I use Advanced Custom Fields 5 Pro for most custom WordPress projects, and the latest version supports saving your Field Groups to JSON. To activate this functionality out of the box, all you have to do is create a folder within your theme called /acf-json/. But I don't like that.
I prefer for these JSON files to be stored outside of the theme, because they're key to the functionality of the website, and functionality === plugin. Storing this data outside of your WordPress theme makes it inherently more portable, and means you can retain the ACF JSON data even if you change theme.
Once the plugin is installed and activate it will automatically save field groups as JSON within the plugin's /acf-json/ folder when they are updated or saved.
This plugin is under active development on Github
Requirements
- ACF 5 Pro (Synchronized JSON requires at least v5.1.5)