promised.js

var Promise = require("bluebird");

/**
 * Promised is a shim that wraps a collaborator that requires
 * asynchronous initialization. Instead of having to wait for
 * the initialization to finish (and do all your code that
 * depends on it in a giant `.then()` call), you get an object
 * you can use immediately.
 *
 * Every method invoked on it will simply return another promise
 * that is fulfilled either
 *
 * a) when the collaborator has finished initializing
 * b) immediately, if the collaborator is already available
 */
function Promised(promise) {
  if (promise instanceof Promise) {
    this.promised = promise;
  } else {
    throw new Error("must provide a promise!");
  }
  
  var methods = Array.prototype.slice.call(arguments, 1),
      self = this;

  methods.forEach(function(method) {
    self[method] = self.exec.bind(self, method);
  });
}

Promised.prototype.exec = function(/* varargs */) {
  var args = Array.prototype.slice.call(arguments),
      method = args.shift(), // first argument is method name
      self = this;

  function apply(obj, method, args) {
    return obj[method].apply(obj, args);
  }

  if (this.promised instanceof Promise) {
    return this.promised.then(function(obj) {
      self.promised = obj;
      return apply(obj, method, args);
    });
  } else {
    try {
      return Promise.resolve(apply(this.promised, method, args));
    } catch (err) {
      return Promise.reject(err);
    }
  }
}

module.exports = Promised;

usage.js

var Promise = require("bluebird"),
    Promised = require("./promised");
    
var promised = new Promised(Promise.resolve("test"));

// promises.exec() always returns a promise that
// resolves to the return value of the executed method
promised.exec("toUpperCase").then(console.log); // prints "FOO"
promised.exec("chatAt", 0).then(console.log); // prints "f"

// promise is rejected if method does not exist, or throws an error
promised.exec("doesntExist").catch(console.error); // prints "TypeError: Cannot call method 'apply' of undefined"

// Nicer version: use method shims
promised = new Promised(Promise.resolve("test"), "toUpperCase", "charAt");

// now you can do:
promised.toUpperCase().then(console.log); // prints "FOO"
promised.charAt(0).then(console.log); // prints "f"