JQuery Array of Deferreds, implementation not working

Question

can anyone see any mistakes in this code? I use this approach throught my application, almost identically, but for some reason I simply cannot seem to resolve the main promise "a";

   Parser.prototype.insertSomeData = function(data)
    {
        var a = $.Deferred(),
            table = "Example",
            columns = ["col1", "col2", "col3"];

        var deferreds = [];

        // insert Data into the database
        for (var i = 0; i < data.length; i++)
        {
            var dfd = $.Deferred();

            deferreds.push(dfd.promise());

            item = data[i];

            database.insert(table, columns, [item.one, item.two, item.three], function(){console.log("resolved"); dfd.resolve()}, dfd.reject);
        }
        $.when.apply(null, deferreds).then(function(){console.log("it worked!"); a.resolve()});

        return a.promise();
    }

both the promises in the deferred array do get resolved. So I think the problem is in the when Any see something I'm missing?

pimvdb · Accepted Answer · 2012-08-23 09:36:14Z

2

You coded dfd.resolve but this doesn't do anything more than just getting the function. You'd have to call it: dfd.resolve().
When all deferreds are finished, you probably want to resolve a, not dfd. When $.when has finished, all dfds have been resolved, and you probably want to resolve the master deferred (a) in that case.

answered Aug 23, 2012 at 9:36

pimvdb

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4 Comments

pimvdb Over a year ago

@CrimsonChin: The dfd is pointing to the last item in the callback due to how scoping works in JavaScript. Wrap the for loop body with (function(i) { ... })(i);.

Jon Wells Over a year ago

Ok that looks like it! I don't understand what the i has to do do with the deferred when we talk about scope here though?

pimvdb Over a year ago

@CrimsonChin: Not only i is scoped to the function but dfd as well. So in the callback you're resolving the very same deferred each time. By wrapping it in a function you create separate variables.

Jon Wells Over a year ago

Oh I follow you, the dfd variable was being hoisted each time losing scope so they all pointed to the same dfd object! I have been doing Javascript for about 5 months now and scoping really busts my balls each time

David Hellsing · Accepted Answer · 2012-08-23 09:32:13Z

0

deferreds only contains promises from the dfd object, so they are the ones getting resolved.

answered Aug 23, 2012 at 9:32

David Hellsing

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