wp_ajax_update_theme() WordPress Function

function wp_ajax_update_theme() {
	check_ajax_referer( 'updates' );

	if ( empty( $_POST['slug'] ) ) {
		wp_send_json_error(
			array(
				'slug'         => '',
				'errorCode'    => 'no_theme_specified',
				'errorMessage' => __( 'No theme specified.' ),
			)
		);
	}

	$stylesheet = preg_replace( '/[^A-z0-9_\-]/', '', wp_unslash( $_POST['slug'] ) );
	$status     = array(
		'update'     => 'theme',
		'slug'       => $stylesheet,
		'oldVersion' => '',
		'newVersion' => '',
	);

	if ( ! current_user_can( 'update_themes' ) ) {
		$status['errorMessage'] = __( 'Sorry, you are not allowed to update themes for this site.' );
		wp_send_json_error( $status );
	}

	$theme = wp_get_theme( $stylesheet );
	if ( $theme->exists() ) {
		$status['oldVersion'] = $theme->get( 'Version' );
	}

	require_once ABSPATH . 'wp-admin/includes/class-wp-upgrader.php';

	$current = get_site_transient( 'update_themes' );
	if ( empty( $current ) ) {
		wp_update_themes();
	}

	$skin     = new WP_Ajax_Upgrader_Skin();
	$upgrader = new Theme_Upgrader( $skin );
	$result   = $upgrader->bulk_upgrade( array( $stylesheet ) );

	if ( defined( 'WP_DEBUG' ) && WP_DEBUG ) {
		$status['debug'] = $skin->get_upgrade_messages();
	}

	if ( is_wp_error( $skin->result ) ) {
		$status['errorCode']    = $skin->result->get_error_code();
		$status['errorMessage'] = $skin->result->get_error_message();
		wp_send_json_error( $status );
	} elseif ( $skin->get_errors()->has_errors() ) {
		$status['errorMessage'] = $skin->get_error_messages();
		wp_send_json_error( $status );
	} elseif ( is_array( $result ) && ! empty( $result[ $stylesheet ] ) ) {

		// Theme is already at the latest version.
		if ( true === $result[ $stylesheet ] ) {
			$status['errorMessage'] = $upgrader->strings['up_to_date'];
			wp_send_json_error( $status );
		}

		$theme = wp_get_theme( $stylesheet );
		if ( $theme->exists() ) {
			$status['newVersion'] = $theme->get( 'Version' );
		}

		wp_send_json_success( $status );
	} elseif ( false === $result ) {
		global $wp_filesystem;

		$status['errorCode']    = 'unable_to_connect_to_filesystem';
		$status['errorMessage'] = __( 'Unable to connect to the filesystem. Please confirm your credentials.' );

		// Pass through the error from WP_Filesystem if one was raised.
		if ( $wp_filesystem instanceof WP_Filesystem_Base && is_wp_error( $wp_filesystem->errors ) && $wp_filesystem->errors->has_errors() ) {
			$status['errorMessage'] = esc_html( $wp_filesystem->errors->get_error_message() );
		}

		wp_send_json_error( $status );
	}

	// An unhandled error occurred.
	$status['errorMessage'] = __( 'Theme update failed.' );
	wp_send_json_error( $status );
}