javascript - NodeJS, Async forEachSeries execution order - Stack Overflow

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Just trying to get my head around using Async module for NodeJS.

I have the following code.

var a1 = [1,2,3,4,5,6,7,8];
async.forEachSeries(a1, function(n1, callback) {
    console.log(n1);
    var a2 = [10,11,12,13,14];
    async.forEachSeries(a2, function(n2, callback) {
        console.log(n1 + " " + n2);
        callback();
    });
    callback();
});

I want to make the process of the above code in such a way that the print out becomes

1
1 10
1 11
1 12
1 13
1 14
2
2 10
2 11
2 12
2 13
2 14
3
3 10
3 11
3 12
3 13
3 14
.....

But instead I'm getting something like..

1
1 10
2
2 10
1 11
3
3 10
2 11
1 12
....

How do I fix this?

Just trying to get my head around using Async module for NodeJS.

I have the following code.

var a1 = [1,2,3,4,5,6,7,8];
async.forEachSeries(a1, function(n1, callback) {
    console.log(n1);
    var a2 = [10,11,12,13,14];
    async.forEachSeries(a2, function(n2, callback) {
        console.log(n1 + " " + n2);
        callback();
    });
    callback();
});

I want to make the process of the above code in such a way that the print out becomes

1
1 10
1 11
1 12
1 13
1 14
2
2 10
2 11
2 12
2 13
2 14
3
3 10
3 11
3 12
3 13
3 14
.....

But instead I'm getting something like..

1
1 10
2
2 10
1 11
3
3 10
2 11
1 12
....

How do I fix this?

• javascript
• node.js
• asynchronous
• async.js

Share Improve this question Follow edited Aug 4, 2013 at 12:07 Bergi 665k161161 gold badges1k1k silver badges1.5k1.5k bronze badges asked Apr 11, 2013 at 7:18 ericbaeericbae 9,6442626 gold badges7777 silver badges109109 bronze badges 7

• 2 If you need synchronous output, why do you perform it async? – NilsH Commented Apr 11, 2013 at 7:22
• 1 hm. doesn't forEachSeries perform in a synchronous way? – ericbae Commented Apr 11, 2013 at 7:26
• Both Node 0.10.3 and 0.8.16 produce the output you want on my Mac. Are you piping the output of the script to a file (or another program)? That changes the behaviour of process.stdout, and therefore console.log as well, so that might result in different output. And forEachSeries is synchronous in that the next iteration will only be called once the previous is done. – robertklep Commented Apr 11, 2013 at 7:44
• Hm.. you're right. On my machine it does in fact produce the output you expected as well. But I would rather call it "in sequence" than "synchronous". I'm using node 0.10.3 btw. – NilsH Commented Apr 11, 2013 at 7:45
• @NilsH I guess it's a matter of semantics :) I'd call it synchronized since the operations are dependent on each other, (N+1) only gets called once (N) has completed. – robertklep Commented Apr 11, 2013 at 8:30

 |  Show 2 more comments

1 Answer 1

Sorted by: Reset to default 17

The forEachMethod also accepts a callback when everything done. So your code should look like this:

var a1 = [1,2,3,4,5,6,7,8];
async.forEachSeries(a1, function(n1, callback_s1) {
    console.log(n1);
    var a2 = [10,11,12,13,14];
    async.forEachSeries(a2, function(n2, callback_s2) {
        console.log(n1 + " " + n2);
        callback_s2();
    }, function () {
        /* Finished the second series, now we mark the iteration of first series done */ 
        callback_s1();
    } );
});

The problem in your code is the fact that you assume async.forEachSeries to be synchronous, but it is not. It guarantees the fact the array will be handled synchronously, but the function itself is asynchronous.

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