javascript Underscore nested loop undefined values

Question

Let's say I have a collection of collections named, collections. Now, let's consider this piece of code.

_.each(collections, function(collection){
        _.each(collection, function(item){
           console.log(item);
        }
});

Several times, it prints undefined values. It seems to print item values before actually setting it from the previous loop. Why is it not retaining the order of execution?

Thanks in advance.

What are in the collections? The implementation of _.each shouldn't give that behaviour. — Gruff Bunny
– Gruff Bunny, Commented Feb 24, 2014 at 11:05
Did any of the answers help you? If so can you mark the correct one as answered. If not, can you clarify where you still have questions? — Jonathan Beebe
– Jonathan Beebe, Commented Mar 16, 2014 at 22:16

Jonathan Beebe · Accepted Answer · 2014-02-24 16:34:55Z

1

Are collections and collection plain arrays? Or are they Backbone.Collections?

The behavior you’re describing is not a bug in underscore. It should be maintaining the order of execution as you expect. This is probably an issue with the data in the collection.

If you want to strip all undefined values from your collection before running .each you can use _.compact(array) (assuming they are plain javascript arrays)

_.each(_.compact(collections), function(collection){
    _.each(_.compact(collection), function(item){
        console.log(item);
    }
});

answered Feb 24, 2014 at 16:34

Jonathan Beebe

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1 Comment

mu is too short Over a year ago

I think you're right about collection being a Backbone collection, in that case collection.each(function(item) { ... }) would be the Right Thing.

htatche · Accepted Answer · 2014-02-24 10:20:49Z

0

I guess that it is just because the inner loop also runs it's callback asynchronously. So the parent loop doesn't wait for it to complete, it runs straightaway the next iteration.

answered Feb 24, 2014 at 10:20

htatche

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1 Comment

Jonathan Beebe Over a year ago

underscore's each implementation does not run it's callbacks asynchronously. It defaults to the native forEach if available.

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