JSFiddle - React, Tailwind, and code Playground
HTML
<a href="http://twitter.com/share" class="twitter-share-button" data-text="">Tweet</a><script type="text/javascript" src="http://platform.twitter.com/widgets.js"></script>
<div id="populateText">
<p>Content from here to populate "data-text"</p>
</div>
JavaScript
$(".twitter-share-button").attr("data-text", $("#populateText>p").text());
alert($(".twitter-share-button").attr("data-text"));