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JavaScript

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;  
  
    // ISO 8601 states that week 1 is the week  
    // with the first thursday of that year.  
    // Set the target date to the thursday in the target week  
    target.setDate(target.getDate() - dayNr + 3);  
  
    // Store the millisecond value of the target date  
    var firstThursday = target.valueOf();  
  
    // Set the target to the first thursday of the year  
    // First set the target to january first  
    target.setMonth(0, 1);  
    // Not a thursday? Correct the date to the next thursday  
    if (target.getDay() != 4) {  
        target.setMonth(0, 1 + ((4 - target.getDay()) + 7) % 7);  
    }  
  
    // The weeknumber is the number of weeks between the   
    // first thursday of the year and the thursday in the target week  
    return 1 + Math.ceil((firstThursday - target) / 604800000); // 604800000 = 7 * 24 * 3600 * 1000  
}  

var today = new Date();  
alert("The current week number is: " + today.getWeek());