Add `rel="noopener"` to all "A" tags for external links. See https://gist.github.com/gormus/575844 for an old, similar gist

.js

/**
 * Add rel="noopener" to all "A" tags for external links.
 *
 * Selection rules:
 *  - "A" tag must have "href" attribute, exclude anchors.
 *  - "href" attribute must not start with a "."
 *  - "href" attribute must not start with a "/"
 *  - "A" tag must not have rel="noopener" set.
 *  - "A" tag must not have rel="noreferrer" set.
 *
 * @todo: Rewrite in plain javascript.
 */

$('a[href]').not("[href^='.'], [href^='/'], [rel=noopener], [rel=noreferrer]").each(function () {
  var a = $(this),
      r = a[0].rel ? a[0].rel + ' ' : '';

  if (a[0].origin != window.location.origin) {
    a[0].rel = r + 'noopener';
  }
});