fix: throw better error message on invalid date formatter

Closes #897

Author: mattlewis92

projects/angular-calendar/src/modules/common/calendar-date.pipe.ts

@@ -25,6 +25,16 @@ export class CalendarDatePipe implements PipeTransform {
     excludeDays: number[] = [],
     daysInWeek?: number
   ): string {
+    if (typeof this.dateFormatter[method] === 'undefined') {
+      const allowedMethods = Object.getOwnPropertyNames(
+        Object.getPrototypeOf(CalendarDateFormatter.prototype)
+      ).filter(iMethod => iMethod !== 'constructor');
+      throw new Error(
+        `${method} is not a valid date formatter. Can only be one of ${allowedMethods.join(
+          ', '
+        )}`
+      );
+    }
     return this.dateFormatter[method]({
       date,
       locale,

projects/angular-calendar/test/calendar-date.pipe.spec.ts

@@ -95,4 +95,15 @@ describe('calendarDate pipe', () => {
       weekStartsOn: 0
     });
   });
+
+  it('should throw when an invalid method is passed', () => {
+    const fixture: ComponentFixture<TestComponent> = TestBed.createComponent(
+      TestComponent
+    );
+    fixture.componentInstance.date = new Date('2016-01-01');
+    fixture.componentInstance.method = 'invalid';
+    expect(() => fixture.detectChanges()).to.throw(
+      /^invalid is not a valid date formatter. Can only be one of/
+    );
+  });
 });