JSFiddle - React, Tailwind, and code Playground

HTML

<div id="nav-show"> <a class="m-btn rnd black mobile-hide current_page_item" href="">Home</a>
 <a class="m-btn rnd black mobile-hide" href="">About</a>
 <a class="m-btn rnd black mobile-hide" href="">Projects</a>
 <a class="m-btn rnd black mobile-hide" href="">Quality</a>
 <a class="m-btn rnd black mobile-hide" href="">Products</a>
 <a class="m-btn rnd black mobile-hide" href="">Contact Us</a>
 <a class="m-btn rnd black mobile-hide" href="">Links</a>


</div>

CSS

#magic-line {
    position: absolute;
    top: 2px;
    left: 0;
    width: 10px;
    height: 12px;
    background: #fe4902;
}

JavaScript

$(function() {

    var $el;
    var leftPos;
    var newWidth;
    var $mainNav = $("#nav-show");
    
    $mainNav.append("<span id='magic-line'></span>");
    var $magicLine = $("#magic-line");
    
    $magicLine
        .width($(".current_page_item").width())
        .css("left", $("a.current_page_item").position().left)
        .data("origLeft", $magicLine.position().left)
        .data("origWidth", $magicLine.width());
        
    $("#nav-show a").hover(function() {
        $el = $(this);
        leftPos = $el.position().left;
        newWidth = $el.width();
        $magicLine.stop().animate({
            left: leftPos,
            width: newWidth
        });
    }, function() {
        $magicLine.stop().animate({
            left: $magicLine.data("origLeft") + "px",
            width: $magicLine.data("origWidth") + "px"
        });    
    });
});