script.js

const date = document.getElementById("date-dd");
const month = document.getElementById("month-dd");
const year = document.getElementById("year-ip");
const mainStyle = document.querySelector("main").style;
const resultStyle = document.getElementById("result").style;

const MONTH_ODD_DAYS = [0, 3, 0, 3, 2, 3, 2, 3, 3, 2, 3, 2, 3];
const DAYS = ["SUNDAY", "MONDAY", "TUESDAY", "WEDNESDAY", "THURSDAY", "FRIDAY", "SATURDAY"];
const NO_OF_DATES = [0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31];

function changeDate(calledBy) {
	if ((calledBy === "month") || (calledBy === "year" && document.getElementById("month-dd").value === "2"))
		document.getElementById("date-dd").selectedIndex = 0;
}

function changeDatesBasedOnMonthAndYear() {
	let inputMonth = month.value, inputYear = year.value;
	const monthCheck = (inputMonth === "select"), yearCheck = (inputYear === "");

	if (monthCheck && yearCheck) {
		alert("Please input the Month and Year before inputting date");
	} else if (monthCheck) {
		alert("Please input Month before inputting date");
	} else if (yearCheck) {
		alert("Please input Year before inputting date");
	} else {
		inputMonth = parseInt(inputMonth);
		inputYear = parseInt(inputYear);

		const d29 = document.querySelector("#date-dd option[value=\"29\"]");
		const d30 = document.querySelector("#date-dd option[value=\"30\"]");
		const d31 = document.querySelector("#date-dd option[value=\"31\"]");

		if (inputMonth === 2) {
			if (inputYear % 4 === 0) {
				const array = Array.from(date.options).map((opt) => opt.value);
				if (!array.includes("29")) date.innerHTML += "<option value=\"29\">29</option>";
				if (array.includes("30")) d30.remove();
				if (array.includes("31")) d31.remove();
			} else {
				const array = Array.from(date.options).map((opt) => opt.value);
				if (array.includes("29")) d29.remove();
				if (array.includes("30")) d30.remove();
				if (array.includes("31")) d31.remove();
			}
		} else {
			const array = Array.from(date.options).map((opt) => opt.value);
			if (!array.includes("29")) date.innerHTML += "<option value=\"29\">29</option>";
			if (!array.includes("30")) date.innerHTML += "<option value=\"30\">30</option>";

			if (NO_OF_DATES[inputMonth] === 30 && array.includes("31")) {
				d31.remove();
			} else if (NO_OF_DATES[inputMonth] === 31 && !array.includes("31")) {
				date.innerHTML += "<option value=\"31\">31</option>";
			}
		}
	}
}

function checkFields() {
	if (document.getElementById("date-dd").value === "select" ||
		document.getElementById("month-dd").value === "select" ||
		document.getElementById("year-ip").value === "") {
		alert("Please input all the fields");
		return false;
	}
	return true;
}

function whatIsDay() {
	if (date.value === "select" || month.value === "select" || year.value === "") {
		alert("Please input all the fields");
	} else {
		const dateValue = parseInt(date.value);
		const monthValue = parseInt(month.value);
		const yearValue = parseInt(year.value) - 1;

		const date_odd_days = dateValue % 7;

		const array = MONTH_ODD_DAYS.slice(0, monthValue);
		let month_odd_days = array.reduce(function (x, y) {
			return x + y;
		}, 0);

		if ((monthValue > 2) && (yearValue + 1) > 400 && (yearValue + 1) % 4 === 0)
			month_odd_days += 1;

		const leap_years = parseInt(yearValue / 4);
		const year_odd_days = (leap_years * 2 + (yearValue - leap_years)) % 7;

		resultStyle.display = "inherit";
		document.getElementById("date-span").innerText = dateValue + " " + month.options[month.selectedIndex].text + " " + (yearValue + 1);

		const today = new Date();
		const currentDate = today.getFullYear().toString() + (today.getMonth() + 1).toString() + today.getDate().toString();
		const dateGiven = (yearValue + 1).toString() + monthValue.toString() + dateValue.toString();
		const verb = document.getElementById("verb");
		if (dateGiven < currentDate) verb.innerText = "was";
		else if (dateGiven === currentDate) verb.innerText = "is";
		else verb.innerText = "will be";

		document.getElementById("day").innerText = DAYS[(date_odd_days + month_odd_days + year_odd_days - 1) % 7];

		mainStyle.paddingBottom = "0";
	}
}

function reset() {
	if (resultStyle.display === "inherit") {
		date.value = "select";
		month.value = "select";
		year.value = "";
		resultStyle.display = "none";
		mainStyle.paddingBottom = "2rem";
	}
}