jQuery addClass example
Change class name on click in jQuery
by 20yco
HTML
<i class='fab fa-tumblr tumblrbc tumblr-post-share' onclick='javascript:window.open(\"https://www.tumblr.com/widgets/share/tool?posttype=photo&canonicalUrl=https://www.example.com/video/434&content=https://www.example.com/video/434&title=title\", tumblr-share-dialog, \"width=540,height=600\");'><p>Tumblr</p></i>
JavaScript
document.addEventListener('DOMContentLoaded', document.querySelector('.tumblr-post-share').classList.add('tumblr-share-button'));