Lesson 8 Cookies

What is a cookie A little “tarball” of information stored on the client machine’s hard drive. –Usually in the cookies.txt file –information can be placed in the cookies.txt file two ways: by the client using Javascript by the server placing the cookie in the HTTP Response Header –in this case, you’ll usually be asked it its OK to save the cookie by your browser –the “tarball” consists of name value pairs

Cookie facts Cookies are domain specific cookie information can only be retrieved by servers in the same domain that set the cookie; If you set a cookie from Javascript it can only be retrieved by Javascript Cookies are properties of documents ( there is a document.cookie) Cookie records can contain: –cookie domain (domain of the server) –whether it need a sucure connection to access the cookie –pathname of URL capable of accessing the cookie –expiration date –name/value pairs

Cookie limitations Not everyone has a cookie-friendly browser (but most people do). Not everyone who has a cookie-friendly browser will accept your cookies (but most people will). Each domain is allotted only 20 cookies (on a specific client machine), so use them sparingly. Cookies must be no larger than 4 KB. That's just over 4,000 characters, which is plenty.

Life Cycle of Cookies When you start up your browser the contents of cookies.txt is loaded into memory during a browser session, cookies (including new ones) are kept in memory for speed reasons in memory cookies are written to cookies.txt when you shut down the browser cookies without expiration dates won’t be written to cookies.txt cookies with expiration dates will be persistent in cookies.txt until Netscape removed (when they expire)

Setting cookies Setting cookies is more frequently done by servers passing a cookie in the HTTP response header assign the information to document.cookie –successive assignments are not destructive, they keep appending to what is already there –Javascript limits what you can assign from the client as follows: Document.cookie = “cookieName = cookieData [ ; expires = timeInGMTString] [ ; path = pathName ] [ ; domain = domainName ] [ ; secure ]

More... Name/data - each cookie must have a name and data; data can contain no spaces (this implies URL encoding, use function escape( ) and unescape( ) expires - must be in GMT format (use toGMTString function); if expires is omitted cookie is temporary and won’t be written to cookies.txt path - for client side cookies just use the default Domain - to help synchronize cookie data across a set of documents for the specified domain (this is known as the cookie domain) SECURE - this is really for server-side cookies, so omit it.

Multiple name/value pairs If you want to save multiple name value pairs make sure you separate them with some character so you can parse them out (like a “:”) Function getCookieData(label) { var labelLen = label.length var cLen = document.cookie.length var I = 0 var cEnd while ( i < cLen) { var j = I + labelLen if ( document.cookie.substring( I, j ) == label ) { cEnd = document.cookie.indexOf (“:”), j) if (cEnd = = -1 ) { cEnd = document.cookie.length } return unescape(document.cookie.substring(j,cEnd) } i++ } return “ “ }