JavaScript (and Python): Get All Sundays From Current Month

JavaScript

Before I start, I was impressed by this brilliancy on StackOverflow. Thus, I also apply the concept to JavaScript. First, I looped through the whole month then picked each Sunday, which was fine. But using that concept, we can loop weekly or every 7 days, which is nicer, I suppose. The thing to pay attention more carefully is to get the correct first Sunday of the month.

Alright, let's go. In JavaScript, Sunday is at index 0 in its weekdays construction, um. Or other wordings. The .getDay() (of a Date object) method:

0 is Sunday
1 is Monday
2 is Tuesday
3 is Wednesday
4 is Thursday
5 is Friday
6 is Saturday

(Same as C language since JavaScript is C based)

(Not the coding below, but the array position above)

function get_sundays(a) {    
    var sundays = [];    
    
    var current = new Date(); // Current date (default)
    if (a) {
        if (a.constructor === Date) { // There's input date
            current = a; // Input date
        }
    }
        
    var date = new Date(current.getFullYear(), current.getMonth(), 1); // First date of current month
    
    var addend = date.getDay();
    if (addend > 0) { // First date of month is NOT Sunday
        addend = 7 - addend;
    }
    date.setDate(date.getDate() + addend); // Important --> first Sunday of current month    
    
    while (date.getMonth() === current.getMonth()) { //  Loop only for current month        
        sundays.push(date.getDate()); // Store Sunday date
        date.setDate(date.getDate() + 7); // Increment weekly (7 days)
    }    
    
    return sundays;
}

Input

It receives Date object only. To construct a new date:

var date_reference = new Date(2018, 11, 1); // Date object of December 1st, 2018
                                           // The month is always minus 1, January is 0 and so on
var sundays = get_sundays(date_reference); // This will get all Sundays dates

Remember, month in JavaScript is always minus 1 the actual month.

October is the 10^th Gregorian dilly, then in JavaScript it's 9. Hahaha. Uhm. Kinda confusing at first, but not if you're used to.

It's related to index of an array. If we have [January, February, March, ..., December], surely the position of "January" in the array will be 0. Month in JavaScript is any integer from 0 to 11, where 0 is January, and 11 is December.

And also [Sunday, Monday, ..., Saturday], therefore "Sunday" is at index 0. Thus, weekdays index in JavaScript ranges from 0 to 6 (integer), where 0 is Sunday and 6 is Saturday.

In English, "weekday" is any day of the week other than Sunday (and Saturday). "Working day" in another words *twinkle twinkle*

Anyhow, you can leave the input argument to be empty or undefined to get current month's Sundays.

Output

It will be array of integers.

For instance, November 2018, will yield [4, 11, 18, 25]

If you look the calendar, November 4, 2018 is Sunday, and so the 11^th, 18^th, and 25^th

Python

In Python, Sunday is at index 6. Because Python is a name of a gigantic snake before it became a TV show then a programming language, so it doesn't have correlation with earlier sentence. What sentence?

Using .weekday() (of a datetime object) method:

0 is Monday
1 is Tuesday
2 is Wednesday
3 is Thursday
4 is Friday
5 is Saturday
6 is Sunday

def get_sundays(a = None):
    from datetime import date, datetime, timedelta # Module import
    
    sundays = []

    current = datetime.now() # Current date (default)
    if isinstance(a, date): # There's date input
        current = a # Input date
    
    date = date(current.year, current.month, 1) # First date of current month    
    date += timedelta(days = 6 - date.weekday()) # First Sunday of current month    
    
    while date.month == current.month: # Loop only for current month
        sundays.append(date.day)  # Store Sunday date
        date += timedelta(days = 7)  # Increment weekly (7 days)
        
    return sundays

Input

Input type must be date object. Or, you can leave it empty to get current month.

from datetime import date
date_reference = date(2019, 8, 1) # Date object of August 1st, 2019

sundays = get_sundays(date_reference) # This will get all Sundays dates

In Python, the month integer is the same as the actual Gregorian. December is 12, September is 9, March is 3, January is 1, and all. Any integer from 1 to 12.

Only weekday is using 0. Any integer from 0 to 6, where 0 is Monday, 6 is Sunday.

Output

Exactly the same as the JavaScript snippet, a list of integers.

More tinkering

It's up to you, whatever modification of input / process / output that suits your progrem. In this post, I shared my new learning of "looping weekly". I suppose the concept can be broadened and applied to other matters, not just calendar related.

How about public holidays?

Well, we need a regularly updated database of course and read that list. Every country has its own different holidays. There're services we can use, free or paid, for instance Google Calendar or TimeAndDate, or others. Some countries (government sites) provide free access to get the list of holidays of current year, perhaps. I mean like an API, not just displaying it on the site's page. But not every country.

Or, we could create our own DB "manually".

Monkey Raptor

Monkey Raptor

Thursday, November 29, 2018

JavaScript (and Python): Get All Sundays From Current Month

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