Question

I am using the excellent caolan “async” module for nodejs:

I have this code:

exports.manageComments = function(req, res) {
    var toDeleteIds = [];
    var deleteFunctions = [];
    if (req.body.doDelete) {
        toDeleteIds = req.body.doDelete;
    }
    var i;
    for ( i = 0; i < toDeleteIds.length; i++ ) {
        var deleteFunction = function(callback) {
            var id = toDeleteIds[i];
            console.log(id);
            Comment.findOne({_id:id}, function(err, found) {            
                if (!err) found.remove(callback);
            });
        }
        deleteFunctions.push(deleteFunction);
    }
    async.parallel(
        deleteFunctions,
        function(err,results) {
            exports.comments(req, res); //render a view
        }
    );
};

My array contains two elements but console.log() keeps telling me “undefined”.

What am I missing?

Answer 1

Your problem is with:

    var deleteFunction = function(callback) {
        var id = toDeleteIds[i];

because at the time each callback function is executed, i will have the same value as toDeleteIds.length. A closure doesn’t “trap” the value that an outer variable had at the time it was created; it “traps” a reference to whatever value the outer variable has at the time it’s executed (which in this case won’t be until well after your for loop has finished.

In order to “trap” the value of i at the time you create your callback function, you need to make i a parameter of a function that you call to create your callback function. You need something like

    var deleteFunction = makeDeleteFunction(i, callback);

And then create a separate function outside the callback:

function makeDeleteFunction(i, callback) {
    return function(callback) {
        var id = toDeleteIds[i];
        console.log(id);
        Comment.findOne({_id:id}, function(err, found){            
            if (!err) found.remove(callback);
        });
     };
}