Coroutines and promises in Node.

coro.js

// Run with `node --harmony coro.js`

"use strict";

const Promise = require('bluebird');

// Takes a function*, `g`, that yields promises.
// Returns a promise that resolves to `g`'s eventual return value.
var coro = function(gStar) {
  const pending = Promise.pending();
  const gArgs = Array.prototype.slice.call(arguments, 1);
  const gen = gStar.apply(undefined, gArgs);

  let result;
  try { result = gen.next(); }
  catch (e) {
    pending.reject(e);
    return pending.promise;
  }

  if (result.done) {
    pending.resolve(result.value);
  } else {
    trampoline(gen, result.value, pending);
  }

  return pending.promise;
};

var trampoline = function(gen, promise, pending) {
  promise.then(function(input) {
    let result;
    try { result = gen.next(input); }
    catch (e) {
      pending.reject(e);
      return;
    }

    if (result.done) {
      pending.resolve(result.value);
    } else {
      trampoline(gen, result.value, pending);
    }
  }).catch(function(e) {
    let result;
    try { result = gen.throw(e); }
    catch (e) {
      pending.reject(e);
      return;
    }

    if (result.done) {
      pending.resolve(result.value);
    } else {
      trampoline(gen, result.value, pending);
    }
  });
};

var sleep = function*(ms) {
  return yield Promise.delay(ms);
};

var apiCall = function*(endpoint) {
  yield* sleep(2000);
  return { users: ["Foo", "Bar", "Baz"] };
};

var buggyApi = function*(endpoint) {
  yield Promise.reject(endpoint + " failed!");
};

var bang = function*() {
  throw "bang!";
};

var outer = function*() {
  yield* inner();
};

var inner = function*() {
  yield* sleep(1000);
  yield* bang();
};

var example = function*(ms) {
  console.log("Let's sleep for", ms, "milliseconds.");
  yield* sleep(ms);

  console.log("And let's now make an expensive api call.");
  const result = yield* apiCall("/users");

  try { yield* bang(); }
  catch (e) { console.log("bang error", e); }

  try { yield* outer(); }
  catch (e) { console.log("outer error", e); }

  try { yield* buggyApi("/badroute"); }
  catch (e) { console.log("buggy api", e) }

  console.log("And let's finish by running some logic.");
  return result.users.map(user => user.toLowerCase());
};

coro(example, 500)
  .then(result => console.log("result =", result))
  .catch(e => console.log("We got an error!", e));

coro(function*() {
  let count = 0;
  while (true) {
    yield* sleep(100);
    console.log("tick");
    count += 1;
    if (count > 50) { throw "time's up!" }
  }
}).catch(e => console.log(e));