Aggregation functions return undefined when they should return null

[kraenhansen]

How frequently does the bug occur?

Seen once

Description

Our types declare that the min, max and sum can return null:

min(property?: string): number | Date | null;
max(property?: string): number | Date | null;
sum(property?: string): number | null;
avg(property?: string): number;

But the implementation shows that min, max and avg actually return undefined when there are no objects in the collection:

const realm = new Realm({
  schema: [{
    name: "Item",
    properties: { value: "int" }
  }],
});
realm.objects("Item").min("value"); // returns `undefined`
realm.objects("Item").max("value"); // returns `undefined`
realm.objects("Item").avg("value"); // returns `undefined`
realm.objects("Item").sum("value"); // returns `0`

We are actually testing, to ensure this behaviour ☝️
https://github.com/realm/realm-js/blob/master/tests/js/list-tests.js#L1389-L1392

Stacktrace & log output

No response

Can you reproduce the bug?

Yes, always

Reproduction Steps

No response

Version

10.21.1

What SDK flavour are you using?

Local Database only

Are you using encryption?

No, not using encryption

Platform OS and version(s)

Tested on Node.js

Build environment

No response

Cocoapods version

No response

[kneth]

The return value from aggregation function is using std::optional<Mixed>, and with zero objects it has no value. The return value is then set to undefined (https://github.com/realm/realm-js/blob/master/src/node/node_return_value.hpp#L119).

[kraenhansen]

@kneth right. I think the easiest would be to update the types.