[#12588] Add unit tests to DatePickerFormatter (#12593)

* Add unit tests to datepickerformatter

* Update wording for comment regarding date

---------

Co-authored-by: Jason Qiu <jason_qiu@hotmail.com>

src/web/app/components/datepicker/datepicker-formatter.spec.ts

@@ -0,0 +1,43 @@
+import { NgbDateStruct } from '@ng-bootstrap/ng-bootstrap';
+import { DatePickerFormatter } from './datepicker-formatter';
+
+describe('DatepickerFormatter', () => {
+    let formatter : DatePickerFormatter;
+
+    beforeEach(() => {
+        formatter = new DatePickerFormatter();
+    });
+
+    it('should create an instance', () => {
+        expect(formatter).toBeTruthy();
+    });
+
+    it('should return an empty string if date is null', () => {
+        // Any is used as the type for date here as it is the only way to pass in null to NgbDateStruct
+        const date: any = null;
+        const formattedDate: String = formatter.format(date);
+        expect(formattedDate).toEqual('');
+    });
+
+    it('should return a properly formatted date string', () => {
+        const date: NgbDateStruct = { year: 2023, month: 12, day: 12 };
+        const formattedDate = formatter.format(date);
+        expect(formattedDate).toEqual('Tue, 12 Dec, 2023');
+    });
+
+    it('should parse the valid date string correctly', () => {
+        const date : string = 'Tue, 12 Dec, 2023';
+        const parsedDate : NgbDateStruct = formatter.parse(date);
+        expect(parsedDate.day).toEqual(12);
+        expect(parsedDate.month).toEqual(12);
+        expect(parsedDate.year).toEqual(2023);
+    });
+
+    it('should return NaN for all the fields if invalid date string format is parsed', () => {
+        const date : string = '12th December 2023';
+        const parsedDate : NgbDateStruct = formatter.parse(date);
+        expect(parsedDate.day).toEqual(NaN);
+        expect(parsedDate.month).toEqual(NaN);
+        expect(parsedDate.year).toEqual(NaN);
+    });
+});