JSFiddle - React, Tailwind, and code Playground

HTML

<a href="url.html?abc=def">1</a>
<a href="url.html">2</a>

JavaScript

var querystring = 'myquerystringtoadd';

$('a').each(function()
{
 var href = $(this).attr('href');
 href += (href.match(/\?/) ? '&' : '?') + querystring;
 $(this).attr('href', href);
});