How do I create a new object of type "T" in a generic class in typescript?
I have the following service
@Injectable()
export class CollectionService<T> {
constructor(protected http: Http) {}
factory<T>(item?: any): T {
let type: new (item?: any) => T;
return new type(item);
}
item(id): Observable<T> {
return this.http.get(`${this.baseUrl}/${id}`)
.map((resp: Response)=> this.factory(resp.json()))
.catch((error: any) => {
return Observable.throw(error);
});
}
}
Compilation completed successfully, but I have the following error in the browser console.
TypeError: type is not a constructor
at PortalVideoService.webpackJsonp.../../../../../src/app/services/collection/collection.service.ts.CollectionService.factory (http://localhost:4200/main.bundle.js:1568:16)
at http://localhost:4200/main.bundle.js:1580:64
at Array.map (native)
at MapSubscriber.project (http://localhost:4200/main.bundle.js:1580:29)
at MapSubscriber.webpackJsonp.../../../../rxjs/operator/map.js.MapSubscriber._next (http://localhost:4200/vendor.bundle.js:24034:35)
at MapSubscriber.webpackJsonp.../../../../rxjs/Subscriber.js.Subscriber.next (http://localhost:4200/vendor.bundle.js:13455:18)
at XMLHttpRequest.onLoad (http://localhost:4200/vendor.bundle.js:101703:38)
at ZoneDelegate.webpackJsonp.../../../../zone.js/dist/zone.js.ZoneDelegate.invokeTask (http://localhost:4200/polyfills.bundle.js:2838:31)
at Object.onInvokeTask (http://localhost:4200/vendor.bundle.js:93420:37)
at ZoneDelegate.webpackJsonp.../../../../zone.js/dist/zone.js.ZoneDelegate.invokeTask (http://localhost:4200/polyfills.bundle.js:2837:36)
How do I create an object of type T?
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The name T
only exists at compile time. You can use it for type checking, but you cannot create an object of type T unless you have access to the constructor.
Modify the definition factory
to use the runtime type constructor as an argument:
factory<T>(type: {new(): T}, item?: any): T {
return new type(item);
}
Now the only problem will be where you get the type argument; you probably also need to pass it to the method item()
so the compiler knows what type to make Observable<T>
.
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You can pass the constructor as a generic class rather than a method.
export class CollectionService<T, CT extends { new(item?: any): T }> {
constructor(protected http: Http, private type: CT) {}
factory(item?: any): T {
return new this.type(item);
}
// ...
}
class CustomClass {
constructor(private item?: any) {
}
};
let cs = new CollectionService<CustomClass, { new(): CustomClass }>(http, CustomClass);
console.dir(cs.factory(123));
// UPDATE
class CustomClassService extends CollectionService<CustomClass, { new(): CustomClass }> {
constructor(protected http: Http) {
super(http, CustomClass);
}
}
let ccs = new CustomClassService(http);
console.dir(ccs.factory(456));
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