Show week number with Javascript

Question

I have the following code that is used to show the name of the current day  followed by a set phrase    lt script type  text javascript  gt        lt  --         Array of day names     var dayNames   new Array       It s Sunday  the weekend is nearly over        Yay  Another Monday         Hello Tuesday  at least you re not Monday         It s Wednesday  Halfway through the week already         It s Thursday          It s Friday - Hurray for the weekend        Saturday Night Fever        var now   new Date        document write dayNames now getDay                    -- gt   lt  script gt    What I would like to do is have the current week number in brackets after the phrase  I have found the following code   Date prototype getWeek   function         var onejan   new Date this getFullYear   0 1       return Math ceil    this - onejan    86400000    onejan getDay   1  7        Which was taken from http   javascript about com library blweekyear htm but I have no idea how to add it to existing javascript code

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I was coding in the dark  a challenge  and couldn t lookup or test my code   I forgot what round up was called  Math celi  So I wanted to be extra sure i got it right and came up with this code     x000D   x000D  var elm   document createElement  input   x000D  elm type    week  x000D  elm valueAsDate   new Date   x000D  var week   elm value split  W   pop   x000D   x000D  console log week  x000D   x000D   x000D   Just a proof of concept of how you can get the week in any other way  But still i recommend any other solution that isn t required by the DOM

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Martin Schillinger s version seems to be the strictly correct one   Since I knew I only needed it to work correctly on business week days  I went with this simpler form  based on something I found online  don t remember where   ISOWeekday    0    InputDate getDay      7   InputDate getDay    ISOCalendarWeek   Math floor      InputDate getTime   -  new Date InputDate getFullYear   0 1   getTime      86400000  - ISOWeekday   10    7      It fails in early January on days that belong to the previous year s last week  it produces CW   0 in those cases  but is correct for everything else

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If you already use Angular  then you could profit  filter  date     For example   var myDate   new Date    var myWeek    filter  date   myDate   ww

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All the proposed approaches may give wrong results because they don   t take into account summer winter time changes  Rather than calculating the number of days between two dates using the constant of 86   400   000 milliseconds  it is better to use an approach like the following one   getDaysDiff   function  dateObject0  dateObject1        if  dateObject0  gt   dateObject1  return 0      var d   new Date dateObject0 getTime         var nd   0      while  d  lt   dateObject1            d setDate d getDate     1           nd              return nd-1

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You could find this fiddle useful  Just finished  https   jsfiddle net dnviti ogpt920w  Code below also    x000D   x000D       x000D     Get the ISO week date week number  x000D        x000D  Date prototype getWeek   function        x000D       Create a copy of this date object   x000D    var target    new Date this valueOf       x000D   x000D       ISO week date weeks start on monday   x000D       so correct the day number   x000D    var dayNr      this getDay     6    7    x000D   x000D       ISO 8601 states that week 1 is the week   x000D       with the first thursday of that year    x000D       Set the target date to the thursday in the target week   x000D    target setDate target getDate   - dayNr   3     x000D   x000D       Store the millisecond value of the target date   x000D    var firstThursday   target valueOf      x000D   x000D       Set the target to the first thursday of the year   x000D       First set the target to january first   x000D    target setMonth 0  1     x000D       Not a thursday  Correct the date to the next thursday   x000D    if  target getDay      4      x000D      target setMonth 0  1     4 - target getDay      7    7     x000D        x000D   x000D       The weeknumber is the number of weeks between the    x000D       first thursday of the year and the thursday in the target week   x000D    return 1   Math ceil  firstThursday - target    604800000      604800000   7   24   3600   1000   x000D      x000D   x000D       x000D    Get the ISO week date year number  x000D       x000D  Date prototype getWeekYear   function       x000D      x000D       Create a new date object for the thursday of this week   x000D    var target    new Date this valueOf       x000D    target setDate target getDate   -   this getDay     6    7    3     x000D   x000D    return target getFullYear      x000D    x000D   x000D       x000D     Convert ISO week number and year into date  first day of week  x000D       x000D  var getDateFromISOWeek   function w  y    x000D    var simple   new Date y  0  1    w - 1    7   x000D    var dow   simple getDay    x000D    var ISOweekStart   simple  x000D    if  dow  lt   4  x000D      ISOweekStart setDate simple getDate   - simple getDay     1   x000D    else x000D      ISOweekStart setDate simple getDate     8 - simple getDay     x000D    return ISOweekStart  x000D    x000D   x000D  var printDate   function    x000D      var dateString   document getElementById  date   value  x000D   var dateArray   dateString split            use this if you have year-week in the same field x000D   x000D    var dateInput   document getElementById  date   value  x000D    if  dateInput         x000D      var date   new Date       get today date object x000D      x000D    else  x000D      var date   new Date dateInput      get date from field x000D      x000D   x000D    var day     0    date getDate    slice -2      get today day x000D    var month     0     date getMonth     1   slice -2      get today month x000D    var fullDate   date getFullYear    -   month   -   day       get full date x000D    var year   date getFullYear    x000D    var week     0     date getWeek     slice -2   x000D    var locale   it-it   x000D     x000D    document getElementById  date   value   fullDate     set input field x000D   x000D    document getElementById  year   value   year  x000D    document getElementById  week   value   week     this prototype has been written above x000D   x000D    var fromISODate   getDateFromISOWeek week  year   x000D     x000D   var fromISODay     0    fromISODate getDate    slice -2   x000D    var fromISOMonth     0     fromISODate getMonth     1   slice -2   x000D    var fromISOYear   date getFullYear    x000D     x000D       Use long to return month like  December  or short for  Dec  x000D      var monthComplete   fullDate toLocaleString locale    month   long       x000D   x000D    var formattedDate   fromISODay    -    fromISOMonth    -    fromISOYear  x000D   x000D    var element   document getElementById  fullDate    x000D   x000D    element value   formattedDate  x000D    x000D   x000D  printDate    x000D  document getElementById  convertToDate   addEventListener  click   printDate   x000D     x000D    font-family  consolas x000D    x000D   lt label for  date  gt Date lt  label gt  x000D   lt input type  date  name  date  id  date  style  width 130px text-align center  value      gt  x000D   lt br   gt  lt br   gt  x000D   lt label for  year  gt Year lt  label gt  x000D   lt input type  year  name  year  id  year  style  width 40px text-align center  value      gt  x000D  - x000D   lt label for  week  gt Week lt  label gt  x000D   lt input type  text  id  week  style  width 25px text-align center  value      gt  x000D   lt br   gt  lt br   gt  x000D   lt label for  fullDate  gt Full Date lt  label gt  x000D   lt input type  text  id  fullDate  name  fullDate  style  width 80px text-align center  value      gt  x000D   lt br   gt  lt br   gt  x000D   lt button id  convertToDate  gt  x000D  Convert Date x000D   lt  button gt  x000D   x000D   x000D    It s pure JS  There are a bunch of date functions inside that allow you to convert date into week number and viceversa

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By adding the snippet you extend the Date object   Date prototype getWeek   function         var onejan   new Date this getFullYear   0 1       return Math ceil    this - onejan    86400000    onejan getDay   1  7       If you want to use this in multiple pages you can add this to a seperate js file which must be loaded first before your other scripts executes  With other scripts I mean the scripts which uses the getWeek   method

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In case you already use jquery-ui  specifically datepicker    Date prototype getWeek   function      return   datepicker iso8601Week this       Usage   var myDate   new Date    myDate getWeek      More here  UI Datepicker iso8601Week  I realize this isn t a general solution as it incurs a dependency  However  considering the popularity of jquery-ui this might just be a simple fit for someone - as it was for me

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With that code you can simply   document write dayNames now getDay             now getWeek               You will need to paste the getWeek function above your current script

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It looks like this function I found at weeknumber net is pretty accurate and easy to use      This script is released to the public domain and may be used  modified and    distributed without restrictions  Attribution not necessary but appreciated     Source  http   weeknumber net how-to javascript      Returns the ISO week of the date  Date prototype getWeek   function       var date   new Date this getTime       date setHours 0  0  0  0        Thursday in current week decides the year    date setDate date getDate     3 -  date getDay     6    7        January 4 is always in week 1    var week1   new Date date getFullYear    0  4        Adjust to Thursday in week 1 and count number of weeks from date to week1    return 1   Math round   date getTime   - week1 getTime      86400000 - 3    week1 getDay     6    7    7       If you re lucky like me and need to find the week number of the month a little adjust will do it      Returns the week in the month of the date  Date prototype getWeekOfMonth   function       var date   new Date this getTime       date setHours 0  0  0  0        Thursday in current week decides the year    date setDate date getDate     3 -  date getDay     6    7        January 4 is always in week 1    var week1   new Date date getFullYear    date getMonth    4        Adjust to Thursday in week 1 and count number of weeks from date to week1    return 1   Math round   date getTime   - week1 getTime      86400000 - 3    week1 getDay     6    7    7

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Some of the code I see in here fails with years like 2016  in which week 53 jumps to week 2   Here is a revised and working version   Date prototype getWeek   function            Create a copy of this date object     var target    new Date this valueOf             ISO week date weeks start on monday  so correct the day number     var dayNr      this getDay     6    7          Set the target to the thursday of this week so the        target date is in the right year     target setDate target getDate   - dayNr   3           ISO 8601 states that week 1 is the week with january 4th in it     var jan4      new Date target getFullYear    0  4           Number of days between target date and january 4th     var dayDiff    target - jan4    86400000         if new Date target getFullYear    0  1  getDay    lt  5           Calculate week number  Week 1  january 4th  plus the            number of weeks between target date and january 4th         return 1   Math ceil dayDiff   7             else       jan 4th is on the next week  so next week is week 1      return Math ceil dayDiff   7

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Consider using my implementation of  Date prototype getWeek   think is more accurate than the others i have seen here     Date prototype getWeek   function           We have to compare against the first monday of the year not the 01 01        60 60 24 1000   86400000         onejan next monday time  reffers to the miliseconds of the next monday after 01 01      var day miliseconds   86400000          onejan   new Date this getFullYear   0 1 0 0 0           onejan day    onejan getDay    0    7   onejan getDay            days for next monday    8-onejan day           onejan next monday time   onejan getTime      days for next monday   day miliseconds              If one jan is not a monday  get the first monday of the year         first monday year time    onejan day gt 1    onejan next monday time   onejan getTime            this date   new Date this getFullYear    this getMonth   this getDate   0 0 0     This at 00 00 00         this time   this date getTime            days from first monday   Math round   this time - first monday year time    day miliseconds         var first monday year   new Date first monday year time           We add 1 to  days from first monday  because if  days from first monday  is  7         then 7 7   1  and as we are 7 days from first monday         we should be in week number 2 instead of week number 1  7 7 1         We consider week number as 52 when  days from first monday  is lower than 0         that means the actual week started before the first monday so that means we are on the firsts        days of the year  ex  we are on Friday 01 01  then  days from first monday  -3         so friday 01 01 is part of week number 52 from past year          days from first monday lt  364  because  364 1  7    52  if we are on day 365  then  365 1  7  gt   52  Math ceil 366 7  53  and thats wrong      return  days from first monday gt  0  amp  amp  days from first monday lt 364    Math ceil  days from first monday 1  7    52      You can check my public repo here https   bitbucket org agustinhaller date getweek  Tests included

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Simply add it to your current code  then call  new Date    getWeek     lt script gt      Date prototype getWeek   function             var onejan   new Date this getFullYear    0  1           return Math ceil    this - onejan    86400000    onejan getDay     1    7              var weekNumber    new Date    getWeek         var dayNames     Sunday    Monday    Tuesday    Wednesday    Thursday    Friday    Saturday        var now   new Date        document write dayNames now getDay             weekNumber           lt  script gt

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If you want something that works and is future-proof  use a library like MomentJS   moment date  week    moment date  isoWeek     http   momentjs com docs   get-set week

[javascript] Show week number with Javascript?

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