No way to get generic type of an abstract class inside a factory function

TypeScript Version: 3.7.3

Search Terms: abstract class, generic, infer

Code

type InstanceViaNew<T> = T extends { new (...args: never[]): infer R } ? R : never
type InstanceViaProto<T> = T extends { prototype: infer R } ? R : never

function classFactory1<T>() {
  class CS {
    x!: T
  }

  return CS
}

const CSS = classFactory1<string>()
type CS1 = InstanceViaNew<typeof CSS>["x"] // ok, string
type CS2 = InstanceViaProto<typeof CSS>["x"] // not ok, any :(

function classFactory2<T>() {
  abstract class ACS {
    x!: T
  }

  return ACS
}

const ACS = classFactory2<string>()

type AC1 = InstanceViaNew<typeof ACS>["x"] // not ok, never (because abstract classes have no new) :(
type AC2 = InstanceViaProto<typeof ACS>["x"] // not ok, any :(

Expected behavior:

There should be a way to make abstract classes keep the generic information and use it.

Actual behavior:

There's no way to make abstract classes keep the generic information and use it (unless they are type-casted to non abstract classes by adding to them a fake constructor).

Related Issues: #26829

I have to add that if in classFactory2 the return statement is changed to

return ACS as typeof ACS & {prototype: typeof ACS }

Then oddly enough the "InstanceViaNew" works, yet "InstanceViaProto" still doesn't (I'd have expected the opposite)

InstanceViaProto ought to work here; we're probably failing to apply the type mapper in the right place?

Ron Buckton (@rbuckton)

Ron Buckton (@rbuckton) I think the PR will certainly make InstanceViaNew work (nice!), but I guess it wouldn't fix the InstanceViaProto case?

Also I found out that if you do something like this:

function classFactory2<T>() {
  abstract class ACS {
    constructor(_y: number) {}
    x!: T
  }

  return ACS
}

const ACS = classFactory2<string>()

// this doesn't work because the abstract has no constructor (expected I guess)
type AC3 = InstanceType<typeof ACS>["x"] // Type 'typeof ACS' does not satisfy the constraint 'new (...args: any) => any'
type AC4 = ConstructorParameters<typeof ACS> // Type 'typeof ACS' does not satisfy the constraint 'new (...args: any) => any'.

// but the intersection of the type with anything makes it somehow work... ?
// as if the intersection made the class get "unabstracted"
type AC1 = InstanceType<(typeof ACS) & {whatever: any}>["x"] // weirdly enough this gives string
type AC2 = ConstructorParameters<(typeof ACS) & {whatever: any}> // weirdly enough this gives [number]

Navigation Menu

Repository navigation

No way to get generic type of an abstract class inside a factory function #35576

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

No way to get generic type of an abstract class inside a factory function #35576

Description

Activity

xaviergonz commented on Dec 9, 2019

RyanCavanaugh commented on Jan 22, 2020

xaviergonz commented on Jan 24, 2020

xaviergonz commented on Jan 24, 2020

Metadata

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

Issue actions