Async.js - Deferring execution while still accessing the correct array index

Question

I'm trying to us Async.js to process an array of items. Is there some cute way to get this to work properly? If you're smarter than I, you'd except that because of the deferred execution, http://3 gets printed three times.

jsFiddle link

var a_servers = ['http://1', 'http://2', 'http://3'];

var a_actions = [];

for (var i = 0; i < a_servers.length; i += 1)
{
    var server = a_servers[i];

    a_actions.push(function(callback)
    {
        document.write(server  + '<br/>');
        callback(false, server );
    });
}

async.series(a_actions, function(err, a_servers)
{
    document.write('processed ' + a_servers.length + ' servers<br>');
    console.info(a_servers);
});

Peter Lyons · Accepted Answer · 2012-04-03 04:13:02Z

1

You have a classic closure scope issue. You need to pass the server variable from the outer scope to the inner scope to get the desired behavior. You can use an IIFE for this.

for (var i = 0; i < a_servers.length; i += 1)
{
    var server = a_servers[i];

    a_actions.push((function(server) {
        return function(callback) {
            document.write(server  + '<br/>');
            callback(false, server );

    })(server));
}

answered Apr 3, 2012 at 4:13

Peter Lyons

147k32 gold badges285 silver badges281 bronze badges

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

2 Comments

Langdon Over a year ago

Gah, thanks, just figured it out and came back to ask if there was an easier way. jsfiddle.net/gYZhw/2

Peter Lyons Over a year ago

My general pattern is to define a top level function that defines and returns the closure, with the closure taking ("closing over") the interesting parameters as part of the closure scope of the top-level function. Then it clearly separates your closure scope complexity from what the loop is trying to accomplish/build. So you end up with a_actions.push(makeWriter(server));, which is clearer I think.

Collectives™ on Stack Overflow

Async.js - Deferring execution while still accessing the correct array index

1 Answer 1

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related