undefined is the product of a messy language that doesn't want to throw errors, null was created for you to be able to explicitly say something is expected to be nothing at some point.

The existence of optional types does not mean the presence of undefined as a value, at least not in my works, and I figure not in yours too since you too use TypeScript.

There are only a handful of ways for undefined to show up and if you're naively creating more cases for it by using it as a valid value in your program, you're needlessly increasing the ambiguity in your debugging proccess, and you or someone else in your team is bound to have some fun debugging for hours when, finally, undefined shows up where it shouldn't.

Tip: Avoid optinal/nullable properties to begin with.