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?