JSFiddle - React, Tailwind, and code Playground

by Torwan

HTML

<ul class="categories">
   <li data-category="category-1">Category 1</li>
   <li data-category="category-1">Category 1</li>
   <li data-category="category-1">Category 1</li>
   <li data-category="category-1">Category 1</li>
   <li data-category="category-1">Category 1</li>
   <li data-category="category-2">Category 2</li>
   <li data-category="category-2">Category 2</li>
   <li data-category="category-2">Category 2</li>
   <li data-category="category-3">Category 3</li>
    <li>Category 4</li>
</ul>
<div id="result"></div>

JavaScript

var categories = {},
    category;
$('.categories li[data-category]').each(function(i, el){
    category = $(el).data('category');
    if (categories.hasOwnProperty(category)) {
        categories[category] += 1;
    }
    else {
        categories[category] = 1;
    }
});

// print results
$('#result').append('<hr>');
for(var key in categories){
    $('#result').append(key + ' (' + categories[key] + ')<br>');
}