I'm having real trouble working out whether JavaScript's getDay() method (.asp) plies with ISO 8601.

getDay() returns 0 for Sunday, 1 for Monday etc..

The documentation remends mapping to day names like this:

var weekday=new Array(7);
weekday[0]="Sunday";
weekday[1]="Monday";
weekday[2]="Tuesday";
...
var n = weekday[(new Date()).getDay()];

I can only see that the ISO standard defines Monday as the first day of the week. I would assume this means Monday should be 0, not Sunday.

Does anyone have any experience with this? Can you clarify and would you remend overriding the method to make Monday the first day of the week?

I'm having real trouble working out whether JavaScript's getDay() method (http://www.w3schools./jsref/jsref_getday.asp) plies with ISO 8601.

getDay() returns 0 for Sunday, 1 for Monday etc..

The documentation remends mapping to day names like this:

var weekday=new Array(7);
weekday[0]="Sunday";
weekday[1]="Monday";
weekday[2]="Tuesday";
...
var n = weekday[(new Date()).getDay()];

I can only see that the ISO standard defines Monday as the first day of the week. I would assume this means Monday should be 0, not Sunday.

Does anyone have any experience with this? Can you clarify and would you remend overriding the method to make Monday the first day of the week?

• javascript

3 Answers 3

var weekday=new Array(7);
weekday[0]="Monday";
weekday[1]="Tuesday";
weekday[2]="Wednesday";
...
var n = weekday[((new Date()).getDay() + 6) % 7 + 1];

or extend the Date prototype:

Date.prototype.getISODay = function(){ return (this.getDay() + 6) % 7 + 1; }

and then use

var n = weekday[(new Date()).getISODay()];

How the weekdays are represented only matters if you want to use a date format where the weekday actually is represented as a number, e.g. the YYYY-Www-D format, or if you want to get the week number for a specific date.

Otherwise it doesn't matter how the weekdays are represented numerically. A monday is still a monday.

The only sources I could find which state which numbers the days of the week should be were Wikipedia and this page. Both suggest the day of week numbers should be 1 to 7 and start from Monday.

I support ic3b3rg's suggestion of extending Date.prototype, although would suggest using UTC and a literal to get the names. The following example uses an Object and an anonymous function so the names only need be defined once.

(function () {
    var dayNames = {1: 'Monday', 2: 'Tuesday', 3: 'Wednesday', 4: 'Thursday', 5: 'Friday', 6: 'Saturday', 7: 'Sunday'};
    // Local time
    Date.prototype.getISODay = function () {return this.getDay() || 7;};
    Date.prototype.getISODayName = function () {return dayNames[this.getDay() || 7];};
    // Date.prototype.getDayName = Date.prototype.getISODayName
    // UTC
    Date.prototype.getUTCISODay = function () {return this.getUTCDay() || 7;};
    Date.prototype.getUTCISODayName = function () {return dayNames[this.getUTCDay() || 7];};
    // Date.prototype.getUTCDayName = Date.prototype.getUTCISODayName;
}());

As the day name would be the same as the ISO day name (assuming English), I've added in extra lines which can be unmented if you want both.