ISO year was calculate depending on ISO 8601 (#431)

* ISO year was calculate depending on ISO 8601

* getIsoYearByDate and getIsoYear test cases added

src/dateRange.ts

@@ -1,4 +1,5 @@
 import {
+	getIsoYear,
 	getWeekIsoNumber,
 	getWeekIsoOfYear,
 	getWeekNumber,
@@ -273,7 +274,7 @@ export class YearWeek extends DateRange implements IYearWeekDateRange {
 
 export class YearWeekIso extends DateRange implements IYearWeekDateRange {
 	constructor(year?: number, week?: number) {
-		const actualYear = year ?? new Date().getFullYear();
+		const actualYear = year ?? getIsoYear();
 		const actualWeek = week ?? getWeekIsoOfYear();
 
 		const startDate = new Date(actualYear, 0, 1 + (actualWeek - 1) * 7);

src/weekUtils.ts

@@ -23,3 +23,12 @@ export const getWeekIsoNumber = (date: Date): number => {
 };
 
 export const getWeekIsoOfYear = (): number => getWeekIsoNumber(new Date());
+
+export const getIsoYearByDate = (date: Date): number => {
+    const dayOfWeek = date.getDay() === 0 ? 7 : date.getDay();
+    const thursday = new Date(date);
+    thursday.setDate(date.getDate() + (4 - dayOfWeek));
+    return thursday.getFullYear();
+  };
+
+export const getIsoYear = () : number => getIsoYearByDate(new Date());

test/weekUtils.spec.ts

@@ -1,4 +1,6 @@
 import {
+	getIsoYear,
+	getIsoYearByDate,
 	getWeekIsoNumber,
 	getWeekIsoOfYear,
 	getWeekNumber,
@@ -31,4 +33,20 @@ describe("weekUtils", () => {
 			expect(getWeekIsoOfYear()).toBe(currentWeekNumber);
 		});
 	});
+
+	describe("getIsoYear", () => {
+		it("should return the Iso year number of the current date", () => {
+			const currentDate = new Date();
+			const isoYearNumber = getIsoYearByDate(currentDate);
+			expect(getIsoYear()).toBe(isoYearNumber);
+		});
+	});
+
+	describe("getIsoYearByDate", () => {
+		it("should return the Iso year number of an specific date", () => {
+			const specificDate = new Date(2024,12,31);
+			const isoCorrectYear = 2025;
+			expect(getIsoYearByDate(specificDate)).toBe(isoCorrectYear);
+		});
+	});
 });