Finding day of week from date (day, month, year) in JavaScript

We are required to write a JavaScript function that takes in three argument, namely:day, month and year. Based on these three inputs, our function should find the day of the week on that date.

For example: If the inputs are −

day = 15,
month = 8,
year = 1993

Output

Then the output should be −

const output = 'Sunday'

Example

The code for this will be −

const dayOfTheWeek = (day, month, year) => {
   // JS months start at 0
   return dayOfTheWeekJS(day, month - 1, year);
}
function dayOfTheWeekJS(day, month, year) {
   const DAYS = [
      'Sunday',
      'Monday',
      'Tuesday',
      'Wednesday',
      'Thursday',
      'Friday',
      'Saturday',
   ];
   const DAY_1970_01_01 = 4;
   let days = day − 1;
   while (month − 1 >= 0) {
      days += daysInMonthJS(month − 1, year);
      month −= 1;
   }
   while (year − 1 >= 1970) {
      days += daysInYear(year − 1);
      year −= 1;
   }
   return DAYS[(days + DAY_1970_01_01) % DAYS.length];
};
function daysInMonthJS(month, year) {
   const days = [
      31, // January
      28 + (isLeapYear(year) ? 1 : 0), // Feb,
      31, // March
      30, // April
      31, // May
      30, // June
      31, // July
      31, // August
      30, // September
      31, // October
      30, // November
      31, // December
   ];
   return days[month];
}
function daysInYear(year) {
   return 365 + (isLeapYear(year) ? 1 : 0);
}
function isLeapYear(year) {
   return year % 4 === 0 && year % 100 !== 0 || year % 400 === 0;
}
console.log(dayOfTheWeek(15, 8, 1993));

Explanation

We want to count how many days until the given date. To do so, we can start at the infamous Unix time 0 (Thursday 1970−01−01) and go from there −

• Count days in complete years

• Count days in month for the incomplete year

• Remaining days for the incomplete month

Output

And the output in the console will be −

Sunday