_2waiting.js

"use strict";

var Q = require('q');

var waitForA = null;
var waitForB = null;

function _a() {
  if (waitForA) return waitForA;
  console.info('call a');
  waitForA = Q.fcall(function() {
    console.info('resolve a');
    return {a:1};
  });
  
  return waitForA;
}

function a() {
  return _a().then(function(aRes) {
    // wait until B will update the result of a()
    if (!waitForB) return aRes;
    return waitForB.then(function() {
      waitForA = null;
      return aRes;
    });
  });
}

function b() {
  console.info('call b');
  return waitForB = Q.fcall(function() {
    console.info('resolve b 1');
    return _a();
  }).then(function(obj) {
    console.info('resolve b 2');
    waitForB = null;
    obj.b = 2;
    return {some_other:1};
  });
}

Q.fcall(function() {
  console.info('test 1');
  return [a(), b()];
}).spread(function(aRes, bRes) {
  console.info('result ', aRes, bRes);
  
  console.info('---------------');
  console.info('test 2');
  return [b(), a()];
}).spread(function(bRes, aRes) {
  console.info('result ', aRes, bRes);

  console.info('---------------');
  console.info('test 3');
  return a();
}).then(function(aRes) {
  console.info('result ', aRes);
  
  console.info('---------------');
  console.info('test 4');
  return b();
}).then(function(bRes) {
  console.info('result ', bRes);
}).done();