JSFiddle - React, Tailwind, and code Playground

HTML

<div id="foo" class="cfoo cbar"></div>
<div id="foobar" class="cbar"></div>

JavaScript

var idArr = [];
var classArr = [];
[].forEach.call(document.querySelectorAll("*"), function(element){
    if (element.id && idArr.indexOf(element.id) == -1) {
        idArr.push(element.id);
    }
    if (element.className) {
        var tempClassArr = element.className.split(" ");
        for (var i = 0; i < tempClassArr.length; i++) {
            if (classArr.indexOf(tempClassArr[i]) == -1) {
                classArr.push(tempClassArr[i]);
            }
        }
    }
});
console.log(idArr);
console.log(classArr);