trepmal/debug-rest-api.php

## debug-rest-api.php
<?php
/**
 * Plugin Name: Debug Bar for REST API
 * Description: Enable Debug Bar for REST API endpoints (add ?debug). Not for production.
 * Author: Kailey "trepmal" Lampert
 */

namespace trepmal\rest_debug;

/**
 * load plugins, template if ?debug
 * disable theme-y css/js, known cruft
 */
function rest_api_init( $wp_rest_server ) {
	if ( ! is_rest_debug() ) {
		return;
	}

	// probably a bad shortcut, helps with authentication-required endpoints
	// as well as Debug Bar Console
	wp_signon();

	do_action( 'template_redirect' );

	foreach( [ 'style_loader_src', 'script_loader_src' ] as $hook ) {
		add_filter( $hook, function( $src, $handle ) {
			if ( false !== strpos( $src, '/themes/' ) ) {
				return false;
			}
			return $src;
		}, 10, 2 );
	}
	remove_action( 'wp_head', 'print_emoji_detection_script', 7 );
	remove_action( 'wp_print_styles', 'print_emoji_styles' );

}
add_action( 'rest_api_init', __NAMESPACE__ . '\rest_api_init' );

/**
 * output json in html
 */
function rest_pre_echo_response( $result, $wp_rest_server, $request ) {
	if ( ! is_rest_debug() ) {
		return $result;
	}

	$result = wp_json_encode( $result, JSON_PRETTY_PRINT );
	header('Content-type: text/html');

?>
<!doctype html>
<html <?php language_attributes(); ?>>
<head>
	<meta charset="<?php bloginfo( 'charset' ); ?>">
	<?php wp_head(); ?>
</head>
<body>
	<pre><?php echo esc_html( $result ); ?></pre>
	<?php wp_footer(); ?>
</body>
</html>
<?php
	die();
}
add_filter( 'rest_pre_echo_response', __NAMESPACE__ . '\rest_pre_echo_response', 10, 3 );


/**
 * helper for debugging arg
 */
function is_rest_debug() {
	return isset( $_GET['debug'] );
}
	<?php
	/**
	* Plugin Name: Debug Bar for REST API
	* Description: Enable Debug Bar for REST API endpoints (add ?debug). Not for production.
	* Author: Kailey "trepmal" Lampert
	*/

	namespace trepmal\rest_debug;

	/**
	* load plugins, template if ?debug
	* disable theme-y css/js, known cruft
	*/
	function rest_api_init( $wp_rest_server ) {
	if ( ! is_rest_debug() ) {
	return;
	}

	// probably a bad shortcut, helps with authentication-required endpoints
	// as well as Debug Bar Console
	wp_signon();

	do_action( 'template_redirect' );

	foreach( [ 'style_loader_src', 'script_loader_src' ] as $hook ) {
	add_filter( $hook, function( $src, $handle ) {
	if ( false !== strpos( $src, '/themes/' ) ) {
	return false;
	}
	return $src;
	}, 10, 2 );
	}
	remove_action( 'wp_head', 'print_emoji_detection_script', 7 );
	remove_action( 'wp_print_styles', 'print_emoji_styles' );

	}
	add_action( 'rest_api_init', __NAMESPACE__ . '\rest_api_init' );

	/**
	* output json in html
	*/
	function rest_pre_echo_response( $result, $wp_rest_server, $request ) {
	if ( ! is_rest_debug() ) {
	return $result;
	}

	$result = wp_json_encode( $result, JSON_PRETTY_PRINT );
	header('Content-type: text/html');

	?>
	<!doctype html>
	<html <?php language_attributes(); ?>>
	<head>
	<meta charset="<?php bloginfo( 'charset' ); ?>">
	<?php wp_head(); ?>
	</head>
	<body>
	<pre><?php echo esc_html( $result ); ?></pre>
	<?php wp_footer(); ?>
	</body>
	</html>
	<?php
	die();
	}
	add_filter( 'rest_pre_echo_response', __NAMESPACE__ . '\rest_pre_echo_response', 10, 3 );


	/**
	* helper for debugging arg
	*/
	function is_rest_debug() {
	return isset( $_GET['debug'] );
	}