Parallel and then series, in JavaScript (Promise, async, generators, POJS)

async.js

// async

async.waterfall([
  function(callback) {
    async.parallel([
      function(callback) {
        _get("/token", callback);
      },
      function(callback) {
        _post("/key", callback);
      }
    ] callback)
  },
  function(data, callback) {
    _post("/login", data, callback);
  },
], function(auth, callback) {
  window.location.href = "/dashboard?" + auth;    
});

generators.js

// using generators

sync(function* (resume, pResume) {
  var [token, key] = yield [_get("/token", pResume()), _post("/key", pResume())]; // parallel
  var auth = _post("/login", data, resume);
  window.location.href = "/dashboard?" + auth;
});

vanilla.js

// vanilla

_get("/token", function(err, token) {
  login.token = token;
  if (login.token && login.key) login();
});

_post("/key", function(err, key) {  
  login.key = key;
  if (login.token && login.key) login();
});

function login() {
  _post("/login", [login.token, login.key], function(err, auth) {
    window.location.href = "/dashboard?" + auth;
  });
}