Source

File: wp-includes/blocks.php

function serialize_block( $block ) {
	$block_content = '';

	$index = 0;
	foreach ( $block['innerContent'] as $chunk ) {
		$block_content .= is_string( $chunk ) ? $chunk : serialize_block( $block['innerBlocks'][ $index++ ] );
	}

	if ( ! is_array( $block['attrs'] ) ) {
		$block['attrs'] = array();
	}

	return get_comment_delimited_block_content(
		$block['blockName'],
		$block['attrs'],
		$block_content
	);
}