Why does await/async not work properly when I pass a function into .exec()?

Question

I'm trying to use await to wait for a database lookup using mongoose and save the response to a variable. For some reason, when I pass a function to .exec(), then await doesn't seem to work.

Way that doesn't work:

    var document = await Collection.findOne({ 'num': num }).populate('field').exec((err, result) => {
            if (err) throw error
    })
    console.log(document) // undefined

Way that does work:

    var document = await Collection.findOne({ 'num': num }).populate('field').exec()
    console.log(document) // The document I want

I don't think the population is what is causing the problem because I tried the code without the population and it worked. Why does this happen and how can I fix this?

You don't return result in your first callback, so it's discarded — Andy Ray, Aug 17 '22 at 20:44
I just tried returning result and it didn't work for me. Perhaps I'm not doing it right. Do you just put return result in the callback as follows: .exec((err, result) => { return result }) console.log(result) // undefined — Jonathan S., Aug 17 '22 at 20:56

score 0 · Accepted Answer · answered Aug 17 '22 at 22:21

0

This is the documented behaviour of mongodb:

NOTE
If you specify a callback, the method does not return a Promise.

^{(Emphasis theirs)}

Also here:

NOTE: All the examples below use async/await syntax.

However, all async API calls support an optional callback as the final argument, if a callback is provided a Promise will not be returned.

answered Aug 17 '22 at 22:21

Bergi

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Awesome! Thanks! Do you know how I could solve this problem? – Jonathan S. Aug 17 '22 at 22:45
I don't see a problem. Just don't use a callback, use the way that does work! – Bergi Aug 17 '22 at 23:02
True, true;). Let's say I wanted to do something with that error, would there be a way to do that? I guess I could always put the whole thing into a try catch block. – Jonathan S. Aug 17 '22 at 23:15
That, or `….exec().catch(err => { … })`. See also [here](https://stackoverflow.com/questions/44663864/correct-try-catch-syntax-using-async-await). – Bergi Aug 17 '22 at 23:17
I like your options. Thank you – Jonathan S. Aug 18 '22 at 00:09

Why does await/async not work properly when I pass a function into .exec()?

1 Answers1