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// create custom selector for external links
$.expr[':'].external = function(obj){
	var $this = $(obj);
  return !$this.is('[href^="mailto:"]') && !$this.is('[href^="javascript:"]') && (obj.hostname != location.hostname) && ($this.data('target') != "top");
};

// DOM is ready
$(function(){

  // apply target="_blank" to each external link on the page

rjv

document.querySelectorAll('[target=_blank]').forEach(function(target) {
  target.setAttribute("rel", "noopener noreferrer");
});

rjv

<?php

function get_thumbnail_url($post_id = null, $size = 'medium')
{
    global $post;

    if (is_string($post_id)) {
        $size    = $post_id;
        $post_id = $post->ID;
    }