Format date using Javascript

A Simple javascript function to format date. Pass a date as an argument and get formatted date.

I needed a JavaScript date format function such as the Visual Basic Format function, in which you can pass a format string and get the formatted string back; my first approach was to issue a series of consecutive and "destructive" replace calls, but upon discovering that the 5.5 (or higher) version of JScript supported the use of a function as the replaceTextargument of the replace method, I got creative.
Here's an example call of what I wanted:

SomeDiv.innerText = (new Date()).format('dddd, mmmm dd, yyyy.');

This would display:

Saturday, July 16, 2005

So in my first approach, I globally and case-insensitively replaced dddd with the corresponding string, which "destroyed" every occurrence, so that later in the code I could replace dd with the date number.

This worked just fine, but I knew that by inspecting the format specifier for a match, I could skip the search of every format specifier; say I only want the month and the date; well, by switching upon the format specifier (or rather "datepart" specifier), the year replacement will never be issued. Get it?

The fun part relies in the use of a function in the replaceText argument of the replace method; this way the $1 property as a function argument always represents the last match.

Other considerations include the format or "datepart" specifiers: none other than yyyy will be parsed as the year; months and days have the usual three flavors of fullname (mmmm), three-letter (mmm) or numeric (mm); hours (hh) can be rectified to the 12-hour format with the a/p specifier, and minutes (nn) and seconds (ss) may also be specified.

Javascript Function

// a global month names array
var gsMonthNames = new Array(
// a global day names array
var gsDayNames = new Array(
// the date format prototype
Date.prototype.format = function(f)
if (!this.valueOf())
return ' ';

var d = this;

return f.replace(/(yyyy|mmmm|mmm|mm|dddd|ddd|dd|hh|nn|ss|a\/p)/gi,
switch ($1.toLowerCase())
case 'yyyy': return d.getFullYear();
case 'mmmm': return gsMonthNames[d.getMonth()];
case 'mmm': return gsMonthNames[d.getMonth()].substr(0, 3);
case 'mm': return (d.getMonth() + 1).zf(2);
case 'dddd': return gsDayNames[d.getDay()];
case 'ddd': return gsDayNames[d.getDay()].substr(0, 3);
case 'dd': return d.getDate().zf(2);
case 'hh': return ((h = d.getHours() % 12) ? h : 12).zf(2);
case 'nn': return d.getMinutes().zf(2);
case 'ss': return d.getSeconds().zf(2);
case 'a/p': return d.getHours() < 12 ? 'a' : 'p';

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