daysInMonth

Gets the number of days in the given month of the specified year.

• Use the new Date() constructor to create a date from the given year and month.
• Set the days parameter to 0 to get the last day of the previous month, as months are zero-indexed.
• Use Date.prototype.getDate() to return the number of days in the given month.

const daysInMonth = (year, month) => new Date(year, month, 0).getDate();

daysInMonth(2020, 12)); // 31
daysInMonth(2024, 2)); // 29