Understanding find in Node/Mongo

Question

I am trying to learn node. Consider this code (based on official MongoDB Node.js driver)

  // Retrieve all the documents in the collection
  collection.find().toArray(function(err, documents) {
    assert.equal(1, documents.length);
    assert.deepEqual([1, 2, 3], documents[0].b);

    db.close();
  });

I have two questions:

• is find synchronous or asynchronous?

• If it is asynchronous, the .toArray function call is confusing me, because normally I would expect something along the lines of

  collection.find(function(err, results){});
  

Specifically I am interested what mechanism allows you to call a .toArray on result of asynchronous function? Because asynchronous functions as I get it rarely return something (I think except promises), rather invoke callbacks passed to them. Can someone clarify this situation with find and .toArray?

---

For example in the accepted answer of this question: How to get a callback on MongoDB collection.find(), you can see author calls find the way I envisioned, and received cursor in callback function. That is fine with me, that is how I expected it to work. But chaining result of asynchronous call find (if it is asynch?), with toArray a bit confuses me.

My speculation is find returns a handle kind of thing, the data at this point hasn't been loaded from DB, only when you call toArray the actual data arrives. Am I right?

Answer 1

I concede you, this case is a bit weird. Here is for the v2.2 of mongodb-native.

First of all, find has two different usages. You can either give a callback function or not. But in any case, it returns synchronously an object. More precisely it's a cursor. We could expect a asynchronous mechanism when passing a callback but not here.

collection.find({ }, function (err, cursor) {
  assert(!err);
});
console.log('This happens after collection.find({ }, callback)');

OR

const cursor = collection.find({});
console.log('Also happening after');

On the other hand, toArray is an asynchronous function and has also two different usages. This time, the returned object is different depending on the arguments.

Are equivalent:

cursor.toArray(function (err, documents) {
  assert.equal(1, documents.length);
});

AND

cursor.toArray()
  .then(documents => {
    assert.equal(1, documents.length);
  });

In the first call, toArray returns undefined whereas in the second, it returns a Promise.