Sketch supports HTTP communication with a polyfill on
fetch, which uses Cocoa APIs underneath. I've been using it for multiple kind of requests (POST, PUT verbs, NSData, Buffer, String, and JSON bodies etc.), and it works well. The code is straightforwad (https://github.com/skpm/sketch-polyfill-fetch/blob/master/lib/index.js) and it also handles Sketch Fibers, which is used to determine when a plugin has truly finished executing by keeping track of async requests.