Configurable authentication

If the pusher js-client could take a function authAdapter returning a promise, then it could be left to the user to perform the authentication in any way fit (different endpoints, headers handling, withCredentials switch, etc).

Should solve https://github.com/pusher/pusher-js/issues/83 and https://github.com/pusher/pusher-js/issues/99.

new Pusher(apiKey, {
  authAdapter: function(channel, ...) {
    return Promise(resolve, reject) {
      // ...
    };
  },
  // other conf...
});

[jackfranklin]

Although I’d personally prefer promises, I would stick to callbacks given our commitment to supporting a bunch of older browsers as well as newer ones.

Or we could make it work with both and ask those who want to use Promises to provide their own polyfill.

Like the idea though 👍

[jpatel531]

I like the idea. I might be sceptical about Promises as we’d have to add another dependency to polyfill older browsers. Auth adapters is definitely something we’d like to tackle after the coming release is out