objectMongodb.js
function getCollection(cb) {
    if (collection) {
      setImmediate(function () { cb(null, collection); });
      return;
    }
    MongoClient.connect(url, function (err, db) {
      if (err) {
        return cb(err);
      }
      collection = db.collection(collectionName);
      cb(null, collection);
    });
  }
function getCollection(cb) {
    if (collection) {
      setImmediate(function () { cb(null, collection); });
      return;
    }
    MongoClient.connect(url, function (err, db) {
      if (err) {
        return cb(err);
      }
      collection = db.collection(collectionName);
      cb(null, collection);
    });
  }
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