Injected dependency is undefined when extending BaseRequestOptions

Question

I am extending `BaseRequestOptions` in Angular2 to add headers for each request. I also have a `Config` class that supplies key/value pairs based on the domain, which I need to inject into my derived class: ``` import { BaseRequestOptions } from '@angular/http'; import { Config } from '../../config/configuration'; export class DefaultRequestOptions extends BaseRequestOptions { constructor(private config: Config) { super(); Object.keys(this.config.api.headers).map((key) => { this.headers.append(key, this.config.api.headers[key]); }); } } ``` In my module, I am specifying the provider as such: ``` @NgModule({ . . ., providers: [ . . ., { provide: RequestOptions, useClass: DefaultRequestOptions } ] }) . . . ``` The problem I'm having is that `this.config` is `undefined` in `DefaultRequestOptions`. I am also using the `Config` class as an injected dependency in other classes, so I'm confident it's working as expected, and if I manually set the values for `this.headers` everything works fi…

Accepted Answer

Provider classes don't require `@Injectable()` unless they require constructor parameters to be injected. In your case it does. This is why Angular recommends to always use it on your providers, whether or not you require injections. Maybe for this exact reason, where you forget, when it's needed.

```
@Injectable()
export class DefaultRequestOptions extends BaseRequestOptions {
```

After testing

It seems this still doesn't work. I've seen this problem is a result of extending a class that already uses `@Injectable()` (which `BaseRequestOptions` does). In this case, the parent constructor is called instead of the extending class'. If you extend `RequestOptions` instead (which is not decorated with `@Injectable()`) then it would work

```
@Injectable()
class DefaultRequestOptions extends RequestOptions {
  constructor(public config: Config) {
    super({method: RequestMethod.Get, headers: new Headers()})

this.headers.append('data', this.config.data);
  }
}
```

Notice the `super()` call. This is all that `BaseRequestOptions` does.

If you wanted to keep it the way it currently is, using `BaseRequestOptions`, then you could use a factory, instead of letting Angular create it

```
{
  provide: RequestOptions,
  deps: [ Config ],
  useFactory: (config: Config) => {
    return new DefaultRequestOptions(config);
  }
}
```

Injected dependency is undefined when extending BaseRequestOptions

Solution

Problem