Confused about a Javascript closure case

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 ca开发者_如何学Cche = 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?

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;

Nothing closes over cache after the function returned from getCollection (usersCollection) executes. There is no function that is returned from that scope.

cache needs to be defined outside of the usersCollection function for any reference to it to be captured.