JSFiddle - React, Tailwind, and code Playground

HTML

<div class="test"></div>
<div class="netest"></div>
<div class="test"></div>
<div class="netest"></div>
<div class="test"></div>
<div class="netest"></div>
<div class="test"></div>
<div class="peretest"></div>

<pre id="result"></pre>

JavaScript

//document.getElementsByClassName('someClass').length

var elems = document.getElementsByTagName('div');
var arr = [];

for (var i = 0; i < elems.length; i++) {
  arr.push(elems[i].classList); 
}

for (var len = arr.length, i = len; --i >= 0;) {
  if (arr[arr[i]]) {
    arr[arr[i]] += 1;
    arr.splice(i, 1);
  } else {
    arr[arr[i]] = 1;
  }
}
arr.sort(function(a, b) {
  return arr[b] - arr[a];
});

console.log(arr);
var stringResult = JSON.stringify(arr, function(k, v) {
  if (k == '') return v;
  return `${v} - ${arr[v]}`;
}, 2);
document.getElementById('result').innerHTML = stringResult;
console.log(stringResult);
console.log(arr.map((el, i, a) => ({
  [el]: a[el]
})));