Check if Element Exists | CSS-Tricks

// in css:
/* Display line under clicked navbar link */
.active {
  text-decoration-line: underline !important;
  text-decoration-thickness: 2px !important;
  color: rgb(20, 19, 19);
}

//in html: 
    <script src="https://ajax.googleapis.com/ajax/libs/jquery/3.5.1/jquery.min.js"></script>

    <script>
      $(document).ready(function () {
        // Underline to remain in navbar after click using URL
        jQuery(function ($) {
          var path = window.location.href; // because the 'href' property of the DOM element is the absolute path
          $('.nav-link').each(function () {
            if (this.href === path) {
              $(this).addClass('active');
            }
          });
        });
      });
    </script>

//Note class in link should be nav-link

$.ajax({
  type: "POST",
  url: url,
  data: data,
  success: success,
  dataType: dataType
});

var allElements = $("*");

$("#slideshow > div:gt(0)").hide();

setInterval(function() { 
  $('#slideshow > div:first')
    .fadeOut(1000)
    .next()
    .fadeIn(1000)
    .end()
    .appendTo('#slideshow');
},  3000);

$(window).bind("pageshow", function() {
    var form = $('form'); 
    // let the browser natively reset defaults
    form[0].reset();
});

<script>
jQuery( document ).ready(function($){
	$(document).on('click','.elementor-location-popup a', function(event){
		elementorProFrontend.modules.popup.closePopup( {}, event);
	})
});
</script>

// THE HTML/PHP

// Categories Nav START
<? $terms = get_terms( array(
    'taxonomy' => 'category-name', // <-- update this
    'orderby' => 'ID',
  )); 
  if ( $terms && !is_wp_error( $terms ) ){ ?>
   <ul class="CHANGEME-categories-nav">
      <? foreach( $terms as $term ) { ?>
        <li id="cat-<?php echo $term->term_id; ?>">
           <a href="#" class="<?= $term->slug; ?> ajax" data-term-number="<?= $term->term_id; ?>" title="<?= $term->name;?>"><?= $term->name; ?></a>
        </li>
      <? } ?>
   </ul>
<? } ?>
// Categories Nav END
                                       
// Results Container START
<div id="CHANGEME-results-container" class="CHANGEME-filter-result">
   <? // post query
     $query = new WP_Query( array(
        'post_type' => 'post-name', // <-- update this
        'posts_per_page' => -1,
      ) ); 
   if( $query->have_posts() ): while( $query->have_posts()): $query->the_post(); ?>
    
      // POST TEMPLATE HERE
    
   <? endwhile; endif; wp_reset_query(); ?>
</div>                      
// Results Container END

// The onpage JS for the page template
<script>
(function($) {
        'use strict';
        function cat_ajax_get(catID) {
            jQuery.ajax({
                type: 'POST',
                url: raindrop_localize.ajaxurl,
                data: {"action": "filter", cat: catID },
                success: function(response) {
                    jQuery("#CHANGEME-results-container").html(response);
                    return false;
                }
            });
        }
        $( ".CHANGEME-categories-nav a.ajax" ).click(function(e) {
            e.preventDefault();
            $("a.ajax").removeClass("current");
            $(this).addClass("current"); //adds class current to the category menu item being displayed so you can style it with css
            var catnumber = $(this).attr('data-term-number');
            cat_ajax_get(catnumber);
        });

    })(jQuery);
</script>
                                       
// Callback function for functions.php or some other functions specific php file like theme.php
// Aside from the inital add actions and a few other things, the actual query and post template should be the same as what is on the page.
                                       
add_action( 'wp_ajax_nopriv_filter', 'CHANGEME_cat_posts' );
add_action( 'wp_ajax_filter', 'CHANGEME_cat_posts' );
                                       
function CHANGEME_cat_posts () {
    $cat_id = $_POST[ 'cat' ];
    $args = array (
	  'tax_query' => array(
		    array(
		      'taxonomy' => 'category-name', // <-- update this
		      'field' => 'term_id',
		      'terms' => array( $cat_id )
		    )
		  ),
	    'post_type' => 'post-name', // <-- update this
	    'posts_per_page' => -1,
	  );
	global $post;
    $posts = get_posts( $args );
    ob_start ();
    foreach ( $posts as $post ) { 
	    setup_postdata($post); ?>

	    // POST TEMPLATE HERE

   <?php } wp_reset_postdata();
   $response = ob_get_contents();
   ob_end_clean();
   echo $response;
   die(1);
}

<script> 
jQuery(document).ready(function($) { 
var delay = 100; setTimeout(function() { 
jQuery('.elementor-tab-title').removeClass('elementor-active');
jQuery('.elementor-tab-content').css('display', 'none'); }, delay); 
}); 
</script>

$('iframe').attr('src', $('iframe').attr('src'));

// Replace source
$('img').on("error", function() {
  $(this).attr('src', '/images/missing.png');
});

// Or, hide them
$("img").on("error", function() {
  $(this).hide();
});

jQuery.extend({

  getQueryParameters : function(str) {
	  return (str || document.location.search).replace(/(^\?)/,'').split("&").map(function(n){return n = n.split("="),this[n[0]] = n[1],this}.bind({}))[0];
  }

});

(function($) {
    
  var allPanels = $('.accordion > dd').hide();
    
  $('.accordion > dt > a').click(function() {
    allPanels.slideUp();
    $(this).parent().next().slideDown();
    return false;
  });

})(jQuery);

(function() {
    
    var mX, mY, distance,
        $distance = $('#distance span'),
        $element  = $('#element');

    function calculateDistance(elem, mouseX, mouseY) {
        return Math.floor(Math.sqrt(Math.pow(mouseX - (elem.offset().left+(elem.width()/2)), 2) + Math.pow(mouseY - (elem.offset().top+(elem.height()/2)), 2)));
    }

    $(document).mousemove(function(e) {  
        mX = e.pageX;
        mY = e.pageY;
        distance = calculateDistance($element, mX, mY);
        $distance.text(distance);         
    });

})();

<ul>
  <li>one</li>
  <li>two</li>
  <li>three</li>
</ul>