$ npm install --save-dev gas-lib
Your code editor will pick up all function and documentation provided.
These Google Apps Script stubs are auto-generated and manualy patched to provide a better context in some cases.
Nothing ensure that code written using this help will work as is in the Google Apps Script environment. Just test it.
This code is also not updated at every changes of the Google Apps Script documentation. Though we will try to keep up with GAS.