Javascript Pushing elements inside a loop, doesn't affect the array outside the loop

Question

I have an empty array that I want to push items onto, inside a loop. Once outside the loop, the array losses all information

var result = [];
        users.find({}, {username: true, isOnline: true, uniq: true, _id: false}, function(err, cursor) {
            cursor.each(function(err, item) {
                result.push(item);
                console.log(result); //every iteration the array shows fine
            });

            console.log(result); //array is empty
        });

        console.log(result); //array is empty

What exactly is `cursor`? does it's `each` method work asynchronously? If not, then your explanation of what is happening doesn't make sense. I wouldn't expect `.each` to be asynchronous. — Kevin B, Apr 30 '15 at 19:57
`each` must be asynchronous. Where does that method come from? You should specify what framework/library you are using. — , Apr 30 '15 at 19:59
This is MingoDB and it says on their site that cursors has the each function. — lazandrei19, May 01 '15 at 18:08

score 1 · Accepted Answer · 2015-04-30T20:32:29.357

It looks like you are using Mongoskin, you can use the toArray method to convert the cursor to an Array, which seems to be what you want. Check this out:

http://www.hacksparrow.com/mongoskin-tutorial-with-examples.html

db.collection('stuff').find().toArray(function(err, result) {
    if (err) throw err;
    console.log(result);
});

So your code would look like this:

var result = [];
        users.find({}, {username: true, isOnline: true, uniq: true, _id: false})
        .toArray(function(err, cursor) {
            // cursor is now an array. forEach is sync.
            cursor.forEach(function(item) {
                result.push(item);
                console.log(result); //every iteration the array shows fine
            });

            console.log(result); // Should not be empty now
            // Add more code in here, if you want to do something with
            // the result array
        });
        // Still empty, because find is async.
        // Your code should go inside of the users.find call
        // and not here
        console.log(result);

This is something you'll be dealing with a lot with node. For async code, the rest of your code must go inside of the async calls. You can keep dealing with callbacks, or use Promises, for instance.

Or if you do not like CB or promises you can use Async/waterfall library — Amiga500, Apr 30 '15 at 20:27

Manuel Bitto · Answer 2 · 2015-04-30T20:02:45.753

0

This could be because users.find and cursor.each functions may be asynchronous, so your second console.log is executed before the execution of cursor.each and your third console.log is executed before users.find

edited Apr 30 '15 at 20:02

answered Apr 30 '15 at 19:56

Manuel Bitto

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second console.log statement is executed in the callback of ```users.find``` – Transcendence Apr 30 '15 at 19:57
this would only be the case if the each function were asynchronous – aw04 Apr 30 '15 at 19:58
aw04 the each function is probably async, in Javascript it's forEach to iterate sync over an array. – Sven Apr 30 '15 at 19:59

Javascript Pushing elements inside a loop, doesn't affect the array outside the loop

2 Answers2