Coding Style
PHP
Starting with Nextcloud 19 there is a shared PHP Coding Standards Fixer configuration you can use to automatically format your app’s source code. For full details see the repository on Github.
JavaScript
There is a shared configuration for eslint that you can use to automatically format your Nextcloud apps’s JavaScript code. It consists of two parts: a config package that contains the formatting preferences and a plugin to detect deprecated and removed APIs in your code. See their readmes for instructions.