Being new to TypeScript, what is the best method to implement a static factory in a base class that instantiates the child class type. For instance, consider a findAll method in a base model class:
class BaseModel {
static data: {}[];
static findAll() {
return this.data.map((x) => new this(x));
}
constructor(readonly attributes) {
}
}
class Model extends BaseModel {
static data = [{id: 1}, {id: 2}];
constructor(attributes) {
super(attributes);
}
}
const a = Model.findAll(); // This is BaseModel[] not Model[]
This returns BaseModel[] rather than Model[].
To answer my own question, this turns out to be a well known issue in TypeScript. The Github issue Polymorphic this for static methods has a long discussion. The solution is as follows:
export type StaticThis<T> = { new (): T };
export class Base {
static create<T extends Base>(this: StaticThis<T>) {
const that = new this();
return that;
}
baseMethod() { }
}
export class Derived extends Base {
derivedMethod() { }
}
// works
Base.create().baseMethod();
Derived.create().baseMethod();
// works too
Derived.create().derivedMethod();
// does not work (normal)
Base.create().derivedMethod();
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