Native mongodb connection for node.js with promises - node.js

Question

I am running node.js express app on IIS with iisnode and i am having the following problem,
i got a file for the mongo repository
var MongoClient = require('mongodb').MongoClient;
var url = MY_URL;
var db = null;
MongoClient.connect(url, function(err, database) {
db = database
})
var repository = {};
repository.getAll = getAll;
module.exports = repository;
function getAll(collectionName)
{
return new Promise(
function(resolve, reject){
db.collection(collectionName).find().toArray(function(err, res) {
if (err) reject(err);
resolve(res);
});
});
}
Sometimes when i am trying to access the getAll function from a http get request i am getting an error
"Cannot read property 'collection' of null"
It happens because db is null.
But when i call it again it always returns the values.
Is it possible that the getAll function is being activated before the connection is being setup?
Or maybe the first call awakes the connection in some way?
It is hard for me to work on the problem since i cant reproduce it in a controlled manner. It usually happens after not using the app for a while ( it seems that sometimes restarting the IIS also triggers it ).

Answer 1

MongoClient.connect returns promise. So does getAll.
To ensure db always exists by the time you try to find something there you need to chain the promises. With few other bits fixed it could be:
const MongoClient = require('mongodb').MongoClient;
const url = MY_URL;
const dbConnected = MongoClient.connect(url); // it is a promise, db is not connected yet
function getAll(collectionName)
{
return dbConnected.then(db => // the connection promise resolved, we are good to use db
db.collection(collectionName).find().toArray() // it returns promise, no need to wrap it with another one
);
}
module.exports = {getAll};