[tshaddox]

It’s not a perfect mapping with JSON. Everyone knows that stuff like functions and dates can’t go over JSON, but there are also subtler things, like the fact that undefined can’t exist as a value in JSON. I’ve seen codebases get pretty mixed up about the semantics of things like properties being optional versus T | undefined.

[nrclark]

> Everyone knows that stuff like functions and dates can’t go over JSON, but there are also subtler things, like the fact that undefined can’t exist as a value in JSON

I use `null` for that purpose, and it's been pretty reliable. What are the situations where that falls down?

[tshaddox]

I also like null | T for required properties, but for whatever reason I have seen that undefined | T is a much more common convention in the TypeScript world. Maybe the reason for that is the semantic about how object access returns undefined, but that’s precisely the source of ambiguity between “object has property X with value undefined” and “object does not have property X”.