JSFiddle - React, Tailwind, and code Playground

HTML

<!DOCTYPE html>
<html>
    <body>
        <input type="text" id="string" />
        <input type="submit" id="calc" value="Get size" />
        <strong id="size"></strong>
    </body>
</html>

JavaScript

$('calc').addEvent('click', function(e) {
    e.preventDefault();
    
    var tmp = new Element('span');
    tmp.set('html', $('string').get('value'));
    
    var length = tmp.get('text').length;
    
    console.log(length);
    
    $('size').set('text', length);
});