如何重写异步函数（node.js，和尚）

Question

I'm trying to write an asynchronous function to give me a random document from a mongodb collection.

var getRandDoc = function(){
    var db = monk('localhost/data');
    var coll = db.get('coll');

    coll.count({}, function(err, count){
        if (err) console.log(err);
        else {
            coll.find({}, {limit:-1, skip:randomNum(0, count)}, function(err, out){
                if (err) console.log(err);
                else{
                    db.close();
                    return out[0]['name'];
                }
            });
        }
    });
}

In another file, I call this function with something like:

console.log(test.getRandDoc());

And I get undefined

What am I doing wrong and how do I fix it?

Answer 1

It's the usual node callback confusion. If you don't want to use promises then getRandDoc() needs to accept a callback and then call it with the results in the coll.find(...) method. So something like this:

var getRandDoc = function(cb){
    var db = monk('localhost/data');
    var coll = db.get('coll');

    coll.count({}, function(err, count){
        if (err) console.log(err);
        else {
            coll.find({}, {limit:-1, skip:randomNum(0, count)}, function(err, out){
                if (err) return cb(err)
                else{
                    db.close();
                    return cb(null, out[0]['name']);
                }
            });
        }
    });
}

You probably want to pass back that error too, so:

test.getRandDoc(function(err, name){
});

A promise based version is going to look something like this:

var getRandDoc = function(){
    var db = monk('localhost/data');
    var coll = db.get('coll');
    var deferred = Q.defer();

    coll.count({}, function(err, count){
        if (err) deferred.reject(err);
        else {
            coll.find({}, {limit:-1, skip:randomNum(0, count)}, function(err, out){

                if (err) deferred.reject(err);
                else{
                    db.close();
                    deferred.resolve((out[0]['name']);
                }
            });
        }
    });

    return deferred.promise;
    }

But it's still not going to give you a straight forward variable assignment. You'll wind up calling it something like this:

test.getRandDoc().then(function(res){}).fail(function(err){});

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