I am using caolan's async.js. I'm confused why the callback is getting called before all of the iterators are done. When I run the following code:

  async.filter(matched_objects.fields, function(item, callback) {
      console.log('checking field: ' + item.id);
      if(item.id == 130 || item.id == 131) {
        console.log('calling field true: ' + item.id);
        callback(true);
      } 
      callback(false);
    },
    function(fieldResults) {
      console.log('fieldsResults.length=' + fieldResults.length);
    });

I get the following output:

checking field: 130 
calling field true: 130 
fieldsResults.length=1 
checking field: 131
calling field true: 131 

It doesn't make any sense to me that the console.log in the callback is getting called before the second results.fields item is checked in the filter loop.

I am using caolan's async.js. I'm confused why the callback is getting called before all of the iterators are done. When I run the following code:

  async.filter(matched_objects.fields, function(item, callback) {
      console.log('checking field: ' + item.id);
      if(item.id == 130 || item.id == 131) {
        console.log('calling field true: ' + item.id);
        callback(true);
      } 
      callback(false);
    },
    function(fieldResults) {
      console.log('fieldsResults.length=' + fieldResults.length);
    });

I get the following output:

checking field: 130 
calling field true: 130 
fieldsResults.length=1 
checking field: 131
calling field true: 131 

It doesn't make any sense to me that the console.log in the callback is getting called before the second results.fields item is checked in the filter loop.

• javascript
• async.js

Share Improve this question Follow edited Nov 2, 2015 at 20:16 Mrchief 76.3k2020 gold badges145145 silver badges192192 bronze badges asked Jan 15, 2013 at 19:15 akronymnakronymn 2,44644 gold badges2626 silver badges4040 bronze badges

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2 Answers 2

Sorted by: Reset to default 6

The problem is the callback(false) is called everytime, even if you hit the if condition. The right approach would be to add a return statement:

if(item.id == 130 || item.id == 131) {
  console.log('calling field true: ' + item.id);
  return callback(true);  // now you won't call callback(false) everytime
} 
callback(false); 

And you can even shorten the filter by saying:

callback(item.id == 130 || item.id == 131);

After more experimentation I've discovered the problem was the callback(false); line. apparently this caused the filter to exit. I guess this makes sense since it forces the callback to be called.