Have you ever used REST API with WordPress? It helps you to get/update data (posts, users, comments) via ajax request in a standard way. You can use it to make themes or apps with WordPress.
To use REST API better, you might need to get custom fields made by the Meta Box plugin. That's why we made the MB REST API extension for Meta Box, which integrates Meta Box and WordPress REST API. All the data of the custom fields are pulled automatically in a field named "meta_box".
The extension is free and available on wordpress.org. Check it out here:
Let us know what you think and feel free to open issues on Github if you find any bug.