CODEWITHSUNDEEP
typescript
Jaime Still
Jaime Still

Reputation: 1998

Generic Object.assign

Scenario
I know the shape of the data that is returning from an API call, and I want it to have some extended functionality via a TypeScript class. I already know that I can do the following to fulfill this requirement: 

this.http
    .get(url)
    .map((res: Response) => {
        return res.json().map((obj: Type) => {
            return Object.assign(new Type(), obj);
        });
    })
    // And so on

Would it be possible to make a generic implementation of this similar to the following: 

get<T>(url: string): Observable<T> {
    return this.http
        .get(url)
        .map((res: Response) => {
            return res.json().map((obj: T) => {
                return Object.assign(new T(), obj); // not sure how to approach this
            });
        });
}

The above works for returning a straight up JavaScript Object, but will I'm not sure how to run assign on a generic type.

Upvotes: 5

Views: 2471

Answers (1)

Jaime Still
Jaime Still

Reputation: 1998

I was able to make it work by creating a factory class for new generic types. If anyone else can propose a better alternative, I'll mark it as the answer.

Here's what I did:

factory.ts 

export class Factory<T> {
  constructor(private type: new () => T) { }

  getNew(): T {
    return new this.type();
  }
}

app.service.ts 

@Injectable()
export class AppService {
  users = new BehaviorSubject<User[]>([]);

  constructor(public http: Http) { }

  getUsers() {
    const userFactory = new Factory<User>(User);    
    this.http.get('https://jsonplaceholder.typicode.com/users')
      .map((res: Response) => this.extractGeneric<User>(res, userFactory))
      .catch(this.handleError)
      .subscribe(
        data => this.users.next(data),
        err => console.log(err)
      )
  }

  extractGeneric<T>(res: Response, factory: Factory<T>) {
    return res.json().map((obj: T) => {
      return Object.assign(factory.getNew(), obj);
    });
  }  

  handleError(error: Response | any) {
    let errMsg: string;    
    if (error instanceof Response) {
      const body = error.json() || '';
      const err = body.error || body.Message || JSON.stringify(body);
      errMsg = `${err}`;
    } else {
      errMsg = error.message ? error.message : error.toString();
    }    
    console.error(errMsg);
    return Observable.throw(errMsg);
  }
}

See this project on StackBlitz

Upvotes: 5

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