Utilizing TypeScript to instantiate a generic type constructor

Question

Utilizing TypeScript to instantiate a generic type constructor

Here is a snippet of code that I am working with:

  public getProducts<T>(): Observable<T[]> {
    return this.httpClient
      .get<T[]>(this.baseUrl + '/users')
      .pipe(
        map((entities) => {
          return entities.map((entity) => {
            return new T(entity);
          })
        }),
        catchError((err) => Observable.throw(err))
      );
  }

I encountered an issue with this line: return new T(entity);

Can anyone provide guidance on how to instantiate a generic type constructor in TypeScript?

angular typescript ionic-framework ionic3

Answer 1

Answer №1

T serves as a placeholder, noting that types are removed at runtime, making them unable to be present in expressions.

To resolve this, you should provide the constructor of T.


 public fetchItems<T>(ctor: new (item: Partial<T>) => T): Observable<T[]> {
    return this.httpClient
      .get<T[]>(this.apiUrl + '/items')
      .pipe(
        map((results) => {
          return results.map((result) => {
            return new ctor(result);
          })
        }),
        catchError((error) => Observable.throw(error))
      );
  }

This assumes that there is a defined class for T. If no class exists, omit the use of new T. In case T represents an interface, the objects received from the server should align with the interface requirements without additional intervention.

Answer 2

T serves as a placeholder, noting that types are removed at runtime, making them unable to be present in expressions.

To resolve this, you should provide the constructor of T.


 public fetchItems<T>(ctor: new (item: Partial<T>) => T): Observable<T[]> {
    return this.httpClient
      .get<T[]>(this.apiUrl + '/items')
      .pipe(
        map((results) => {
          return results.map((result) => {
            return new ctor(result);
          })
        }),
        catchError((error) => Observable.throw(error))
      );
  }

This assumes that there is a defined class for T. If no class exists, omit the use of new T. In case T represents an interface, the objects received from the server should align with the interface requirements without additional intervention.

Utilizing TypeScript to instantiate a generic type constructor

Answer №1

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