Typescript: What's the difference between an optional field and a union with undefined?

Question

I want to define an interface with optional fields. I thought the following were synonymous.

(A):

interface State {
  userDetails: unde         


        

Answer 1

Even at runtime there is a difference between a key with an undefined value and a key that doesn't exist. That TypeScript allows differentiating is not surprising.

Try this:

Object.keys({a: undefined});

If leaving a out and setting it's value to be undefined were the same thing then we'd expect the above to return an empty array, but in fact it returns

[ 'a' ]