Node.js Promises: Push to array asynchronously & save

Question

I am currently trying to push to an array (attribute within a Mongo Model), from a list of items I receive through a request. From those items, I loop through them to see which one is currently in the db and if that is not the case, then I create a new Item and try to save it. I am using promises to accomplish this task but I am unable to figure out why the array is empty after all the promises have fulfilled.

var q     = require('q');

var items_to_get = ['1', '2', '3']; // example array

var trans = new Transaction({
    items   : []
});

var promises = [];

for (var i = 0; i < items_to_get.length; i++) {

  var ith = i; //save current i, kinda hacky
  var deferred = q.defer(); //init promise

  //find an existing item
  Item.findOne({simcode: items_to_get[ith]}, function(err, item) {
      trans.items.push(item); // push item to transaction
      deferred.resolve(item); // resolve the promise
  });
  promises.push(deferred); // add promise to array, can be rejected or   fulfilled
};

q.allSettled(promises).then(function(result) {
  console.log(trans.items); //is empty
  trans.save();
}

EDIT Resolved: Code bellow, based on http://jsbin.com/bufecilame/1/edit?html,js,output .. credits go to @macqm

var items_to_get = ['1', '2', '3'];
var promises     = []; //I made this global

items_to_get.forEach(item) {
  upsertItem(item);
}

q.allSettled(promises).then(function(result) {
  //loop through array of promises, add items 
  result.forEach(function(res) { 
    if (res.state === "fulfilled") {
      trans.items.push(res.value);
    }
  });
  trans.save();
  promises = []; //empty array, since it's global.
}

//moved main code inside here
function upsertItem(item) {
  var deferred = q.defer(); //init promise
  //find an existing item
  Item.findOne({simcode: item}, function(err, item) {
    deferred.resolve(item); // resolve the promise
    // don't forget to handle error cases
    // use deffered.reject(item) for those
  });
  promises.push(deferred); // add promise to array
}

Francesco Casula · Accepted Answer · 2017-06-23 16:59:33Z

This is how I did it without any 3rd party libraries.

Since I just needed to defer and I'm on ES2017 I figured it was best not to over complicate things with unnecessary dependencies.

'use strict';

/**
 * @param {function(*)} callee
 * @param {Array} args
 * @returns {Promise.<*>}
 */
const defer = (callee, args) => {
    return new Promise(resolve => {
        resolve(callee(...args));
    });
};

/**
 * @param {Number} one
 * @param {Number} two
 * @param {Number} timeout
 * @returns {Promise.<Number>}
 */
const asyncFunction = (one, two, timeout) => {
    return new Promise(resolve => {
        setTimeout(resolve, timeout, one + two);
    });
};

let promises = [];
promises.push(defer(asyncFunction, [3, 7, 0])); // returns immediately
promises.push(defer(asyncFunction, [10, 20, 100])); // returns after 100ms
promises.push(defer(asyncFunction, [55, 45, 50])); // returns after 50ms

Promise.all(promises).then(results => {
    console.log(results);
});

Run the above code and you'll get [ 10, 30, 100 ].

Peter Wagener · Accepted Answer · 2015-03-21 18:48:50Z

1

Instead of

promises.push(deferred);

... try ...

promises.push(deferred.promise);

Also, since your promises are resolving to the item that was saved anyway, you can use the result of q.allSettled(...) as your items:

q.allSettled(promises).then(function(results) {
    trans.items = results;
    trans.save();
});

answered Mar 21, 2015 at 18:48

Peter Wagener

2,09214 silver badges20 bronze badges

1 Comment

makumo Over a year ago

Seems like each result will be an object of {state: ..., value: ...} structure, where value is an item you want, so you may have to do an extra step to get them out.

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