Asynchronous IO and iteration variables in Javascript

Question

I am working with NodeJS and MongoDB and on start I create collections and fill them in if they don't already exist. Right now this is just development , but with backups and things I could end up doing the same thing or something similar in production.

My question is what will happen to the iteration variable(i)? Will the callback use the correct value for that iteration? Or will it get some other value further down the list, or perhaps none at all?

ldb.createCollection('people', { strict: true }, function(err, col){
    if(err) { console.log('createCollection( people ) ', err); } 
    else { 
        for(var i in people){
            col.insert(people[i], { w: 1 }, function(err, doc){
                people[i]._id = doc._id;
            }
        }
    }
});

Edit: This is an object, not an array.

Since insert seems to be asynchronous, then yes i will change before the callback is executed. See Javascript closure inside loops - simple practical example — Bergi
– Bergi, Commented May 1, 2014 at 23:26

SimpleJ · Accepted Answer · 2014-07-29 16:56:06Z

1

When the callback passed into insert is called, the value of i will not be preserved (it will most likely be the last key in people). If you want to preserve i, you could give it its own functional scope like this:

people.forEach(function(person, i) {
    col.insert(person, { w: 1 }, function(err, doc) {
        person._id = doc._id;
    });
});

edit: Using an anonymous function instead of the forEach method:

for(var i in people) {
    function(i) {
        col.insert(people[i], { w: 1 }, function(err, doc){
            people[i]._id = doc._id;
        });
    }(i);
}

edited Jul 29, 2014 at 16:56

answered May 1, 2014 at 22:45

SimpleJ

15k13 gold badges62 silver badges98 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

Arlen Beiler Over a year ago

What about (function(var1, var2){ /* do something */ }(this.var1, this.var2);? Seems like I've seen that used in cases? How would that be done? Would it work? This is an object, not an array, so it doesn't have a foreach function.

SimpleJ Over a year ago

Yes. That would probably be a more precise solution to your problem. I will add an example of that to my answer.

SimpleJ Over a year ago

The only reason I would suggest using a forEach call is it's cleaner and avoids common issues associated with for..in loops (such as requiring the use of the hasOwnProperty method).

dandavis Over a year ago

my $0.02: i find it easer to scan when you use a different variable name inside the wrapper than out of it. repeating "i" works and preserves the orig, but it can be confusing...

Collectives™ on Stack Overflow

Asynchronous IO and iteration variables in Javascript

1 Answer 1

4 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

4 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related