Typescript passes type differently to function when using generic

Question

I have the following piece of code

// Address and User are two classes, bothe of which have an id
type Q = Address | User

// This works just fine 
async function c<EntityType = Q>(ids: Q["id"][]) {
}

// This gives me the error described bellow
async function c<EntityType = Q>(ids: EntityType["id"][]) {
}

The problem I get if I use the second function definition is:

Type '"id"' cannot be used to index type 'EntityType'

Further when I in vscode hover over ids in the first function I get (parameter) ids: number[] (Which is expected as Q.id is a number) but if I hover over ids in the second function I get ids: EntityType["id"][]

How can I fix this problem??

A call like c<{hello: string}>(someArray) will not work with the second definition. Or even c<string>(someArray). Remember that it's the caller who determines the generic type argument. You cannot assume that it will be any specific thing when you have not constrained it. — VLAZ
– VLAZ, Commented Oct 11, 2022 at 20:08
@VLAZ would you mind elaborating how I could constrain this in my related post here stackoverflow.com/questions/74033593/…? It seems I made a wrong assumption in the post above which Dylan picked up on — sev
– sev, Commented Oct 11, 2022 at 20:11
It's exactly the same answer. You still have unconstrained EntityType in the second case. You still just need to constrain it with extends. It's basically the same problem. Just different manifestation. — VLAZ
– VLAZ, Commented Oct 11, 2022 at 20:13
a good practice that might help is instead of making Q a union of your classes, make Q an inferface which includes id: number as a property and have your classes implement Q. — Dylan Dang
– Dylan Dang, Commented Oct 11, 2022 at 20:17

Dylan Dang · Accepted Answer · 2022-10-11 20:01:09Z

1

Although you set the generic to type Q by default, the generic could still be any. To fix this, you need to limit the type of the generic by using the extends keyword like so:

async function c<EntityType extends Q>(ids: EntityType["id"][]) {
}

answered Oct 11, 2022 at 20:01

Dylan Dang

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