The Cursor#each method only accepts a single argument, a function which might be called multiple times, and thus lacks a Node.js-style callback argument that is called exactly once.

An Array#forEach style method with an additional `callback` argument would give asynchronous iteration a proper Node.js callback by getting called exactly once:

cursor.forEach(iterator, callback);
function iterator(document){
    // Called zero or more times, once per document in result set
}
function callback(err){
    // Called exactly once, after all documents have been iterated over, or when there is an error
}

Using this function would increase readability and save a couple lines of code.

The functionality would probably be equivalent to:

Cursor.prototype.forEach = function forEach(iterator, callback){
    this.each(function(err, doc){
        if(err) callback(err);
        else if(doc) iterator(doc);
        else callback(null);
    });
}