Pack an Array to a Buffer following given format string

pack.js

/* Extend Array to allow packing to Buffer 
 * 
 * Inspired by ruby array pack method
 * 
 * Takes a format fmt and walks the array
 * C = Element is interpreted as ASCII character (unsigned char)
 * a = Element is interpreted as ASCII string 
 * n = Element is interpreted as Short generating a 2 octets  
 * 
 * Just for the hack of it ;)
 * b = Element is binary but utf8 encoded as string 
 * 	   this is mainly a hack to get hashes from crypto into the buffer since 
 *     they are passed as strings, by first converting them into an array and 
 *     then writing the array into the buffer via buffer.copy
 *
 * 
 * The last fmt element is repeated for the length of the array
 */

Array.prototype.pack = function(fmt) {
	// Fill format sring to length 
	while(fmt.length < this.length) fmt += fmt[fmt.length-1];
	var a = new Array();
	var len = 0;
	this.forEach(function(el, idx, ary) {
		switch(fmt[idx]) {
			case 'C':
				var buf = new Buffer(1);
				buf[0] = el;
				len += 1;
				break;
			case 'a':
				var buf = new Buffer(el);
				len += el.length;
				break;
			case 'n':
				var buf = new Buffer(2);
				// Set lower bits
				buf[1] = el & 0xff;
				// Set higher bits
				buf[0] = el >> 8;
				len += 2;
				break;
			case 'b':
				var buf = new Buffer(1);
				buf[0] = el.charCodeAt(el);
				len += 1;
				break;
			default:
				throw 'Format invalid';
		}
		a.push(buf);
	});
	var res = new Buffer(len);
	var off = 0;
	a.forEach(function(el, idx, ary) {
		el.copy(res, off, 0);
		off += el.length;
	});
	return res;
}