Develop Reference

Get the current week using JavaScript without additional libraries ? [SO examples are broken]

I built a calendar control and was adding the week numbers as a final touch, and encountered a problem with every script example I could find on SO and outside of SO (most of which one has copied from the other).
The issue is that when dates fall in partial months, the week calculation seems to mess up and either continue counting when it is the same week in a new month, or it thinks the last full week in a previous month is the same week number as the first full new week in the following month.
Following is a visual demonstration of one of the libraries (they all have their inaccuracies as they generally base their week calculation off a fixed number and build from there) :
You can view the codepen here as the project is rather complex, I have the Date.prototype.getWeek function at the start to play with this easier. Feel free to swap in any code from the samples found here on SO as they all end up funking out on some months.
Some of the calculations used :
Show week number with Javascript?
Date get week number for custom week start day
w3resource.com ISO86901
epoch calendar - getting ISO week
Get week of year in JavaScript like in PHP
When running the most current example (2017) from "Get week of year in JavaScript like in PHP", the week returned right now is 42. When you look on my calendar, the week in October right now is showing as 42 which is correct according to here https://www.epochconverter.com/weeks/2018.
Given the example, there are full weeks sharing the same week number - so I don't see how 42 can even be accurate.
Date.prototype.getWeek = function (dowOffset) {
/*getWeek() was developed by Nick Baicoianu at MeanFreePath: http://www.epoch-calendar.com */
dowOffset = typeof(dowOffset) == 'int' ? dowOffset : 0; //default dowOffset to zero
var newYear = new Date(this.getFullYear(),0,1);
var day = newYear.getDay() - dowOffset; //the day of week the year begins on
day = (day >= 0 ? day : day + 7);
var daynum = Math.floor((this.getTime() - newYear.getTime() -
(this.getTimezoneOffset()-newYear.getTimezoneOffset())*60000)/86400000) + 1;
var weeknum;
//if the year starts before the middle of a week
if(day < 4) {
weeknum = Math.floor((daynum+day-1)/7) + 1;
if(weeknum > 52) {
nYear = new Date(this.getFullYear() + 1,0,1);
nday = nYear.getDay() - dowOffset;
nday = nday >= 0 ? nday : nday + 7;
/*if the next year starts before the middle of
the week, it is week #1 of that year*/
weeknum = nday < 4 ? 1 : 53;
}
}
else {
weeknum = Math.floor((daynum+day-1)/7);
}
return weeknum;
};
Here is some code (also tried this) that is Sunday specific (see near the bottom). I am also pasting the relevant snip here :
/* For a given date, get the ISO week number
*
* Based on information at:
*
* http://www.merlyn.demon.co.uk/weekcalc.htm#WNR
*
* Algorithm is to find nearest thursday, it's year
* is the year of the week number. Then get weeks
* between that date and the first day of that year.
*
* Note that dates in one year can be weeks of previous
* or next year, overlap is up to 3 days.
*
* e.g. 2014/12/29 is Monday in week 1 of 2015
* 2012/1/1 is Sunday in week 52 of 2011
*/
function getWeekNumber(d) {
// Copy date so don't modify original
d = new Date(Date.UTC(d.getFullYear(), d.getMonth(), d.getDate()));
// Set to nearest Thursday: current date + 4 - current day number
// Make Sunday's day number 7
d.setUTCDate(d.getUTCDate() + 4 - (d.getUTCDay()||7));
// Get first day of year
var yearStart = new Date(Date.UTC(d.getUTCFullYear(),0,1));
// Calculate full weeks to nearest Thursday
var weekNo = Math.ceil(( ( (d - yearStart) / 86400000) + 1)/7);
// Return array of year and week number
return [d.getUTCFullYear(), weekNo];
}

The algorithm is to use the week number of the following Saturday. So get the following Saturday, then use it's year for the 1st of Jan. If it's not a Sunday, go to the previous Sunday. Then get the number of weeks from there. It might sound a bit convoluted, but it's only a few lines of code. Most of the following is helpers for playing.
Hopefully the comments are sufficient, getWeekNumber returns an array of [year, weekNumber]. Tested against the Mac OS X Calendar, which seems to use the same week numbering. Please test thoroughly, particularly around daylight saving change over.
/* Get week number in year based on:
* - week starts on Sunday
* - week number and year is that of the next Saturday,
* or current date if it's Saturday
* 1st week of 2011 starts on Sunday 26 December, 2010
* 1st week of 2017 starts on Sunday 1 January, 2017
*
* Calculations use UTC to avoid daylight saving issues.
*
* #param {Date} date - date to get week number of
* #returns {number[]} year and week number
*/
function getWeekNumber(date) {
// Copy date as UTC to avoid DST
var d = new Date(Date.UTC(date.getFullYear(), date.getMonth(), date.getDate()));
// Shift to the following Saturday to get the year
d.setUTCDate(d.getUTCDate() + 6 - d.getUTCDay());
// Get the first day of the year
var yearStart = new Date(Date.UTC(d.getUTCFullYear(), 0, 1));
yearStart.setUTCDate(yearStart.getUTCDate() - yearStart.getUTCDay());
// Get difference between yearStart and d in milliseconds
// Reduce to whole weeks
return [d.getUTCFullYear(), (Math.ceil((d - yearStart) / 6.048e8))];
}
// Helper to format dates
function fDate(d) {
var opts = {weekday:'short',month:'short',day:'numeric',year:'numeric'};
return d.toLocaleString(undefined, opts);
}
// Parse yyyy-mm-dd as local
function pDate(s){
var b = (s+'').split(/\D/);
var d = new Date(b[0],b[1]-1,b[2]);
return d.getMonth() == b[1]-1? d : new Date(NaN);
}
// Handle button click
function doButtonClick(){
var d = pDate(document.getElementById('inp0').value);
var span = document.getElementById('weekNumber');
if (isNaN(d)) {
span.textContent = 'Invalid date';
} else {
let [y,w] = getWeekNumber(d);
span.textContent = `${fDate(d)} is in week ${w} of ${y}`;
}
}
Date:<input id="inp0" placeholder="yyyy-mm-dd">
<button type="button" onclick="doButtonClick()">Get week number</button><br>
<span id="weekNumber"></span>

Going through weeks is automatically changing on wednesday rather than sunday/monday

I'm looking for some help with my coursework as I've spent the last 8 hours trying to find a way to do what i'm trying to accomplish.
Now I have to use JavaScript, cannot use plugins but i can use the jQuery Library.
I'm basically trying to assign weeks to the semesters of the university year, so we say 29th September 2014 (Monday) to 26th January which is around 15 weeks.
Now I'm using something like this:
Date.prototype.getWeek = function() {
var firstJan = new Date(2014,0,1);
var today = new Date(2014, 9, 14);
var firstMonSem1 = new Date(2014, 8, 29);
var firstMonSem2 = new Date(this.getFullYear(), 1, 1);
var dayOfYear = ((today - firstJan + 86400000)/86400000);
var startWeekSem1 = ((firstMonSem1 - firstJan + 86400000)/86400000);
var startWeekSem2 = ((firstMonSem2 - firstJan + 86400000)/86400000);
var currentWeekNumber = Math.ceil(dayOfYear/7);
var startWeekNumberSem1 = Math.ceil(startWeekSem1/7);
var startWeekNumberSem2 = Math.ceil(startWeekSem2/7);
var output = {current: currentWeekNumber, sem1: startWeekNumberSem1, sem2: startWeekNumberSem2};
return output;
};
So i'm returning the current week of the year, semester start date for semester 1 and 2.
But for this example let's only focus on the first is 29th September (Monday) until 26th January which is around 15 weeks.
Now i've worked out based on the values i get when the semester starts and finishes including a 4 week holiday over the christmas period. And then I have an array for the index of which week of the semester we are in, in comparison to the week of the year (this way it could be week 41 in the year and based on the index array I would know that this is week 2).
Here's the code (note: it works!)
var today = new Date();
var info = today.getWeek();
if (info['current'] >= info['sem1'] && info['current'] <= (info['sem1'] + 18)) {
// Semester 1
var index = [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15];
var yearWeeks = [];
var count = 1;
for (var i = info['sem1']; i < (info['sem1'] + 18); i++) {
if (count !== 12 && count !== 13 && count !== 14) {
yearWeeks.push(i);
}
count++;
}
var rightIndex = $.inArray(info['current'], yearWeeks);
var x = 1;
while (x <= 15) {
if (x >= rightIndex) {
var weekOption = $("<option/>", {
"value" : x,
html : "Week "+x
}).appendTo(week);
}
x++;
}
}
So what I do here is append to a select all the weeks that are greater than the current week, so if we are in week 5 then there won't be a week 1,2,3,4 for the sake of this example (it's because in the project we are selecting a week to make a booking on but that's irrelevant).
Now, the problem.
This all works fantastically but there is a major catch, the week changes on Wednesday for example:
Week 1 is commencing 29th of September (Monday)
Week 2 starts 8th October (Wednesday)
Week 3 starts 15th October (Wednesday)
So every time the week changes on the Wednesday, now as the idea is that this system is being implemented so you can book room for that given week aka Monday - Friday, I need the weeks to update every Monday rather than Wednesday.
I really need help with this, and I know it might be a lot to grasp. However, I appreciate all the help I can get.
Thank you.

I think you need to replace this line:
var currentWeekNumber = Math.ceil(dayOfYear/7);
To be:
var currentWeekNumber = Math.ceil((dayOfYear + 5)/7);
To make a week start from Wednesday. In such case, the week of Day 2 (Tuesday) is Math.ceil((2+5)/7)=1, while Day 3 (Wednesday) is Math.ceil((3+5)/7)=2.

I worked it out some and hope it helps. This is not just for 2014 that you use, but for the current year. With some adaptation you can adjust.
You get the first of the year and get the offset of the day-number. Then you calculate the current day and subtract. The result will help you get the weeknr.
The week starts on Monday (where in other regions it may be on Sunday).
I created a fiddle, so you can play with it:
http://jsfiddle.net/djwave28/r0m4sokv/
This is an abstract from that fiddle:
// set an offset
var offset = 0;
var now = new Date();
var testday = new Date(now.getTime() + offset * 24 * 60 * 60 * 1000);
var firstofyear = new Date(testday.getFullYear(), 0, 1);
var firstdayoffset = firstofyear.getDay();
var daynr = Math.ceil((testday - firstofyear) / 86400000);
var weeknr = (daynr - firstdayoffset) / 7;

Looking to add 1 month to current date inside a div

i am looking to display the date plus 1 month inside a div which will be used to display next invoice date. i have seen a few examples in various places, but could not implement. I also saw that there were many solutions, and some controversy surrounding each one.

Once you have a date object, just call setMonth(), passing in the current number of months plus 1.
var CurrentDate = new Date();
CurrentDate.setMonth(CurrentDate.getMonth() + 1);

You can either use this : http://jsfiddle.net/dfA8b/ if you need same date of the next month
var invoiceDt = new Date();
invoiceDt.setMonth(invoiceDt.getMonth()+1);
$('#invoiceDate').text(invoiceDt.toDateString());
Or
You can use this : http://jsfiddle.net/hjSDu/ if you need 30 days month(mostly used for invoice purposes)
var invoiceDt = new Date();
var days = 30;
invoiceDt.setDate(invoiceDt.getDate()+days);
$('#invoiceDate').text(invoiceDt.toDateString());
For the formatting purpose :
http://jsfiddle.net/7bU6n/
var invoiceDt = new Date();
invoiceDt.setMonth(invoiceDt.getMonth()+1);
$('#invoiceDate').text((invoiceDt.getMonth()+1) +"-"+ invoiceDt.getDate() +"-" + invoiceDt.getFullYear());
also see : https://stackoverflow.com/a/1643468/3603806

1 month is not so clear... what You mean? 31, 30 days or if exists simply same date of the following month?
1st case: (assuming 31 days)
var d = new Date(),
dplus31 = d.getDate() + 31;
d.setDate(dplus31);
console.debug(d, dplus31);
2nd case: one month
var d = new Date(),
dplus1 = d.getMonth() + 1;
d.setMonth(dplus1);
console.debug(d, dplus1);
..but even in this case there are some edge cases (ex. 31 January)
hope it helps

function addMonthsNoOverflow(dateParam, intParam) {
var sum = new Date(new Date(dateParam.getTime()).setMonth(dateParam.getMonth() + intParam);
if (sum.getDate() < dateParam.getDate()) { sum.setDate(0); }
return(sum);
}
Notes:
It handles cases where 29, 30 or 31 turned into 1, 2, or 3 by eliminating the overflow
Day of Month is NOT zero-indexed so .setDate(0) is last day of prior month.

Calculate the Date based on Current Date and No of Days using Javascript/Jquery

I need a help..
I have a Current Date and No of days column.
When i enter number of days,i should add current date plus no of days entered.
For example,
todays date 5th jan + 20(no of days) = 25th Jan 2011 in another column.
Kindly help me.
Thanks in Advance.

Date.js is fantastic for this.
Date.today().add(5).days();

As you are learning JavaScript you may find the w3schools site useful for simple examples of objects and functions that are exposed and how they may be used.
http://www.w3schools.com/jsref/jsref_obj_date.asp
You can calculate the date as follows:
var d = new Date(); // Gets current date
var day = 86400000; // # milliseconds in a day
var numberOfDays = 20;
d.setTime(d.getTime() + (day*numberOfDays)); // Add the number of days in milliseconds
You can then use one of the various methods of displaying the date:
alert(d.toUTCString());

You could do something like
Date.today().add(X).days();
Where X is the number of days the user has entered.

You can add dates like this in js:
var someDate = new Date();
var numberOfDaysToAdd = 6;
someDate.setDate(someDate.getDate() + numberOfDaysToAdd);
var month = someDate.getMonth() + 1; //Add 1 because January is set to 0 and Dec is 11
var day = someDate.getDate();
var year = someDate.getFullYear();
document.write(month + "/" + day + "/" + year);
See this p.cambell's answer here: How to add number of days to today's date?

Using answers from prompt for a calculation

Total newbie at JavaScript.
I would like to calculate how many days one has been alive by asking the user their date of birth via prompts/alerts, then obviously subtracting their date of birth from today's date.
I've made a bit of a start...
var month=prompt("Please enter month of birth"," ");
var day=prompt("Please enter day of birth"," ");
var year=prompt("Please enter your year of birth"," ");
var curdate = this is the bit i need help with
var birth = this is the bit i need help with
var milliDay = 1000 * 60 * 60 * 24; // a day in milliseconds;
var ageInDays = (curdate - birth) / milliDay;
document.write("You have been alive for: " + ageInDays);
Any advice or help would be much appreciated.

You need to use the Date object (MDN). They can be created from a month, a day, and a year, and added/subtracted.
Typically :
var curDate = new Date();
var birth = new Date(year, month, day);
var ageInDays = (curdate.getTime() - birth.getTime()) / milliDay;
Be aware of the fact that months starts at 0, e.g. January is 0.

var curDate = new Date();
gives you the current date.
var birthdate = new Date(year, month-1, day);
gives you a Date from the separate variables. NB the month is zero-based.

end = Date.now(); // Get current time in milliseconds from 1 Jan 1970
var date = 20; //Date you got from the user
var month = 8-1; // Month, subtracted by one because month starts from 0 according to JS
var year = 1996; // Year
//Set date to the old time
obj = new Date();
obj.setDate(date);
obj.setMonth(month);
obj.setYear(year);
obj = obj.getTime(); //Get old time in milliseconds from Jan 1 1970
document.write((end-obj)/(1000*60*60*24));
Simply subtract current time from Jan 1 1970 in milliseconds from their birthdate's time from Jan 1 1970 in milliseconds. Then convert it to days. Look at MDN's Docs for more info.
See JSFiddle for a working example. Try entering yesterday's date. It should show 1 day.

Read some of this: http://www.w3schools.com/js/js_obj_date.asp