Asynchronous array problems

Question

Near the end of the look where you see console.log(authors), the array is constantly overwriting itself. The async part of this loop is usually outside the loop. I did this to debug the reason why I get a big array full of the same username, rather than an array full of a series of different usernames. This is the code I have:

while (i >= 0) {
  var search = User.find({'twitter.id' : collected[i].author});

  search.limit(1);
  searches.push(function(cb) {
    search.exec(function (err, hold){
      if (err) cb(err, null);
      else {
        cb(null, hold[0].twitter.username);
      }
    });
  });
  i = i - 1;

  async.parallel(searches, function( err, authors) {
    if ( err ) return console.error( err );
    else {
      console.log(authors);
    }
  });
}

These are the results I get:

It's been a long day, I'm not quite seeing where I'm going wrong.

Shouldn't your logic be in the callback function of User.find? As in User.find({'twitter.id':collected[i].author}, function(user) { /* logic */ }); — nathanhleung
– nathanhleung, Commented Nov 23, 2015 at 20:51
@epascarello: "The async part of this loop is usually outside the loop. I did this to debug the reason" — Felix Kling
– Felix Kling, Commented Nov 23, 2015 at 20:53
@nathanleung I'm prepared the query beforehand, it still works the same way in execution. — Gregory Worrall
– Gregory Worrall, Commented Nov 23, 2015 at 20:54
Sounds like the classic: stackoverflow.com/questions/750486/… — epascarello
– epascarello, Commented Nov 23, 2015 at 20:56

Community · Accepted Answer · 2017-05-23 10:27:19Z

1

The issue is that at the moment the task function (function(cb) { ... }) is called, search already has a different value, the value of the last iteration. All the tasks refer to the same search variable and therefore perform the same search.

This is the typical "closure inside loop" problem.

You need to create scope per iteration. See JavaScript closure inside loops – simple practical example for solutions.

edited May 23, 2017 at 10:27

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answered Nov 23, 2015 at 20:53

Felix Kling

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3 Comments

Gregory Worrall Over a year ago

Could you give a little example on how this would be coded?

Felix Kling Over a year ago

There are so many examples in the linked question. E.g. stackoverflow.com/a/19324832/218196 .

Gregory Worrall Over a year ago

Code's working great now, very quick solution. Lesson learned. Closures and scopes.

Tomer W · Accepted Answer · 2015-11-23 20:54:43Z

0

you are not clearing Searches...

so every itteration go like:

add a search to searches
run all searches (write to log)

1st itteration: 1st search.

2nd itteration: 1st search + 2nd search.

3rd itteration: 1st search + 2nd search + 3rd search.

you might wanted to run the async.parallel() after the while ends.

answered Nov 23, 2015 at 20:54

Tomer W

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