In my service, I am using a `http` post. I want to set the URL as a constant. 

```
return this.http.get(this.config.API_URL+'users', options).map(res=>res.json());

```

I tried with a service:

```
import {Injectable} from '@angular/core';

@Injectable()
export class AppService {
  API_URL :String;

  constructor() {
    this.API_URL = 'some url';
  }
}

```

Is there any other method to make a constant value in Angular4 ?