Question

I am writing some node.js code using the node-mongodb driver. I decided to cache the collection objects when I obtain them like this:

var db = connectionObject;

function getCollection(collectionName) {
    return function(callback) {
        var cache;

        if (cache) return callback(null, cache);

        db.collection(collectionName, function(err, collection) {
            return err ? callback(err) : callback(null, cache = collection);
        });
    }
}

var usersCollection = getCollection('users');
usersCollection(function(err, collection) {
    collection.find({}); // Rest of code here ...
});

Repeated calls of the usersCollection function should use the cached collection object, except that it doesn’t – the cache variable is always undefined. Changing the code to this fixes the problem:

return function(callback) {
    var cache = arguments.callee;

    if (cache.cached) return callback(null, cache.cached);

    db.collection(collectionName, function(err, collection) {
        return err ? callback(err) : callback(null, cache.cached = collection);
    });
}

I am still confused about why the ‘cache’ variable goes out of scope. What am I doing wrong?

Answer 1

I think you want this:

function getCollection(collectionName) {
    var cache;
    return function(callback) {

instead of what you have right now:

function getCollection(collectionName) {
    return function(callback) {
        var cache;