propagate async callback usefull when using async.waterfall with mongoose

async-propagate-callback.js

// to propagate async callback
// usefull when using async.waterfall
// with mongoose

async.waterfall([
    task1(),
    task2(),
    task3()
  ], callFinalCallback());
  
function task1() {
  return function (next) {
    MyModel1.find(propagateCallback(next));
  }
}

function task2() {
  return function (model1, next) {
    MyModel2.find(propagateCallback(next, model1));
  }
}

function task3() {
  return function (model2, model1, next) {
    MyModel.find(function (err, data) {
      next(null); // call final callback
    });
  }
}

function propagateCallback () {
  var args = [];
  var callback;

  for (var i = 0; i < arguments.length; i++) {
    args.push(arguments[i]);
  }

  if (typeof args[0] === 'function') {
    callback = args.shift();
  } else {
    throw new Error('no callback to propagate');
  }

  return function (err, data) {
    if (err) {
      return callback(err);
    }
    args.unshift(data);
    args.unshift(null);
    return callback.apply(null, args);
  };
}