Variable Scope in Asynchronous Node.js Loop

Question

I am trying to run some database queries (using sails.js) on an array and upon the queries' return, do something. I figured the best way to do so would be to use a for loop and resolve the promises async, and once they've all resolved, continue on. However, only the last promise in my array is resolving, and it is resolving multiple times because in each 'User.findOne...' then function, the index is array.length-1.

My questions:

1. How does variable scope in asynchronous loops work? Best resources to explain this?
2. What is the best way to solve my problem? Why?
3. Are there any other patterns I should use or not use? I am fairly new to promises and async js, so any tips would be helpful!

Main tutorials I've checked

• https://github.com/kriskowal/q
• https://github.com/kriskowal/q/wiki/API-Reference
• https://github.com/bellbind/using-promise-q/

Thank you for your help!

---

My simplified code:

functionWhichReturnsPromise()
.then(function(user){
  var promises = [];
  Q.try(function(){
    for (var index in array) {
      var fbid = array[index];// Get fbid from array
      promises.push(Q.defer().promise); // Add promise to promise array

      // Find userid from fbid; resolve respective promise when finished
      User.findOne({facebook_id: fbid}).then(function(userSeen){
        promises[index].resolve(userSeen.id);
        sails.log('resolved where id=' + userSeen.id); // correct
        sails.log('resolved where index=' + index); // PROBLEM: always last index
      });
    }
  }).then(function(){

    // For debugging purposes
    Q.delay(1000).then(function(){
      sails.log(promises[0]); // Unresolved
      sails.log(promises[1]); // Unresolved
      sails.log(promises[2]); // Only last promise in array is resolved
    });

    // When the userids have been extracted from above (promises fulfilled)...
    Q.all(promises).then(function(seenids){
      // Do stuff here (Doesn't get here)
    });
  });
});

Answer 1

In Javascript, variable's scope is function and not curly braces.

Therefore in the following code, the scope of var index is not the for loop's curly braces, the scope is actually the function in which the for loop exists.

Q.try(function(){
    for (var index in array) {
      var fbid = array[index];// Get fbid from array
      promises.push(Q.defer().promise); // Add promise to promise array

      // Find userid from fbid; resolve respective promise when finished
      User.findOne({facebook_id: fbid}).then(function(userSeen){
        promises[index].resolve(userSeen.id);
        sails.log('resolved where id=' + userSeen.id); // correct
        sails.log('resolved where index=' + index); // PROBLEM: always last index
      });
    }
  })

Within for loop you call the async function, in your case its mongodb call (findOne). You should always assume that these async function can take any number of milliseconds to run (depends on the function). But in general, usually the loop would have completed before the async functions run. Your for loop fires all those async functions even before those functions start running. The issue is that all those async functions which area pending are still pointing towards that variable index. And that variable is common to all of them because index was in scope of the outer function.

This is a problem created somewhat because of closures in Javascript. And to solve this, we need to use more closures.

There are many resources on the topic of closures that you can google. But go thru the MDN's description of it.

If you capture value of index within another function inside the loop, then you will be good to go.

Here is my suggested solution to your issue. I haven't tested it though, but you get the idea.

Q.try (function () {
    array.forEach( function(ele, idx, array) {
        (function(index) {
            var fbid = array[index]; // Get fbid from array
            promises.push(Q.defer().promise); // Add promise to promise array

            // Find userid from fbid; resolve respective promise when finished
            User.findOne({
                facebook_id : fbid
            }).then(function (userSeen) {
                promises[index].resolve(userSeen.id);
                sails.log('resolved where id=' + userSeen.id); // correct
                sails.log('resolved where index=' + index); // PROBLEM: always last index
            });
        })(idx);
    })
})

Hope this helps.

Also Note: it is incorrect to use for...in for iterating through arrays.