Suggestion: Improve type of constructor on the instance type of a class

[rbuckton]

Search Terms

constructor type

Suggestion

This was previously discussed in this thread: DefinitelyTyped/DefinitelyTyped#36660 (comment)

I could see a possible future mechanism for this.constructor that returned the static side of the containing class without call or construct signatures (but retaining the apparent type of Function).

Currently, the type of the constructor property on most objects is Function. It has been suggested that for any class C {}, the type of the constructor property on the instance should be typeof C. However this suffers a significant drawback in that it severely limits subclasses as any subclass of C must have the same (or compatible) construct signatures as C.

Here is an example of this issue: https://www.typescriptlang.org/play/index.html#code/MYGwhgzhAEDKCuAHApgJwMLitA3gKGmjAC5oIAXVASwDsBzAbgOmAHsaLV5hzXUAKEmUq06ASlzNC5ABZUIAOjDQAvESaEAvnm15QkGAgBGmA9GQAPcshoATQ0jSns+QkdKdRGlu07deAqyI5BCkOEQeIvQANNDuwtT00JoSroRkjoHBimBi3tJyikaq0EEhCkbe2rq01qgAZmDAyHCZzjBphGwclP58pOQAniis9a0oGFgQTDU0dY3NrSZTkuk+PVw8-dBDI2PG7TNAA

Instead, I would suggest a mechanism to type constructor as all of the static members of typeof C but none of the call/construct signatures of typeof C, yet still having an apparent type of Function.

Use Cases

In @ljharb's qs, he'd like to be able to use the constructor property of a Buffer-like object to access the isBuffer method on the constructor in a type-safe way (i.e. obj.constructor.isBuffer).

Examples

/// <reference types="node" />
function isBuffer(obj: { constructor: { isBuffer(obj: any): obj is Buffer; } }) {
  return obj.constructor.isBuffer(obj);
}
const buf = Buffer.alloc(10);
isBuffer(buf); // Buffer class would have a constructor that is `Buffer`

Workaround

There exists a possible workaround for this currently, though it is somewhat complicated:

type StaticMembers<TClass extends Function> = Pick<TClass, keyof TClass> & Function;

class Buffer extends Uint8Array {
  ...
  static isBuffer(obj: any): obj is Buffer;
  ...
}

interface Buffer {
  readonly constructor: StaticMembers<typeof Buffer>;
}

Playground link: https://www.typescriptlang.org/play/index.html#code/MYGwhgzhAEDKCuAHApgJwMLitA3gKGmjAC5oIAXVASwDsBzAbgOmAHsaLV5hzXUAKEmUq06ASlzNC5ABZUIAOjDQAvESaEAvnm15QkGAgBGmA9GQAPcshoATQ0jSns+QkdKdRGlu07deAqyI5BCkOEQeIvQANNDuwtT00JoSroRkjoHBimBi3tJyikaq0EEhCkbe2rrkAJ4ocORg5FTAALLIALZGaBAAPAAqzjCW1nYwAGLwNDxU7AB8JQAKrQDWg8Oxq8i1rABm0ENYEIsAZNBTMy3sTHi01qh7YMDIcJnDkuk+HJT+fKSwJotdpdHqofp1FD7N4oDDHeZMXT3NBPF5vEzHT7pNg-Lg8f6NZqtDrdXp9SHIaHGYYInRAA

Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
  • Whether this is a breaking change needs to be tested.
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

[ljharb]

That sounds great; I'm not concerned with actually constructing or calling the constructor; just about using .constructor to get at static properties.

[fatcerberus]

this suffers a significant drawback in that it severely limits subclasses as any subclass of C must have the same (or compatible) construct signatures as C

Would it be feasible to simply ignore .constructor when doing a structural type check between known class types? AFAIK .constructor is non-writable and non-configurable so the compiler seems to be in a good position there--we already know a priori that it refers back to the class.

edit: I was being stupid, ignore this.

[ljharb]

@fatcerberus It's very much writable and configurable, everywhere.

[fatcerberus]

Hmm, you're right:

> Object.getOwnPropertyDescriptor(C.prototype, 'constructor')
{
  value: [Function: C],
  writable: true,
  enumerable: false,
  configurable: true
}

I could have sworn I remembered it being nonconfigurable. I guess I was thinking of the prototype itself:

> Object.getOwnPropertyDescriptors(C)
{
  length: { value: 0, writable: false, enumerable: false, configurable: true },
  prototype: {
    value: C {},
    writable: false,
    enumerable: false,
    configurable: false
  },
  name: {
    value: 'C',
    writable: false,
    enumerable: false,
    configurable: true
  }
}

My mistake, carry on 👍

[canonic-epicure]

This will definitely improve the quality of life of those who use static class methods. Currently one have to either cast the constructor property manually, or apply various workarounds as in the example.

What is the point of having the static methods, which you can't (according to the type-checker) call from instances?

[canonic-epicure]

As always there are lengthy discussions about the simple, quality of life proposal:

for every class:

class SomeClass {
}

define the type of constructor property as typeof SomeClass

interface SomeClass {
    readonly constructor : typeof SomeClass
}

Can we have this simple improvement?

[canonic-epicure]

This could be easily done with a macros. Then people would start using it right now, finding corner cases, making evaluation. At some point it would be clear if its good change for everyone or its someones, very specific requirement. In the 1st case this "macros" could be included in the TS core, in the 2nd - nobody cares and TS team does not have to care as well.

But TS team does not want to support macroses out of the box to allow language experiments in the user space. Instead they want to proceed through lengthy discussions, which lasts for years.

[Araxeus]

I've encountered problems with accessing static private fields
detailed in #48476

class Test {
    static #staticPrivateField = 'test' ; // (1) #staticPrivateField' is declared but its value is never read.ts(6133)

    constructor() {
        console.log(
            this.constructor.#staticPrivateField // (2) Property '#staticPrivateField' does not exist on type 'Function'.ts(2339)
        )
    }
}

new Test();

note: (1) is a warning that shows up in vscode even if the file is .js, which is pretty annoying

Please fix? 😬

[rbuckton]

Keep in mind this example wouldn't work if you subclass Test, since the subclass constructor will not have that private field. It's much safer to reference Test.#staticPrivateField in this case.

[hasezoey]

I would also be interested in this so i can generically get the static version without typeof Class everywhere, with for example:

type TypeofClass<T extends { constructor: new (...args: any[]) => any }> = T extends { constructor: infer S } ? S : never;

[mmmmmrob]

Another vote for typing obj.constructor for classes to allow access to static members of the class.