Question

Here I have below a full html page for a calendar creation, copy it over to see what it does.
I have two buttons below that can increment month by one. My problem is because I can instantiating var month= date.getMonth() at the top of the page, the global variable month only allows me to increment and decrement by 1, how can I let month be set as a local variable so I am able to increment to more than just 1. Here is the code: I’m sure copy paste will work:

Below I have posted my my button handlers to increase month/decrease month.

<script type="text/javascript"> 

printCalendar(year,month,divDestination);
function printCalendar(year,mth,divDestination){
var currentmonth = mth;
var myOutput=document.getElementById(divDestination)
myOutput.innerHTML=makeCalendar(year,mth)
}
function prevMonth(m){
    m--;
    var currentmonth = m;
    alert(m);
    if(m < 0)
    {
        m = 11;
        year--;
    }

    printCalendar(year,currentmonth,divDestination);
}
function nextMonth(m){
    m++;    
    var currentmonth = m;
    if(m > 11)
    {
        m=0;
        year++;
    }
    printCalendar(year,currentmonth,divDestination);
}
function test(div){
var myOutput=document.getElementById("divCalendar")
myOutput.innerHTML += eventmonth;
}
</script>

<div align="center" id="divCalendar"></div>
<div align="center" id="myButtons">
<input type="button" onclick="test('divCalendar')" value="Say hi" />
<input type="button" onclick="prevMonth(month,'divCalendar')" value="Previous"/>
<input type="button" onclick="nextMonth(month,'divCalendar')" value="Next" />
<input type="button" onclick="printCalendar(year,month,'divCalendar')" value="Reload Calendar" />

Here below is my makeCalendar function

var myDate = new Date();
    var month = myDate.getMonth();
    var year = myDate.getFullYear();
    var divDestination="divCalendar";

    function leapYear(yr) { 
        if (yr < 1000) yr+=1900
        return((yr%4 == 0) && ((yr%100 == 0) || (yr%400 ==0)))
    }

    function startCol(width, height, color){
        return('<TD WIDTH=' + width + ' HEIGHT=' + height + '>' + '<FONT COLOR="' + color + '">');
    }

    function getHoliday(monthSelected,theday)
    {
        monthSelected = monthSelected + 1
        var holiday = ""
        var HolidayName = new Array (1, 1, "New Year's Day",7, 1, "Canada Day",12, 25, "Christmas Day",12, 26, "Boxing Day", 2,14,"Valentine's Day")
        for(var index = 0; HolidayName.length >= index; index++)
        {   
            if(HolidayName[index] == monthSelected && HolidayName[index+1] == theday)
            {
                holiday = HolidayName[index+2]
            }
        }
        return holiday
    }

    function makeCalendar(yr, mth)
    {
        var monthSelected = mth
        var months    = new Array("Jan", "Feb", "Mar", "Apr", "May", "June", "July", "Aug", "Sept", "Oct", "Nov", "Dec")
        var days      = new Array(31, leapYear(yr)?29:28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31)
        var weekDays  = new Array("Sun", "Mon", "Tues", "Wed", "Thurs", "Fri", "Sat")

        var mthSz         = days[mth]
        var mthName       = months[mth]
        var firstDyofMnth = new Date(yr, mth, 1)
        var firstDay      = firstDyofMnth.getDay() + 1
        var numRows       = Math.ceil((mthSz + firstDay-1)/7)
        var mthNameHeight = 50

        var borderWidth   = 2
        var cellSpacing   = 4 
        var cellHeight    = 80 

        var hdrColor      = "midnightblue" 
        var hdrSz         = "+3" 
        var colWidth      = 100 

        var dayCellHeight = 25 
        var dayColor      = "black" 
        var dayCtr        = 1

        // Build the HTML Table 
        var txt = '<CENTER>'
        txt += '<TABLE BORDER=' + borderWidth + ' CELLSPACING=' + cellSpacing + '>' 

        //Show Month Name and Year
        txt += '<TH COLSPAN=7 HEIGHT=' + mthNameHeight + '>' 
        txt += '<FONT COLOR="' + hdrColor + '" SIZE=' + hdrSz + '>' 
        txt += mthName + ' ' + year + '</FONT>' + '</TH>'

        // Show Days of the Week 
        txt += '<TR ALIGN="center" VALIGN="center">'
        for (var dy = 0; dy < 7; ++dy) {
            txt += startCol(colWidth, dayCellHeight, dayColor) + weekDays[dy] + '</FONT></TD>' 
        }
        txt += '</TR>'

        // Show Dates in Calendar
        for (var row=1; row <= numRows; ++row) 
        {
            txt += '<TR ALIGN="right" VALIGN="top">'
            for (var col = 1; col <= 7; ++col) 
            {
                if (((col < firstDay) && (row==1)) || (dayCtr>mthSz))
                    {txt += '<TD BGCOLOR="Gainsboro"><BR></TD>'}
                else
                    {
                        var event = getHoliday(monthSelected, dayCtr)
                        txt += '<TD HEIGHT=' + cellHeight + '><FONT COLOR="' + dayColor + '"> <B>'
                        txt += dayCtr
                        txt += '</B></FONT><BR>' + event + '</TD>'
                        dayCtr++;
                    }       
            }
            txt += '</TR>'
        }
        // close all basic table tags and output txt string
        txt += '</TABLE></CENTER>'  
        return txt
        }

Answer 1

Lets try to trim this down further to the money bits. Here’s basically what you are doing.

var month = new Date().getMonth()

var nextMonth = function(m) {
  m++;
  setMonthSomewhere(m);
};

nextMonth(month);

So what happens is that you pass your global, which is accessible everywhere anywhere, to the nextMonth() function. And function arguments basically declare a local variable, which is sets to the passed in value. You now have a global month variable set to 1, and a totally different m local to the nextMonth() function also set to 1. You then increment the local variable m to 2, which has no effect on the global variable.

Instead, simply don’t pass it in. Manipulate the global directly.

var month = new Date().getMonth()

var nextMonth = function() {
  month++;
  setMonthSomewhere(month);
};

nextMonth();

Now nextMonth() increments the global month variable directly, changing it’s value.