I've investigated this a bit. The problem seems to be:

1. On line 784, it checks whether the new value is the same as the existing value. If it is, it considers no need to do anything. In this case, the existing value is the not-set-value. So it SHOULD still do something because the not-set isn't there yet.
2. On line 98 if no value is set, it returns undefined. It should just return itself.

This is an edge case to how update works with identity functions and default values.

See documentation discussion: http://facebook.github.io/immutable-js/docs/#/Map/updateIn

If the updater function returns the same value it was called with, then no change will occur. This is still true if notSetValue is provided.

@tolmasky thanks for investigating, I added some additional tests for this case and unfortunately I think the changes you made would have broken them. Specifically, that first line you reference is handling the identity function case, and the second line - NOT_SET implies the value doesn't exist anymore, so actually notSetValue should be used there instead of this.

If the current behavior is what is intended, then I must say it is very surprising.

I see that something is indeed mentioned under "updateIn", but the documentation for Map.update does not mention this. It should be mentioned there as well, and the line saying that update is

Equivalent to: map.set(key, updater(map.get(key, notSetValue))).

should be removed, since it's clearly not true.

Thank you

I agree this corner isn't a great one. I'm hoping to improve it in a future major version. In the meantime these are great documentation suggestions