Edit in JSFiddle

// file product.model.ts

export interface Product {
  price: number;
  name: string;
};

//file product.service.ts - Correct Approach

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

import { Hero } from './product.model;';

@Injectable()
export class ProductCheckoutService {
  // has-a relation
  product: Product;

  constructor() {}
}

//file product.service.ts - Weird incorrect approach, let always avoid!!!

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

import { Hero } from './product.model;';

@Injectable()
export class ProductCheckoutService implements Product {
  // is-a relation
  price: number;

  name: string;

  constructor() {}
}