books.service.ts

import { Injectable, HttpException } from '@nestjs/common';
import { BOOKS } from '../mocks/books.mock';
@Injectable()
export class BooksService {
    books = BOOKS;
    ...
    addBook(book): Promise {
        return new Promise(resolve => {
            this.books.push(book);
            resolve(this.books);
        });
    }
}