Typescript static class generic functions

Question

Trying to understand typescript generics + static class functions a bit more thoroughly, and I am running into to some type problems:

class Base {
  value : string = 'default'

  static identity<M extends typeof Base>() : M  {
    return this // Type Error: Type 'typeof Base' is not assignable to type 'M'
  }
}

class Extended extends Base {
  otherValue = 'extended'

  constructor() {
    super()
  }
}

let baseClass = Base.identity()
let extendedClass = Extended.identity()
expect(new baseClass().value).to.eq('default')

// Type Error: Property 'otherValue' does not exist on type 'Base'
// How do I make Extended.identity() be `typeof Extended` without
// an explicit cast?
expect(new extendedClass().otherValue).to.eq('extended')

If I ignore type errors and run the output code, everything runs as expected and expectations are met. I think this is probably due to some understanding gaps with static functions, but any help in understanding the problem would be much appreciated.

jcalz · Accepted Answer · 2017-12-21 13:56:21Z

It looks like what you really want is polymorphic this on static methods, which TypeScript doesn't directly support. What it does seem to support (and it's mentioned in that Github issue), is using this parameters on the method:

class Base {
  value : string = 'default'

  // specify that this is M
  static identity<M extends typeof Base>(this: M) : M  {
    return this; 
  }
}

class Extended extends Base {
  otherValue = 'extended'

  constructor() {
    super()
  }
}

The static identity() method will only work if the object on which you call it is assignable to a subtype of the Base constructor type. The compiler apparently will use type inference to determine the right value for M when you call it. You can verify that things behave as expected:

let baseClass = Base.identity() // Base.identity<typeof Base>
let extendedClass = Extended.identity() // Base.identity<typeof Extended>

It's not well documented, and it frankly surprised me that you can actually get it to work. If you really want polymorphic this in static methods to work without the above voodoo, you might want to go to Microsoft/TypeScript#5863 and give the issue a 👍 or describe your use-case if you think it's particularly compelling.

Hope that helps; good luck!

Vincas Stonys · Accepted Answer · 2017-12-20 18:53:38Z

1

Your problem is that M is more concrete than Base, and you are trying to assign Base to M. Imagine this:

You have your class Base
Another class extends Base -> Base+A (A indicates additional properties that the less generic class has)
You invoke Base.identity method on Base+A class:
```
identity<Base+A extends typeof Base>(): Base+A {
  return this; // This is a Base class method, so it's typeof Base
}
```
Compiler does not find the A part in the return value's type, so it's throwing an error.

answered Dec 20, 2017 at 18:53

Vincas Stonys

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Comments

Kindy · Accepted Answer · 2019-06-11 11:11:12Z

0

Extends @jcalz 's answer to support instance type (works on [email protected]):

class A {
  static a<T extends typeof A>(this: T) {
    return new this();
  }

  static b<T extends typeof A, I = InstanceType<T>>(this: T): I {
    return (new this()) as I;
  }
}

class B extends A {}

let a = A.a();  // type: A
let b = B.a();  // type: A

let a2 = A.b(); // type: A
let b2 = B.b(); // type: B

edited Jun 11, 2019 at 11:11

answered Jun 11, 2019 at 10:58

Kindy

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1 Comment

Guido Tarsia Over a year ago

I'm on typescript 4.7.4, and this would work if the second generic argument of b was I extends A = InstanceType<T>.

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