dayOfYear

Gets the day of the year (number in the range 1-366) from a Date object.

• Use new Date() and Date.prototype.getFullYear() to get the first day of the year as a Date object.
• Subtract the first day of the year from 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.

const dayOfYear = date =>
  Math.floor((date - new Date(date.getFullYear(), 0, 0)) / 1000 / 60 / 60 / 24);

dayOfYear(new Date()); // 272