How to select a key from a discriminated union type?-CodePudding

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?

CodePudding user response：

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 Values<T> = T[keyof T]

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

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

CodePudding user response：

In order to get the required key, you have to somehow tell the Typescript compiler

Hey compiler, I want to know about this object, under a very specific case

In this case, you want to know about the object, when the type ==='anon'. So, taking your example,

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

type SpclCase = MyDUnion & {type: "anon"}

By doing so, you're getting information about when theses two cases overlap. Now you can just index the name key directly.

type Name = SpclCase['name']

If you want to make this more robust, we can make sure that the narrowing type we use ({type: 'anon'} in this case), satisfies the required shape. Here's a little impromptu solution for that.

type DiscriminatorObj = { type: 'anon' }
type RequiredCase = DiscriminatorObj extends Pick<MyDUnion, "type"> ? DiscriminatorObj: never;
type SpclCase = (RequestBody & RequiredCase);
type Name = SpclCase['name']

I know its a little rough around the edges, but you can always extract this into generic functions and use them as you please. I merely showed you the fundamental logic. You can use it to make it more aesthetically pleasing.