JSFiddle - React, Tailwind, and code Playground

HTML

<ul id="nav_main">
    <li>
        <a href="index.php?pag=THIS">THIS</a>
    </li>
    <li>
        <a href="index.php?pag=whatever">whatever</a>
    </li>
    <li>
        <a href="index.php?pag=blah">blah</a>
    </li>
</ul>

CSS

.current
{
    background-color:green;
}

JavaScript

var foo = 'whatever';
$("#nav_main li a").filter(function(index){
    return foo === this.href.match(/pag=(.*)/)[1];
}).parent().addClass("current");