Since switching to TypeScript I have been using a lot of optional properties, for example:

type store = {
  currentUserId?: string
}

function logout () {
  store.currentUserId = undefined
}

However my coworkers and I have been discussing whether null is a more appropriate type instead of undefined, like this:

type store = {
  currentUserId: string | null
}

function logout () {
  store.currentUserId = null
}

It seems like the use of undefined in TypeScript differs slightly from in Javascript.

Do you guys/girls use undefined or null more often? And, which of the examples above do you think is better?

Hey! What are your thoughts on explicit returning undefined in a function vs returning null, this in the absence of what you asked the function for.
Let's take these functions as an example
type getUser = (id: string): User | undefined vs type getUser = (id: string): User | NULL
What are your thoughts on this? Good and bad etc