Add spec for calculator service

src/app/courses/services/calculator.service.spec.ts

@@ -0,0 +1,35 @@
+import { CalculatorService } from "./calculator.service";
+import { TestBed } from "@angular/core/testing";
+import { LoggerService } from "./logger.service";
+
+describe('CalculatorService', () => {
+    let calculator: CalculatorService,
+        loggerSpy: any;
+
+    beforeEach(() => {
+        loggerSpy = jasmine.createSpyObj('LoggerService', ['log']);
+        TestBed.configureTestingModule({
+            providers: [
+                CalculatorService,
+                {provide: LoggerService, useValue: loggerSpy}
+            ]
+        });
+
+        calculator = TestBed.get(CalculatorService);
+    });
+
+    it('should add two numbers', () => {
+        const result = calculator.add(2, 3);
+
+        expect(result).toBe(5);
+        expect(loggerSpy.log).toHaveBeenCalledTimes(1);
+    });
+
+    it('should subtract two numbers', () => {
+        const result = calculator.subtract(4, 3);
+
+        expect(result).toBe(1, 'unexpected subtraction result');
+        expect(loggerSpy.log).toHaveBeenCalledTimes(1);
+    });
+
+});