Inserter

Inserter allows you to create custom templates from your dashboard and insert the in your pages, replacing a CSS selector of your choosing.

You can use the WordPress Templates Interpolation for underscore.js and include JS-based logic in your templates.

You can expose data using the REST API, using the global $post object, or provide your own JSON-formatted data.
To see the available data, you can add this line in your template and then check the console when your site loads:
<# console.log( data ); #>

You can read more about how to use the plugin on https://aristath.github.io/wordpress/introducing-wordpress-inserter-plugin.