NodeJS V8 Pass additional argument to callback - node.js

Question

I write an application using NodeJS, ExpressJS, MongoDB (with Mongoose)...
Everything work perfectly, but, when I have a loop for fetch records and do something with the results, like this:
for(var i = 0; i < 10; i++) {
recods.findOne({number: i}, function(err,doc){
...
});
}
The variable "i" in the scope of callback function is passed by reference and the result is not the desired.
When the callback is called the loop has already run and the variable has changed.
If I try to pass argument as anonymous function, does not work, because it replace the needed arguments:
for(var i = 0; i < 10; i++) {
records.findOne({number: i}, (function(err,doc){
...
})(i));
}
In this way, I lost the "err,doc" arguments,
What can I do to solve this big problem?

Answer 1

You can bind it to your callback to create a partial function with its first argument set to i:
for (var i = 0; i < 10; i++) {
records.findOne({number: i}, function(i, err, doc) {
...
}.bind(records, i));
}

Answer 2

You're applying an anonymous function in the wrong place. It should be applied outside of the function that uses i, not to the callback function.
for (var i = 0; i < 10; i++) {
(function(i) {
records.findOne({number: i}, function(err, doc) {
...
});
}(i));
}

Answer 3

While with a few simple fix-ups to capture the value of i in a closure works as shown in the other answers, you might also consider using Mongoose in a different and likely more efficient way:
var numbers = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9];
records.find( { number : { $in : numbers } }, function(err, allDocs) {
if (err) { throw err; }
// allDocs are now available in an array (they aren't ordered)
// allDocs.length
// allDocs[0].number ...
});
Using the $in operator (reference) makes just one call to the Database, and finds all matching documents, rather than doing individual calls.