Function arraySum

src/piper.js:3856–3860

(array, index)

		}
		var isLeapYear = (year) => year % 4 === 0 && (year % 100 !== 0 || year % 400 === 0);
		var arraySum = (array, index) => {
			var sum = 0;
			for (var i = 0; i <= index; sum += array[i++]) {}
			return sum;
		};
		var MONTH_DAYS_LEAP = [31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31];
		var MONTH_DAYS_REGULAR = [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31];
		var addDays = (date, days) => {