JavaScript function for the get a specifed weekday n-weeks before / in the future I'm currently working on a intranet application.

It shall have a GUI on which you can choose a weekday and a positive / negative integer. The integer stands for weeks.

So let's say someone chooses the Wednesday and (-7) then it shall display the date of the Wednesday seven weeks ago. 

Tuesday and 3 => Tuesday in three weeks.

Here's the code which shall calculate the date: 

    // Returns a Date Object which contains
    // the date before/in n-days and 
    // before/in n-weeks.

    // -- Parameter ----------------------------
    // 1. String - The desired weekday as an 
    //   abbreviation.
    //   Assign for ...
    //     Monday    : 'mo'
    //     Tuesday   : 'tu'
    //     Wednesday : 'we'
    //     Thursday  : 'th'
    //     Friday    : 'fr'
    //     Saturday  : 'sa'
    //     Sunday    : 'su'

    // 2. Number - Positive number =&gt; n-weeks in
    //   the future.
    //   Negative number =&gt; n-weeks before.

    // -- Return -------------------------------
    // (Date) Object 

    function getDayBeforeIn(day, weeks) {
      var currentDate = new Date();
      var currentStamp = currentDate.getTime();   
      var weekdays = {
        'mo': 1,
        'tu': 2,
        'we': 3,
        'th': 4,
        'fr': 5,
        'sa': 6,
        'su': 0
      }   
      var milliSecondsDay = 24 * 60 * 60 * 1000;

      day   = day   || 'mo';
      weeks = weeks || 0;

      currentStamp += weeks * 7 * milliSecondsDay;
      currentStamp +=
        (weekdays[day] - currentDate.getDate()) *
         milliSecondsDay;     

      return new Date(currentStamp);
    }

    // Returns the Day, Month, Year of an 
    // JavaScript Date Object as as String
    // in the Date format DMY.

    // -- Parameter ----------------------------
    // 1. Date-Object
    // 2. String - Is used for separating 
    // Day-Year and Month-Year.

    // -- Return -------------------------------
    // String

    function getDayMonthYearDate(dateObject, separator) {
      var day   = '0' + dateObject.getDate();
      var month = '0' + (dateObject.getMonth() + 1);
      var year  = dateObject.getFullYear();

      separator = separator || '.';

      return day.slice(-2) + separator +
             month.slice(-2) + separator + year;
    }

    // -- TEST -----------------
    for (var i = 0; i &lt; 10; i++) {
      var result = getDayBeforeIn('th', i * (- 1));

      console.log(getDayMonthYearDate(result));
    }
    /* -- Results ---
        04.02.2016
        28.01.2016
        21.01.2016
        14.01.2016
        07.01.2016
        31.12.2015
        24.12.2015
        17.12.2015
        10.12.2015
        03.12.2015  
    */


All suggestions and hints concerning improvements welcome.
var milliSecondsDay = 24 * 60 * 60 * 1000;


This is a constant. Use all caps and underscore to indicate such. If you can use ES6, use const instead of var.

var currentDate = new Date();
var currentStamp = currentDate.getTime(); 


Use Date.now() to get the current timestamp. This saves you from creating a Date object.

return day.slice(-2) + separator + month.slice(-2) + separator + year;


Appears that you are using zero-padding. Create a separate function for it to make it reusable.

getDayBeforeIn


You aren't really returning the day, you're returning a Date object. Either you rename this to reflect the return value, like getDateBeforeIn or return a representation of the day of the week.