Trannosaur/getNextDayOfTheWeek.html

## getNextDayOfTheWeek.html
<html>
    <body>
        <div id="content"></div>
            <script>
            const DaysOfTheWeek =
            {
                SUNDAY: 0,
                MONDAY: 1,
                TUESDAY: 2,
                WEDNESDAY: 3,
                THURSDAY: 4,
                FRIDAY: 5,
                SATURDAY: 6,
            }
            Object.freeze(DaysOfTheWeek)

            function getNextDayOfWeek(sourceDate, targetDayOfWeek, forceToNextWeek = false)
            {
                var resultDate = new Date(sourceDate) // force a copy so we don't modify the original object

                var dayOfWeek = targetDayOfWeek % 7 // fix input if it's too large - ideally we'd throw an exception, but it's not like this is formal code

                var dayOffset = (7 /* go to next week */ + dayOfWeek /* go to the day we want */ - sourceDate.getDay() /* minus what we already are */)

                if (!forceToNextWeek)
                {
                    dayOffset = dayOffset % 7
                }

                var dayOfMonthForNextWeekday = resultDate.getDate() + dayOffset

                // this is magic, if your specified date goes into next month, it does the maths for us :)
                resultDate.setDate(dayOfMonthForNextWeekday);

                return resultDate;
            }

            // TEST CODE, you don't need this:

            function printTestResult(testResult)
            {
                if (testResult) return "✅"
                return "❌"
            }

            function appendText(html)
            {
                document.getElementById("content").innerHTML += html
            }

            var startDate = new Date() // today
            var firstMonth = startDate.getMonth()
            var testResult = ""

            appendText("Today's date:<br />" + startDate + "<br />");

            // These aren't... super rigorous, but whatever.
            var firstSunday = getNextDayOfWeek(startDate, DaysOfTheWeek.SUNDAY)
            appendText("<br />This better be a sunday:  " + printTestResult(firstSunday.getDay() == DaysOfTheWeek.SUNDAY) +"<br />" + firstSunday + "<br />")

            var nextSunday = getNextDayOfWeek(firstSunday, DaysOfTheWeek.SUNDAY)
            appendText("<br />Make sure this is the same date as above, because we're not forcing the next week: ")
            appendText(printTestResult(firstSunday.getTime() == nextSunday.getTime()) + "<br />" + nextSunday + "<br />");

            var forcedNextSunday = getNextDayOfWeek(nextSunday, DaysOfTheWeek.SUNDAY, true)
            appendText("<br />Make sure this is the sunday after the week above, because we're forcing the week: ")
            appendText(printTestResult(forcedNextSunday > nextSunday));
            appendText("<br />" + nextSunday + "<br />");

            nextWednesday = getNextDayOfWeek(forcedNextSunday, DaysOfTheWeek.WEDNESDAY, true)
            appendText("<br />Make sure this is a Wednesday: ")
            appendText(printTestResult(nextWednesday.getDay() == DaysOfTheWeek.WEDNESDAY));
            appendText("<br />" + nextWednesday + "<br />")

            nextSunday = getNextDayOfWeek(nextWednesday, DaysOfTheWeek.SUNDAY, true)

            // guarantee we cross over to the next month:
            nextSunday = getNextDayOfWeek(nextSunday, DaysOfTheWeek.SUNDAY, true)
            nextSunday = getNextDayOfWeek(nextSunday, DaysOfTheWeek.SUNDAY, true)
            appendText("<br />Make sure it crosses over month since first date: " + printTestResult(nextSunday.getMonth != firstMonth) + "<br />" + nextSunday + "<br />");

            // Just sanity checking:
            appendText("<br />This should not be a tick: " + printTestResult(false))
        </script>
    </body>
</html>
	<html>
	<body>
	<div id="content"></div>
	<script>
	const DaysOfTheWeek =
	{
	SUNDAY: 0,
	MONDAY: 1,
	TUESDAY: 2,
	WEDNESDAY: 3,
	THURSDAY: 4,
	FRIDAY: 5,
	SATURDAY: 6,
	}
	Object.freeze(DaysOfTheWeek)

	function getNextDayOfWeek(sourceDate, targetDayOfWeek, forceToNextWeek = false)
	{
	var resultDate = new Date(sourceDate) // force a copy so we don't modify the original object

	var dayOfWeek = targetDayOfWeek % 7 // fix input if it's too large - ideally we'd throw an exception, but it's not like this is formal code

	var dayOffset = (7 /* go to next week / + dayOfWeek / go to the day we want / - sourceDate.getDay() / minus what we already are */)

	if (!forceToNextWeek)
	{
	dayOffset = dayOffset % 7
	}

	var dayOfMonthForNextWeekday = resultDate.getDate() + dayOffset

	// this is magic, if your specified date goes into next month, it does the maths for us :)
	resultDate.setDate(dayOfMonthForNextWeekday);

	return resultDate;
	}

	// TEST CODE, you don't need this:

	function printTestResult(testResult)
	{
	if (testResult) return "✅"
	return "❌"
	}

	function appendText(html)
	{
	document.getElementById("content").innerHTML += html
	}

	var startDate = new Date() // today
	var firstMonth = startDate.getMonth()
	var testResult = ""

	appendText("Today's date:<br />" + startDate + "<br />");

	// These aren't... super rigorous, but whatever.
	var firstSunday = getNextDayOfWeek(startDate, DaysOfTheWeek.SUNDAY)
	appendText("<br />This better be a sunday: " + printTestResult(firstSunday.getDay() == DaysOfTheWeek.SUNDAY) +"<br />" + firstSunday + "<br />")

	var nextSunday = getNextDayOfWeek(firstSunday, DaysOfTheWeek.SUNDAY)
	appendText("<br />Make sure this is the same date as above, because we're not forcing the next week: ")
	appendText(printTestResult(firstSunday.getTime() == nextSunday.getTime()) + "<br />" + nextSunday + "<br />");

	var forcedNextSunday = getNextDayOfWeek(nextSunday, DaysOfTheWeek.SUNDAY, true)
	appendText("<br />Make sure this is the sunday after the week above, because we're forcing the week: ")
	appendText(printTestResult(forcedNextSunday > nextSunday));
	appendText("<br />" + nextSunday + "<br />");

	nextWednesday = getNextDayOfWeek(forcedNextSunday, DaysOfTheWeek.WEDNESDAY, true)
	appendText("<br />Make sure this is a Wednesday: ")
	appendText(printTestResult(nextWednesday.getDay() == DaysOfTheWeek.WEDNESDAY));
	appendText("<br />" + nextWednesday + "<br />")

	nextSunday = getNextDayOfWeek(nextWednesday, DaysOfTheWeek.SUNDAY, true)

	// guarantee we cross over to the next month:
	nextSunday = getNextDayOfWeek(nextSunday, DaysOfTheWeek.SUNDAY, true)
	nextSunday = getNextDayOfWeek(nextSunday, DaysOfTheWeek.SUNDAY, true)
	appendText("<br />Make sure it crosses over month since first date: " + printTestResult(nextSunday.getMonth != firstMonth) + "<br />" + nextSunday + "<br />");

	// Just sanity checking:
	appendText("<br />This should not be a tick: " + printTestResult(false))
	</script>
	</body>
	</html>