testing with Jasmine and Karma

cart.component.spec.ts

import { CartComponent } from './cart.component';
import { ComponentFixture, TestBed } from '@angular/core/testing';
import { HttpClientTestingModule } from '@angular/common/http/testing'
import { BookService } from '../../services/book.service';
import { CUSTOM_ELEMENTS_SCHEMA, NO_ERRORS_SCHEMA } from '@angular/core';

    describe('cart.component', () => {
        let component: CartComponent;//declaramos el componente de tipo que queremos testear y lo importamos
        let fixture: ComponentFixture<CartComponent>;//declaramos una variable para poder extraer el servicio que requiere el tipo que                                                          va a tener. 

        beforeEach(() => {
            TestBed.configureTestingModule({
                imports:[
                    HttpClientTestingModule
                ],
                declarations:[
                    CartComponent
                ],
                providers:[
                    BookService
                ],
                schemas:[CUSTOM_ELEMENTS_SCHEMA, NO_ERRORS_SCHEMA]
            }).compileComponents();
      });
      beforeEach(() => {//to instantiate the component
          fixture = TestBed.createComponent(CartComponent);
          component = fixture.componentInstance;
          fixture.detectChanges();//el component entrara por el método onInit

      }); 
      it('should create', () => {
          //esperamos a que ocurra algo con expect. Que el componet este instanciado correctamente
          expect(component).toBeTruthy();
          //expect(component).toBeFalse();
      });
      //provamos a lanzar el test
    });

cart.component.spec2.ts

//Method to test comes from cart.component.ts
    /*public getTotalPrice(listCartBook: Book[]): number {
        let totalPrice = 0;
        listCartBook.forEach((book: Book) => {
          totalPrice += book.amount * book.price;
        });
        return totalPrice;
      }*/
    //to test this method we create a const of the array
    const listBook: Book[] = [
    {
        name: '',
        author: '',
        isbn: '',
        price: 15,
        amount: 2
    },
    {
        name: '',
        author: '',
        isbn: '',
        price: 30,
        amount: 5
    },
    {
        name: '',
        author: '',
        isbn: '',
        price: 9,
        amount: 3
        }
];
    it('getTotalPrice return an amount', () => {
        const totalPrice = component.getTotalPrice(listBook);
        //this method returned a value greater than 0.
       // expect(totalPrice).toBeGreaterThan(0);
       //with this method you will check that it is not 0. You can expect several options.
        expect(totalPrice).not.toBe(0);
        //expect(totalPrice).toBeNull();
    })

cart.component.ts

import { Component, OnInit } from '@angular/core';
import { BookService } from '../../services/book.service';
import { Book } from '../../models/book.model';

@Component({
  selector: 'app-cart',
  templateUrl: './cart.component.html',
  styleUrls: ['./cart.component.scss']
})
export class CartComponent implements OnInit {

  public listCartBook: Book[] = [];
  public totalPrice = 0;
  public Math = Math;

  constructor(
    private readonly _bookService: BookService
  ) { }

  ngOnInit(): void {
    this.listCartBook = this._bookService.getBooksFromCart();
    this.totalPrice = this.getTotalPrice(this.listCartBook);
  }

  public getTotalPrice(listCartBook: Book[]): number {
    let totalPrice = 0;
    listCartBook.forEach((book: Book) => {
      totalPrice += book.amount * book.price;
    });
    return totalPrice;
  }

  public onInputNumberChange(action: string, book: Book): void {
    const amount = action === 'plus' ? book.amount + 1 : book.amount - 1;
    book.amount = Number(amount);
    this.listCartBook = this._bookService.updateAmountBook(book);
    this.totalPrice = this.getTotalPrice(this.listCartBook);
  }

  public onClearBooks(): void {
    if (this.listCartBook && this.listCartBook.length > 0) {
      this._clearListCartBook();
    } else {
       console.log("No books available");
    }
  }

  private _clearListCartBook() {
    this.listCartBook = [];
    this._bookService.removeBooksFromCart();
  }
}