Iterating over a mongodb cursor serially (waiting for callbacks before moving to next document)

A more modern approach that uses async/await:

const cursor = db.collection("foo").find({});
while(await cursor.hasNext()) {
  const doc = await cursor.next();
  // process doc here
}

Notes:

• This may be even more simple to do when async iterators arrive.
• You'll probably want to add try/catch for error checking.
• The containing function should be async or the code should be wrapped in (async function() { ... })() since it uses await.
• If you want, add await new Promise(resolve => setTimeout(resolve, 1000)); (pause for 1 second) at the end of the while loop to show that it does process docs one after the other.

If you don't want to load all of the results into memory using toArray, you can iterate using the cursor with something like the following.

myCollection.find({}, function(err, resultCursor) {
  function processItem(err, item) {
    if(item === null) {
      return; // All done!
    }

    externalAsyncFunction(item, function(err) {
      resultCursor.nextObject(processItem);
    });

  }

  resultCursor.nextObject(processItem);
}  

since node.js v10.3 you can use async iterator

const cursor = db.collection('foo').find({});
for await (const doc of cursor) {
  // do your thing
  // you can even use `await myAsyncOperation()` here
}

Jake Archibald wrote a great blog post about async iterators, that I came to know after reading @user993683's answer.