Moment.js get the week number based on a specific day (also past years)

to get week number by current date

moment(moment().toDate(), "MM-DD-YYYY").isoWeek()

  $(document).ready(function(){
    var weeknumber = moment("12-25-1995", "MM-DD-YYYY").week();
    console.log(weeknumber);
  });

According momentjs docs:

Because different locales define week of year numbering differently, Moment.js added moment#week to get/set the localized week of the year.

The week of the year varies depending on which day is the first day of the week (Sunday, Monday, etc), and which week is the first week of the year.

For example, in the United States, Sunday is the first day of the week. The week with January 1st in it is the first week of the year.

So, if you are having problems getting the right week number use .isoWeek()

$(document).ready(function(){
  var weeknumber = moment("11-26-2016", "MMDDYYYY").isoWeek();
  alert(weeknumber);
});

Example

You can also use format()

Examples:

moment().format('w') // as .week() like '1'
moment().format('W') // as .isoWeek() like '1'
moment().format('ww') // as .week() (2 digits) like '01'
moment().format('WW') // as .isoWeek() (2 digits) like '01'

ISO Week date: https://en.wikipedia.org/wiki/ISO_week_date

More info: https://momentjs.com/docs/#week-year-week-and-weekday-tokens