How to check if last day of month is on friday Javascript

Question

I'm supposed to write a code for codewars to find out the number of times a month ends with a Friday within a range of years.

To start off, I did research and found out several solutions but I still couldn't figure out the results in the console.log.

The first solution is from this tutorial: In this code, the solution is

let LastDay = new Date(1998, 5 + 1, 0).getDate();

I was able to get the date, but it wasn't clear which day the date falls upon.

Then I found another solution at w3schools. This solution also set the date to be the last day of this month:

var d = new Date();
d.setMonth(d.getMonth() +1, 0);
document.getElementById("demo").innerHTML = d;

However, it works if it displays it as innerHTML = Sat Nov 30 2019 00:57:09 GMT-0500 (Eastern Standard Time). However, when I tried to rewrite the code and console.log it like in this example:

let d = new Date();
  let month = d.getMonth()+1;
let lastday = d.setMonth(month, 0);
  console.log(lastday);

The result I got was 1575093343211. I don't understand how it displays those numbers instead of the dates I was expecting. I thought that if it does display the dates, starting with the day, I can convert the date to string or array and check if the first element is Friday and then add it to the counter in the code I'm writing. How do I get the code to display the way I want it to.

Answer 1

something like this will work...

function LastDayOfMonth(Year, Month) {
  return new Date((new Date(Year, Month, 1)) - 1);
}

var d = LastDayOfMonth(new Date().getYear(), new Date().getMonth())
//var d = LastDayOfMonth(2009, 11)

var dayName = d.toString().split(' ')[0];
console.log(dayName)

Answer 2

The result I got was 1575093343211. I don't understand how it displays those numbers instead of the dates I was expecting

Because you console.log the output of the setMonth method, not the date object:

let lastday = d.setMonth(month, 0);
console.log(lastday);

According to the documentation, the setMonth method returns:

The number of milliseconds between 1 January 1970 00:00:00 UTC and the updated date.

Instead you should use that output to create a new instance of the date object:

let lastday = new Date(d.setMonth(month, 0));
console.log(lastday);

Answer 3

Algorithms to get the last day of the month are generally based on setting a date to day 0 of the following month, which ends up being the last day of the required month.

E.g. to get the last day for June, 2019 (noting that 6 is July, not June):

let endOfJune = new Date(2019, 6, 0):

Once you have the date, you can get the day where 0 is Sunday, 1 is Monday, etc. and 5 is Friday:

let endOfJuneDay = endOfJune.getDay();

The set* methods modify the Date they're called on and return the time value for the modified date. So you don't need to assign the result to anything:

let d = new Date();
let month = d.getMonth() + 1;
// Set date to the new month
d.setMonth(month, 0);
console.log(d);

So if you want to loop over the months for a range of years and get the number that end with a Friday (or any particular day), you might loop over the months something like:

/*
** @param {number} startYear - start year of range
** @param {number} endYear   - end year of range
** @param {number} dat       - day number, 0 = Sunday, 1 = Monday, etc.
**                             default is 0 (Sunday)
*/
function countEOMDay(startYear, endYear, day = 0) {
  // startYear must be <= end year
  if (startYear > endYear) return;
  
  // Start on 31 Jan of start year
  let start = new Date(startYear, 0, 31);
  
  // End on 31 Dec of end year
  let end = new Date(endYear, 11, 31);
  let count = 0;
  
  // Loop over months from start to end
  while (start <= end) {
    // Count matching days
    if (start.getDay() == day) {
      ++count;
    }
    // Increment month to end of next month
    start.setMonth(start.getMonth() + 2, 0);
  }
  return count;
}

console.log(countEOMDay(2019, 2019, 5)); // 1
console.log(countEOMDay(2018, 2019, 5)); // 3

Answer 4

You can use setMonth() method to set the month of a date object. The return value of setMonth() method is milliseconds between the date object and midnight January 1 1970. That's what you get from console.log(lastday);

Your return value,

1575093343211

is milliseconds between your date object (d) and midnight January 1 1970.

If you want to get the expected date, you have to console log your date object instead the lastday, as follows:

  let d = new Date();
  let month = d.getMonth()+1;
  let lastday = d.setMonth(month, 0);
  console.log(d);

output: Sat Nov 30 2019 00:02:47 GMT+0530 (India Standard Time)

This is an alternative solution I wrote to solve your problem. This will return the number of times a month ends with a Friday within a range of years. Hope this will help you :)

var days = [];
var count = 0;

    function getLastFridaysCount(startYear, endYear) {

          for (var year = startYear; year <= endYear; year++) {

            days = [
                31,
                0 === year % 4 && 0 !== year % 100 || 0 === year % 400 ? 29 : 28,
                31, 30, 31, 30, 31, 31, 30, 31, 30, 31
            ];

            for (var month = 0; month <= 11; month++) {
              var myDate = new Date();
              myDate.setFullYear(year);
              myDate.setMonth(month);
              myDate.setDate(days[month]);

              if(myDate.getDay() == 5)
              {
                count++;
              }

      } 
          }

        return count;
    }

    console.log("count", getLastFridaysCount(2014, 2017));

Answer 5

this is the solution, in the code can find the comments "//" explaining of what happens in each iteration.

function lastDayIsFriday(initialYear, endYear) {
  let count = 0;
  //according to when the year ends starts the loop
  if (endYear !== undefined) {
    
    let start = new Date(initialYear, 0, 31);
    let end = new Date(endYear, 11, 31);
    
    while(start <= end) { //check if the start date is < or = to the end
      //The getDay() method returns the day of the week (from 0 to 6) for the specified date.
      if(start.getDay() === 5) { //if = to FriYAY!!!
        count++; //count the day
      }
      start.setMonth(start.getMonth()+2, 0);// returns the month (from 0 to 11) .getMonth
    }                                      //& sets the month of a date object  .setMonth
    return count;
  } else {
    let start = new Date(initialYear, 0, 31);
    console.log(start.toString());
    for(let i = 0; i < 12; i++) {
      if(start.getDay() === 5) {
        count++;
      }
      start.setMonth(start.getMonth() + 2, 0);
      // console.log(start.toString());
    }
    return count;
  }
  }