Javascript - Date isSameWeek() function

isSameWeek() function

The javascript function isSameWeek() checks if two dates are in the same week. 
The extra logic in the code validades when the week begins in a year and ends in another one.

The getWeek() function is found at http://javascript.about.com/library/blweekyear.htm

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script.js

Date.prototype.resetTime = function(){
    this.setHours(0, 0, 0, 0);

    return this;
}

Date.prototype.getWeek = function(){
    var date = this.resetTime(); 
    var onejan = new Date(date.getFullYear(), 0, 1);
    return Math.ceil((((date - onejan) / 86400000) + onejan.getDay() + 1) / 7);
}

Date.prototype.isLastWeekOfTheYear = function(){
    return this.getWeek() == 53;
}

Date.prototype.isFirstWeekOfTheYear = function(){
    return this.getWeek() == 1;
}

Date.prototype.isSameWeek = function(dateToCompare){
    var sameWeek = this.getWeek() == dateToCompare.getWeek() && this.getFullYear() == dateToCompare.getFullYear();
    
    if (!sameWeek && this.isLastWeekOfTheYear() && dateToCompare.isFirstWeekOfTheYear()){
        sameWeek = this.getDay() < dateToCompare.getDay();
    }
    
    return sameWeek;
}

var date1 = new Date(2012, 11, 30);
var date2 = new Date(2013, 0, 5);

console.log('Same week? ' + date1.isSameWeek(date2));