CalendarMonth.js

/* Expects month to be in 1-12 index based. */
var monthInformation = function(year, month){
    /* Create a date. Usually month in JS is 0-11 index based but here is a hack that can be used to calculate total days in a month */
    var date = new Date(year, month, 0);
    /* Get the total number of days in a month */
    this.totalDays = date.getDate();
    /* End day of month. Like Saturday is end of month etc. 0 means Sunday and 6 means Saturday */
    this.endDay = date.getDay();
    date.setDate(1);
    /* Start day of month. Like Saturday is start of month etc. 0 means Sunday and 6 means Saturday */
    this.startDay = date.getDay();
    /* Here we generate days for 42 cells of a Month */
    this.days = [];
    /* Here we calculate previous month dates for placeholders if starting day is not Sunday */
    var prevMonthDays = 0;
    if(this.startDay !== 0) prevMonthDays  = new Date(year, month - 1, 0).getDate() - this.startDay;
    /* This is placeholder for next month. If month does not end on Saturday, placeholders for next days to fill other cells */
    var count = 0;
    // 42 = 7 columns * 6 rows. This is the standard number. Verify it with any standard Calendar
    for(var i = 0; i < 42; i++) {
        var day = {};
        /* So start day is not Sunday, so we can display previous month dates. For that below we identify previous month dates */
        if(i < this.startDay) {
            day.date = (prevMonthDays = prevMonthDays + 1);
        /* belong to next month dates. So, month does not end on Saturday. So here we get next month dates as placeholders */
        } else if(i > this.totalDays + (this.startDay - 1)) { 
            day.date = (count = count + 1);
        /* belong to current month dates. */    
        } else {
            day.date = (i - this.startDay) + 1;
        }
        this.days[this.days.length] = day.date;
    }
};


/* Usage below */
monthInformation.call(this/* Context */, 4, 2015);