Gutenberg - Test case custom block for passing content to render callback

block.js

const el = wp.element.createElement,
	registerBlockType = wp.blocks.registerBlockType,
	InnerBlocks = wp.blocks.InnerBlocks;

registerBlockType( 'gutenberg-boilerplate-es5/hello-world-step-01', {
	title: 'Hello World (Step 1)',

	icon: 'universal-access-alt',

	category: 'layout',

	attributes: {
		foo: {
			type: 'string',
		}
	},

	edit: function( { attributes, setAttributes } ) {
		return [
			el( 'input', {
				value: attributes.foo || '',
				onChange: e => setAttributes( { foo: e.target.value } ),
				key: 1,
			} ),
			el( InnerBlocks, { key: 2 } ),
		];
	},

	save: function( { attributes } ) {
		console.log( InnerBlocks.Content );

		return el( 'div', {}, [
			el( 'p', {}, attributes.foo, { key: 1 } ),
			el( InnerBlocks.Content, { key: 2 } ),
		]);
	},
} );

plugin.php

<?php

function gutenberg_boilerplate_block() {
	wp_register_script(
		'gutenberg-boilerplate-es5-step01',
		plugins_url( 'block.js', __FILE__ ),
		array( 'wp-blocks', 'wp-element' )
	);

	register_block_type( 'gutenberg-boilerplate-es5/hello-world-step-01', array(
		'editor_script' => 'gutenberg-boilerplate-es5-step01',
		'render_callback' => 'my_plugin_render_block_latest_post',
	) );
}
add_action( 'init', 'gutenberg_boilerplate_block' );


function my_plugin_render_block_latest_post( $attributes, $content = '' ) {
	// var_dump( $content );
	return '<p>' . $attributes['foo'] . '</p>' . '<pre><code>' . htmlspecialchars( $content ) . '</code></pre>';
}