How to select a key from a discriminated union type?

Question

I have a disciminated union

type MyDUnion = { type: "anon"; name: string } | { type: "google"; idToken: string };

I want to access the name key from the discriminative union, from the MyDUnion type directly. Something like this

type Name = MyDUnion['name']

But typescript won't allow that

Property 'name' doesn't exist on type '{ type: "anon"; name: string } | { type: "google"; idToken: string }'

How can I access it?

To be clear, this is not a valid solution:

type MyName = string;
type MyDUnion = { type: "anon"; name: MyName } | { type: "google"; idToken: string };
type Name = MyName; // this line would be in a different file

This is not valid, because then I would have to export both MyName and MyDUnion types to be used elsewhere.

Any ideas?

Answer 1

In order to filter union of objects, usually you need to use Extract: The easy way:

type Result = Extract<MyDUnion , {type: "anon"}>

The more robust:

type MyDUnion = { type: "anon"; name: string } | { type: "google"; idToken: string };

type Filter<Union, Type extends Partial<Union>> = Extract<Union, Type>

type Result = Filter<MyDUnion, { type: 'anon' }>