elm_sub.elm

subscribe : SelectionSet response RootSubscription -> Session -> Cmd msg
subscribe query session =
    let
        headers : List ( String, String )
        headers =
            [ ( "authorization", JWT.authorizationHeader session.jwt )
            ]

        queryString : String
        queryString =
            Document.serializeSubscription query
    in
    Ports.subscribeTo
        { endpoint = Env.graphqlEndpoint
        , headers = headers
        , query = queryString
        }

elm_subs.js

app.ports.subscribeTo.subscribe(function(subscriptionRequest) {
  var xhr = new XMLHttpRequest();
  var linesSeen = 0;

  xhr.open("POST", subscriptionRequest.endpoint, true);

  xhr.setRequestHeader("Accept", "application/json");
  xhr.setRequestHeader("Content-Type", "application/json");

  subscriptionRequest.headers.forEach(function([header, value]) {
    xhr.setRequestHeader(header, value);
  });

  xhr.onreadystatechange = function(event) {
  };

  xhr.onprogress = function(event) {
    if (xhr.responseText !== "" && xhr.responseText !== undefined) {
      try {
        var oldLinesSeen = linesSeen;
        var lines = xhr.responseText.split("\n");
        var linesSeen = lines.length;

        lines.forEach(function(line, index) {
          if (index <= oldLinesSeen) { return };
          if (line == "") { return };
          app.ports.notifySubscription.send(line);
        });

      } catch (error) {
        console.log('Error processing json', error);
      }
    }
  };
  xhr.send(JSON.stringify({ query: subscriptionRequest.query }));
});