Class that allows you to async and defer scripts in WordPress by adding data.

defer-async-scripts.php

<?php

class WP_Scholar_Defer_Scripts {

	public static function initialize() {
		add_filter( 'script_loader_tag', array( __CLASS__, 'defer_scripts' ), 10, 2 );
		add_filter( 'script_loader_tag', array( __CLASS__, 'async_scripts' ), 10, 2 );
	}

	public static function defer_scripts( $tag, $handle ) {
		if ( wp_scripts()->get_data( $handle, 'defer' ) ) {
			$tag = str_replace( '></', ' defer></', $tag );
		}

		return $tag;
	}

	public static function async_scripts( $tag, $handle ) {
		if ( wp_scripts()->get_data( $handle, 'async' ) ) {
			$tag = str_replace( '></', ' async></', $tag );
		}

		return $tag;
	}

}

usage.php

<?php

add_action( 'login_enqueue_scripts', function () {
	global $wp_scripts;
	wp_enqueue_script( 'recaptcha', 'https://www.google.com/recaptcha/api.js' );
	$wp_scripts->add_data( 'recaptcha', 'async', true );
	$wp_scripts->add_data( 'recaptcha', 'defer', true );
} );

Very cool Micah, we could probably be using wordpress wrapper functions instead of accessing the global directly. What do you think?

defer-async-scripts.php

class WP_Scholar_Defer_Scripts {
    public static function initialize() {
        add_filter( 'script_loader_tag', array( __CLASS__, 'defer_scripts' ), 10, 2 );
        add_filter( 'script_loader_tag', array( __CLASS__, 'async_scripts' ), 10, 2 );
    }
    public static function defer_scripts( $tag, $handle ) {
        if ( wp_scripts()->get_data( $handle, 'defer' ) ) {
            $tag = str_replace( '></', ' defer></', $tag );
        }
        return $tag;
    }
    public static function async_scripts( $tag, $handle ) {
        if ( wp_scripts()->get_data( $handle, 'async' ) ) {
            $tag = str_replace( '></', ' async></', $tag );
        }
        return $tag;
    }
}

usage.php

add_action( 'login_enqueue_scripts', function () {
    wp_enqueue_script( 'recaptcha', 'https://www.google.com/recaptcha/api.js' );
    wp_script_add_data( 'recaptcha', 'async', true );
    wp_script_add_data( 'recaptcha', 'defer', true );
} );

Yes, indeed. I've updated the code. Thanks!

This is now a full-fledged Composer friendly library: https://github.com/wpscholar/wp-async-defer-scripts