homework.js

d1 = new Date('2017-09-26')
d2 = new Date('2017-09-27')
d3 = new Date('2017-09-28')
d4 = new Date('2017-09-30')
d5 = new Date('2017-10-01')
d6 = new Date('2017-10-02')
d7 = new Date('2017-10-04')
d8 = new Date('2017-10-05')
d9 = new Date('2017-10-07')

dates = [d1, d2, d3, d4, d5, d6, d7, d8, d9];
result = "";

var toKoreanWeekDay = function(date) {
	return date.toLocaleString('ko-kr', { weekday : 'short' });
}

var getTomorrow = function(date) {
	tomorrow = date + 1;
	return tomorrow == 7 ? 0 : tomorrow;
}

var getYesterday = function(date) {
	yesterday = date - 1;
	return yesterday == -1 ? 6 : yesterday;
}

var hasTomorrow = function(date, compare_date) {
	return date == getYesterday(compare_date);
}

var hasYesterday = function(date, compare_date) {
	return date == getTomorrow(compare_date);
}

var appendComma = function() {
	return result += ",";
}

var appendTilt = function() {
	return result += "~";
}

var appendWeekDay = function(date) {
	return result += toKoreanWeekDay(date)
}

var main = function() {
	for (var i = 0; i < dates.length; i++) { 
		if (i == 0) {
			appendWeekDay(dates[i]);
			continue;
		}

		date = dates[i].getUTCDay();
		date_before = dates[i-1].getUTCDay();

		if (i == dates.length - 1) {
			if (hasYesterday(date, date_before)) {
				appendTilt();
			}

			appendWeekDay(dates[i]);
			continue;
		}

		date_after = dates[i+1].getUTCDay();

		if (hasYesterday(date, date_before)) {
			if (hasTomorrow(date, date_after)) {
				continue;
			} else {
				appendTilt();
				appendWeekDay(dates[i]);
				appendComma();
			}
		} else {
			appendWeekDay(dates[i]);
		}
	}

	return result;
}

main();