title: dayOfYear tags: date,beginner

Gets the day of the year from a Date object.

Use new Date() and Date.prototype.getFullYear() to get the first day of the year as a Date object, subtract it from the provided date and divide with the milliseconds in each day to get the result. Use Math.floor() to appropriately round the resulting day count to an integer.

function isString<T = any>(str: string | T): str is string {
  return typeof str === "string";
}
function isValidDate(date: Date) {
  return date instanceof Date && !isNaN(date.getTime());
}
const dayOfYear = (date: Date | string): number => {
  if (isString<Date>(date)) {
    date = new Date(date);
  }
  if (!isValidDate(date)) throw new Error(`Invalid Date string`);
  return Math.floor(
    (date.getTime() - new Date(date.getFullYear(), 0, 0).getTime()) /
      1000 /
      60 /
      60 /
      24
  );
};

dayOfYear(new Date()); // 180
dayOfYear(new Date(2020, 5, 28)); //180
dayOfYear("2020-06-28T10:46:39.856Z"); // 180
dayOfYear("2020-06-28T1s"); // Error: Invalid Date string
// invalid date
assertThrows(
  () => {
    dayOfYear("2020-06-28T1s"); // Error: Invalid Date string
  },
  Error,
  "Invalid Date string"
);