Preload any footer JS files in the header

functions.php

/**
 * Preload any footer JS files in the header
 *
 * originally from: https://macarthur.me/posts/preloading-javascript-in-wordpress
 */

add_action('wp_head', function () {

global $wp_scripts;

foreach($wp_scripts->queue as $handle) {
	$script = $wp_scripts->registered[$handle];

	//-- check if group exists
	if (isset($script->extra['group'])) {

		//-- Weird way to check if script is being enqueued in the footer.
		if( $script->extra['group'] == 1) {

			//-- If version is set, append to end of source.
			$source = $script->src . ($script->ver ? "?ver={$script->ver}" : "");

			//-- Spit out the tag.
			echo "<link rel='preload' href='{$source}' as='script'/>\n";
		}
	}
}
}, 1);