JSFiddle - React, Tailwind, and code Playground

JavaScript

var mystr = '<div class="class1 class2"><p id="id1">some <span class="class3">text about class="class4"</span></p></div>';
var tags = mystr.split("class=\"");
var classes = new Array();
var endclass;
for (var i=0; i<tags.length; i++) {
  endclass = tags[i].indexOf("\">");
  if (endclass > -1) {
    var newclass = tags[i].substring(0,endclass).split(" ");
    classes = classes.concat(newclass);
  }
}

for (i=0; i<classes.length; i++) {
  alert(classes[i]);
}