将静态属性的类型设置为实例类型的数组

Question

I have a class Foo that has a static instances property holding references to all created instances. Then, I have a class Bar extending Foo :

class Foo {
    static instances: Foo[];

    fooProp = "foo";

    constructor() {
        let ctor = this.constructor as typeof Foo;
        ctor.instances.push(this);
    }
}

class Bar extends Foo {
    barProp = "bar";
}

However, the type of Bar.instances is Foo[] , rather than Bar[] , which causes the following error:

let foo = new Foo();
let bar = new Bar();

Foo.instances[0].fooProp; // works
Bar.instances[0].barProp; // Property 'barProp' does not exist on type 'Foo'. ts(2339)

I tried using InstanceType<this> for the type of instances , but it still wouldn't work:

class Foo {
    static instances: InstanceType<this>[]; // A 'this' type is available only in a non-static member of a class or interface. ts(2526)
    ...
}

It looks like my problem is related to this issue in the TypeScript repo regarding a polymorphic "this". Is this true and what I want to do is currently impossible, or I'm missing sometihng?

Playground link here .

Answer 1

TypeScript has correctly identified the error. In your case, static instances array will hold instances of Foo and Bar alike.

Consider adding this to your example and examine the logs:

console.log(Foo.instances[0] instanceof Foo); // true
console.log(Foo.instances[0] instanceof Bar); // false, barProp will not exist on this instance

console.log(Foo.instances[1] instanceof Foo); // true
console.log(Foo.instances[1] instanceof Bar); // true, barProp would be 'bar'

Playground link

If you want to store all instances of Foo in one array and all instances of Bar in another, you need to override the static instances in the Bar class:

class Bar extends Foo {
    static instances: Bar[] = []

    barProp = 'bar';
}

console.log(Foo.instances.length); // 1
console.log(Foo.instances[0].fooProp); // 'foo'

console.log(Bar.instances.length); // 1
console.log(Bar.instances[0].barProp); // 'bar'

Playground link

Answer 2

That Bar is extending Foo doesn't mean, that already declared variables change their type. So the static variable 'instance' will always be of type Foo[] because you declared it as that in your Foo class. I think 'this' will not work, because 'this' refers to an object and in the static context their is no object to which this can point to.