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Polymorphic "this" for static members

See original GitHub issue

When trying to implement a fairly basic, but polymorphic, active record style model system we run into issues with the type system not respecting this when used in conjunction with a constructor or template/generic.

I’ve posted before about this here, #5493, and #5492 appears to mention this behavior also.

And here is an SO post this that I made:

I have recycled my example from #5493 into this ticket for further discussion. I wanted an open ticket representing the desire for such a thing and for discussion but the other two are closed.

Here is an example that outlines a model Factory which produces models. If you want to customize the BaseModel that comes back from the Factory you should be able to override it. However this fails because this cannot be used in a static member.

// Typically in a library
export interface InstanceConstructor<T extends BaseModel> {
    new(fac: Factory<T>): T;

export class Factory<T extends BaseModel> {
    constructor(private cls: InstanceConstructor<T>) {}

    get() {
        return new this.cls(this);

export class BaseModel {
    // NOTE: Does not work....
    constructor(private fac: Factory<this>) {}

    refresh() {
        // get returns a new instance, but it should be of
        // type Model, not BaseModel.
        return this.fac.get();

// Application Code
export class Model extends BaseModel {
    do() {
        return true;

// Kinda sucks that Factory cannot infer the "Model" type
let f = new Factory<Model>(Model);
let a = f.get();

// b is inferred as any here.
let b = a.refresh();

Maybe this issue is silly and there is an easy workaround. I welcome comments regarding how such a pattern can be achieved.

Issue Analytics

  • State:open
  • Created 8 years ago
  • Reactions:295
  • Comments:181 (26 by maintainers)

github_iconTop GitHub Comments

paul-gocommented, Feb 8, 2016

Is there a reason this issue was closed?

The fact that polymorphic this doesn’t work on statics basically makes this feature DOA, in my opinion. I’ve to date never actually needed polymorphic this on instance members, yet I’ve needed every few weeks on statics, since the system of handling statics was finalized way back in the early days. I was overjoyed when this feature was announced, then subsequently let down when realizing it only works on instance members.

The use case is very basic and extremely common. Consider a simple factory method:

class Animal
    static create(): this
        return new this();

class Bunny extends Animal

Bunny.create().hop() // Type error!! Come on!!

At this point I’ve been either resorting to ugly casting or littering static create() methods in each inheritor. Not having this feature seems like a fairly large completeness hole in the language.

FishOrBearcommented, Dec 18, 2018

Three years

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