Some code I use in tests to drop collections between tests.

drop_collections.js

var async = require('async');
var collections = ['projects', 'users', 'drafts', 'share_docs', 'share_docs_ops'];

module.exports = function(db, cb) {
  function dropCollection(name, cb) {
    db.collection(name, function(err, collection) {
      if(err) return cb(err);
      collection.drop(cb);
    });
  }
  async.each(collections, dropCollection, cb);
};