.2)}`;

<div id="main"></div>