How many days a week do you have in a month?

Question

Hello, good night!

I did a lot of research on the Internet and found nothing concerning this doubt! I would like to get how many days of the week there are within a month using javascript! Example:

Quantos sábados existe no mês de atual? = 5
Quantas terças-feiras existes no mês atual? = 4

I would like a script where you inform me the amount of days that exists in the current month.

Thank you in advance...

Answer 1

Using a while adding up days will solve the problem:

 while (month === date.getMonth()) {
   if (date.getDay() === type) count++;
   date.setDate(date.getDate() + 1); // testar o próximo dia
 }

But the increment of date.getDate() + 1 will generate increments and on average 30 loops, most of which are unnecessary if the required rule is a specific day of the week just add up +7 inside the loop instead of +1 and check if it’s the same month after and then yes make the sum count++, the function a little optimized can look like this (as the test done at the end of the answer became almost 3 times faster):

function getDaysByType(type, month, year) {
  const date = new Date(year ? year : new Date().getFullYear(), month, 1, 0, 0, 0);

  var count = 0, increment = 1;

  while (month === date.getMonth()) {
    if (date.getDay() === type) {
        count++;
        increment = 7;
    }

    date.setDate(date.getDate() + increment); // testar o próximo dia
  }
  return count;
}

console.log('Quantidade de Sábados em Janeiro:', getDaysByType(0, 0));
console.log('Quantidade de Sábados em Fevereiro:', getDaysByType(0, 1));
console.log('Quantidade de Sábados em Março:', getDaysByType(0, 2));
console.log('Quantidade de Sábados em Abril:', getDaysByType(0, 3));
console.log('Quantidade de Sábados em Maio:', getDaysByType(0, 4));
console.log('Quantidade de Sábados em Junho:', getDaysByType(0, 5));
console.log('Quantidade de Sábados em Julho:', getDaysByType(0, 6));

This way it will be 4 to 11 loops instead of 28 to 31 loops, which made ~70% faster.

---

However I have another proposal, to use the BASIC MATHEMATICS, because if we know that a day of the week can appear between 4 and 5 times in a same month with that we have already obtained by a calculation, we will only need to know which the first day of the week of a month and which the last

Thus became ~90% faster:

Recalling that 0 = Saturday, 1 = Sunday, 2 = Monday, 3 = Tuesday, 4 = Wednesday, 5 = Thursday, 6 = Friday

Month 0 (January) to 11 (December)

function sumWeekDaysInMonth(weekday, month, year)
{
    if (!year) year = new Date().getFullYear();

    // Obtem o ultimo dia do mês
    var daysInMonth = new Date(year, month + 1, 0).getDate();

    // Obtem primeiro dia da semana do mês
    var firstWeekDay = new Date(year, month, 1).getDay();

    return Math.floor((daysInMonth + (weekday + firstWeekDay) % 7) / 7);
}

console.log('Quantidade de Sábados em Janeiro (ano atual):', sumWeekDaysInMonth(6, 0));
console.log('Quantidade de Sábados em Fevereiro (ano atual):', sumWeekDaysInMonth(6, 1));
console.log('Quantidade de Sábados em Março (ano atual):', sumWeekDaysInMonth(6, 2));

console.log('----');

console.log('Quantidade de Sábados em Janeiro 1988:', sumWeekDaysInMonth(6, 0, 1988));
console.log('Quantidade de Sábados em Fevereiro 1988:', sumWeekDaysInMonth(6, 1, 1988));
console.log('Quantidade de Sábados em Março 1988:', sumWeekDaysInMonth(6, 2, 1988));

To better understand, we use the % to get what is left in a division, then:

day of the week + first day of the week of the month / total days week (7)

In January it was (6 + 3) %7 = 2 (would be as 6+3=9 and 9/7=1.2, rounding is 2)

Then sum the number of days in the month and divide by 7, 4 was the remaining result of division:

(31 + 2) / 7 = 4,714285714285714

Then use Math.floor() to return the smallest integer that will be "4" for the month of January 2020

View the benchmark of all codes: https://jsbench.me/lwkdh223r9/1

• With while optimized managed to run almost 3 times faster

• With "math" was the fastest performing almost 10 times faster

Answer 2

I made a library to generate dates, number of week etc that I use in my projects, can be useful to you.

In this case it is simple and you can do it in Javascript. Note that this script is insensitive to holidays :), you have to add this logic by hand.

function getWorkingDays(month) {
  const date = new Date(2020, month, 1, 0, 0, 0);
  let workingdays = 0;
  while (month === date.getMonth()) {
    const currentDay = date.getDay();
    const isWeekend = (currentDay === 6) || (currentDay === 0);
    if (!isWeekend) workingdays++; // se não é fds aumentar o nr
    date.setDate(date.getDate() + 1); // testar o próximo dia
  }
  return workingdays;
}

// lembra-te que meses têm base 0
// ou seja: janeiro é o mes 0, fevereiro o mes 1, etc...

console.log(getWorkingDays(0));
console.log(getWorkingDays(1));

To know the number of specific days you can make an adaptation of the code:

function getDaysByType(month, type) {
  const date = new Date(2020, month, 1, 0, 0, 0);
  let count = 0;
  while (month === date.getMonth()) {
    if (date.getDay() === type) count++;
    date.setDate(date.getDate() + 1); // testar o próximo dia
  }
  return count;
}

// sábado tem o nr 6
// domingo tem o nr 0
// segunda tem o nr 1
// etc

console.log(getDaysByType(0, 0));
console.log(getDaysByType(0, 1));
console.log(getDaysByType(0, 2));
console.log(getDaysByType(0, 3));
console.log(getDaysByType(0, 4));
console.log(getDaysByType(0, 5));
console.log(getDaysByType(0, 6));