index.js

const INFO = {
  APP_ID: "MY-APP-ID-HERE",
  isMessenger: null,
  thread_type: null,
  tid: null,
  psid: null,
};

/**
 * Thread types returned when getting the context
 * @enum {String}
 */
const ThreadType_T = {
  USER_TO_PAGE: "USER_TO_PAGE",
  USER_TO_USER: "USER_TO_USER",
  GROUP: "GROUP",
};

/**
 * @typedef {{thread_type: ThreadType_T, tid: String, psid: String, signed_request: String}}
 */
let ThreadContext_T;

// set up the Messenger Chat Extension SDK
(function(d, s, id){
  let js, fjs = d.getElementsByTagName(s)[0];
  if (d.getElementById(id)) {return;}
  js = d.createElement(s); js.id = id;
  js.src = "//connect.facebook.net/en_US/messenger.Extensions.js";
  fjs.parentNode.insertBefore(js, fjs);
}(document, 'script', 'Messenger'));

window.extAsyncInit = function() {
  // The Messenger Extensions JS SDK is done loading, call setup.
  setUp();
};

/**
 * Sets up whatever is needed to run Sunny.
 */
function setUp() {
  INFO.isMessenger = window.name === "messenger_ref";
  if (!INFO.isMessenger) {
    // Check that if it's not messenger, it's Facebook.
    console.log("should be true (isFacebook):", window.name === "facebook_ref");
  }

  // retrieve the context and figure out what to do about it
  MessengerExtensions.getContext(
    INFO.APP_ID,
    function success(thread_context) {
      const {thread_type, tid, psid} = thread_context;
      // Update info before handling the thread based on the thread type.
      INFO.thread_type = thread_type;
      INFO.tid = tid;
      INFO.psid = psid;

      switch (thread_type) {
        case ThreadType_T.USER_TO_PAGE:
          handleUserToPageThreadContext(thread_context);
          break;
        case ThreadType_T.USER_TO_USER:
          handleUserToUserThreadContext(thread_context);
          break;
        case ThreadType_T.GROUP:
          handleUserToGroupThreadContext(thread_context);
          break;
        default:
          handleUnknownThreadContext(thread_context);
      }
    },
    function failure(error) {
      console.error("The following error happened:");
      console.error(error);
    },
  );
}

/**
 * @param {ThreadContext_T} thread_context
 */
function handleUserToPageThreadContext(thread_context) {
  const {thread_type, tid, psid, signed_request} = thread_context;
  // TODO: This should explain how to use Sunny... It should explain that
  //  one needs to sign up as a group.
}

/**
 * @param {ThreadContext_T} thread_context
 */
function handleUserToUserThreadContext(thread_context) {
  const {thread_type, tid, psid, signed_request} = thread_context;
  // TODO:
}

/**
 * @param {ThreadContext_T} thread_context
 */
function handleUserToGroupThreadContext(thread_context) {
  const {thread_type, tid, psid, signed_request} = thread_context;
  // TODO: 
}

/**
 * @param {ThreadContext_T} thread_context
 */
function handleUnknownThreadContext(thread_context) {
  const {thread_type, tid, psid, signed_request} = thread_context;
  // TODO:
}