Nodejs parallel with promise

Question

I have a dict like this:

{go: ['went', 'run'], love: ['passion', 'like']}

The value of a key is its synonyms. And 'getSynonymWords(word)' is a async function that returns a promise in which Its value is a list of synonym words corresponding with the parameter passed. How can I loop through the object to get another object recursively like this:

{went: [], run: [], passion: [], like: []}

This is my piece of code:

function getRelatedWords(dict) {
  return new Promise(function(resolve) {
    var newDict = {}; 
    for(var key in dict){
      if (dict.hasOwnProperty(key)) {
        var synonyms = dict[key];
        Promise.map(synonyms, function (synonym) {
          return getSynonymWords(synonym).then(function (synonyms) {
            newDict[synonym] = synonyms;
            return newDict;
          }); 
        }).then(function () {
          resolve(newDict);
        }); 
      }   
    }   
  }); 
}

It is incorrect because some tasks are not finished, But I don't know how to run tasks parallel nested with promises. I'm using Bluebird library. Could you help me?

Maybe it's part of bluebird, I don't know, but Promise.map is not a function in vanilla JS. — Brandon Nozaki Miller, Dec 10 '15 at 09:53
Looks like you need to do something with the result of `Promise.map`. Add it to an array, then *after* the `for` loop finishes return `Promise.all(arrayOfPromiseMapResults)`. That would also negate the need for your `new Promise` which is a bit of [an anti-pattern](http://stackoverflow.com/questions/23803743/what-is-the-explicit-promise-construction-antipattern-and-how-do-i-avoid-it) — CodingIntrigue, Dec 10 '15 at 10:07
@JaromandaX no need to nitpick, OP means concurrency and not parallelism. — Benjamin Gruenbaum, Dec 10 '15 at 10:20
@RGraham Because all tasks in the `for` loop run asynchonously, so how can I know when they finished. Could you explain to me more. Thank you so much! — nguyenngoc101, Dec 10 '15 at 10:20
@nguyenngoc101 Something like [this example](http://jsfiddle.net/63d33ng3/1/). I am reluctant to put this as an answer because I don't understand your use-case fully so there's probably a more succinct way to achieve this. — CodingIntrigue, Dec 10 '15 at 10:24

Benjamin Gruenbaum · Accepted Answer · 2015-12-10T11:41:51.230

First of all, avoid explicit construction. Now that we're over that - we can do this with no nesting and 4 lines of code by first getting all the words, then getting all the synonyms, then folding them back to a dictionary.

function getRelatedWords(dict) {
   // first we get all the synonyms
   var synonyms = Object.keys(dict).map(x => dict[x]).reduce((p, c) => p.concat(c), []);
   // second we get all the synonyms for each word with the word itself
   var withSynonyms = Promise.map(synonyms, s => Promise.all([s, getSynonymWords(s)]));
   // then we fold it back to an object with Promise.reduce
   var asDict = withSynonyms.reduce((p, c) => p[c[0]] = c[1]), {});
   // and return it
   return asDict; 
}

If we wanted to be "clever" we can opt to a one liner, I'm going to use ES2016 here for fun:

let {entries} = Object;
let {reduce, all} = Promise;
const getRelatedWords = dict => reduce(entries(dict), (p, c) => p.concat(c), []).map(s => [s, getSynonymWords(s)]).map(all).reduce((p, [s, syns]) => p[s] = syns, {});

The better solution btw is probably to use something like wordnet that lets you specify the distance and make a single call.

Thank you so much. Yours code is so beautiful. But I have a question how to make sure that all `getSynonymWords` are executed before getting `asDict`. — nguyenngoc101, Dec 10 '15 at 10:42
@nguyenngoc101 I'm using `Promise.reduce` and not `reduce` which waits for the values if they're promises (that's a bluebird method). I'll edit it to use the not-static version as that's slightly nicer. — Benjamin Gruenbaum, Dec 10 '15 at 11:41

Nodejs parallel with promise

1 Answers1