Avoiding explicit promise construction antipattern. Added bluebird.js secret sauce.

fun-with-bluebird-promise-patterns.js

// Promises are great, but bluebird covers :allthethings
var Promise       = require('bluebird');
var appContext    = {ready: false, error: false};
const INVALID_CTX = {ready: false, error: true, message: 'App or User not initialized'};

// Bluebird-ify around `fetch` API
function getUser(username) {return Promise.resolve('users/' + username + '.json').then(fetch);}

function initApp(username) {
  // Use bluebird to do some real-world-ish code: 
  // First, get a user, cache them, check an inbox, then filter msgs already viewed
  return getUser(username)
    .timeout(2500)                    // easy to declare expected performance, seriously awesome
    .bind(appContext)                 // subsequent Promise methods can share access to appContext obj via `this`
    .tap(user => this.user = user)    // use `.tap` to cache the result without modifying the promise result/chain
    .then(checkInbox)                 // send the returned `user` to `checkInbox` - returns an array of msgs
    .tap(messages => this.messages = messages.length) // cache the # of msgs for UI
    .filter(msg => !msg.viewed)       // applied to current array excludes previously viewed msgs
    .tap(unreadMsgs => {
      // update UI without changing promise's return value
      if (unreadMsgs.length >= 1) {
        showToast(`${unreadMsgs.length} New Messages: ${unreadMsgs[0].subject}...`);
      } else {
        showToast('No new messages');
      }
    })
    .tap(() => this.ready = true) // at the end of the line, set `ready` flag
    .then(checkContext) // return the app's context for further chaining
    .catch(console.error.bind(console, `Err! not very fetching: ${url}`))
}
function checkContext() { 
  if ( !this.user || !this.messages ) {
    // uh oh, override context
    return INVALID_CTX;
  }
  // otherwise ok, return ctx
  return this;
}