add simple test to reproduce groupBy regression

[giggo1604]

Hello,

While updating our codebase to the latest version of Immutable, we encountered what seems to be a regression in the behavior of groupBy.

This change was introduced in Commit 53237d2.

Previously, grouping by a function that might return undefined was tolerated, but in the latest version, this now throws. While the new behavior arguably makes sense—since grouping by undefined isn’t very clean—it does appear to be a breaking change, violating semantic versioning.

If this change is considered intentional, it should also be reflected in the type system: the grouper function should not be allowed to return undefined.

We’d like to clarify the intended direction:

• Was this change considered a bug fix?
• Is there any possibility that the previous behavior might be restored (i.e., loosening the new sanity check)?
• Or should we assume this is the expected behavior going forward and update our code accordingly?

Thanks in advance for your guidance, and thanks for maintaining the project!

[jdeniau]

Hi,

Thanks for the bug report. I added that because I trusted typescript, but as immutable accept any value as key, having undefined as key should not be a problem.

See https://immutable-js.com/play/#IE1hcChbW3VuZGVmaW5lZCwgJ2EnIF1dICkKLmdldCh1bmRlZmluZWQp

TS might be more strict on undefined values though. I will check that, but it's clearly a regression.
I do not have access to a computer in the next few days though.

[giggo1604]

Hi,

Thanks for your quick answer.

I added a quick fix and a test that caught the regression. Used @ts-expect-error for now since doing a proper fix would probably be a lot more work.

Long-term, it might make more sense to disallow undefined keys and introduce a breaking change. As this is more in line with how typescript works.

[jdeniau]

Output diff failing test is under control as we change a TS file that does change the builded output.
This has been set to check that file migration from JS to TS does not alter the output in v5.

[jdeniau]

Thanks, this has been released in 5.1.2