Tiny WordPress plugin for how to force external links to open in a new tab.

external-links-new-tab.js

/* global externalLinksNewTab */

jQuery( document.body ).on( 'click', 'a', function( event ) {
	if ( ! this.href ) {
		return;
	}

	// If a target is already specified, let that win.
	if ( this.target ) {
		return;
	}

	let internalHosts = [
		window.location.host.toLowerCase(),
		externalLinksNewTab.siteUrlDomain.toLowerCase()
	];
	let hrefHost = new URL( this.href, window.location ).hostname.toLowerCase();

	if ( -1 === internalHosts.findIndex( host => host === hrefHost ) ) {
		// External!
		event.preventDefault();
		event.stopPropagation();
		window.open( this.href, '_blank' );
		return false;
	}
} );

external-links-new-tab.php

<?php

/**
 * Plugin name: Open External Links in a New Tab!
 * Author: George Stephanis
 * Author URI: https://georgestephanis.wordpress.com
 * Version: 1.0
 * License: GPLv2+
 * License URI: http://www.gnu.org/licenses/gpl-2.0.txt
 */

add_action(
	'wp_enqueue_scripts',
	function() {
		wp_enqueue_script(
			'external-links-new-tab',
			plugins_url( 'external-links-new-tab.js', __FILE__ ),
			array(
				'jquery',
			),
			filemtime( __DIR__ . '/external-links-new-tab.js' ), // version
			true // in footer
		);

		wp_localize_script(
			'external-links-new-tab',
			'externalLinksNewTab',
			array(
				'siteUrlDomain' => wp_parse_url( site_url(), PHP_URL_HOST ),
			)
		);
	}
);