From http://www.webtoolkit.info/javascript-url-decode-encode.html

url.js

/**
*
*  URL encode / decode
*  http://www.webtoolkit.info/
*
**/
 
var Url = {
 
	// public method for url encoding
	encode : function (string) {
		return escape(this._utf8_encode(string));
	},
 
	// public method for url decoding
	decode : function (string) {
		return this._utf8_decode(unescape(string));
	},
 
	// private method for UTF-8 encoding
	_utf8_encode : function (string) {
		string = string.replace(/\r\n/g,"\n");
		var utftext = "";
 
		for (var n = 0; n < string.length; n++) {
 
			var c = string.charCodeAt(n);
 
			if (c < 128) {
				utftext += String.fromCharCode(c);
			}
			else if((c > 127) && (c < 2048)) {
				utftext += String.fromCharCode((c >> 6) | 192);
				utftext += String.fromCharCode((c & 63) | 128);
			}
			else {
				utftext += String.fromCharCode((c >> 12) | 224);
				utftext += String.fromCharCode(((c >> 6) & 63) | 128);
				utftext += String.fromCharCode((c & 63) | 128);
			}
 
		}
 
		return utftext;
	},
 
	// private method for UTF-8 decoding
	_utf8_decode : function (utftext) {
		var string = "";
		var i = 0;
		var c = c1 = c2 = 0;
 
		while ( i < utftext.length ) {
 
			c = utftext.charCodeAt(i);
 
			if (c < 128) {
				string += String.fromCharCode(c);
				i++;
			}
			else if((c > 191) && (c < 224)) {
				c2 = utftext.charCodeAt(i+1);
				string += String.fromCharCode(((c & 31) << 6) | (c2 & 63));
				i += 2;
			}
			else {
				c2 = utftext.charCodeAt(i+1);
				c3 = utftext.charCodeAt(i+2);
				string += String.fromCharCode(((c & 15) << 12) | ((c2 & 63) << 6) | (c3 & 63));
				i += 3;
			}
 
		}
 
		return string;
	}
 
}