24 October 2009
HTTP Cookies: What's the difference between Max-age and Expires?
Expiressets an expiry date for when a cookie gets deleted
Max-agesets the time in seconds for when a cookie will be deleted
- Internet Explorer (ie6, ie7, and ie8) does not support “max-age”, while (mostly) all browsers support expires
Max-age vs Expires, let’s dive in a little deeper:
expires parameter was part of the original cookies baked up by Netscape. In HTTP version 1.1,
expires was deprecated and replaced with the easier-to-use
max-age—instead of having to specify a date, you can just say how long the cookie can live. By setting either of these, the cookie will persist until its time runs out, otherwise—if you set neither—the cookie will last until you close your browser (a “session cookie”).
Setting a cookie for “foo=bar” to last 5 minutes, using
var d = new Date(); d.setTime(d.getTime() + 5*60*1000); // in milliseconds document.cookie = 'foo=bar;path=/;expires='+d.toGMTString()+';';
And the same with
document.cookie = 'foo=bar;path=/;max-age='+5*60+';';
Unfortunately, none of the current versions of Internet Explorer support
max-age, so if you want proper cookie persistence cross-browser, then stick to
Let’s open this up to some fake Q&A…
Q. What if I set both expires and max-age in a cookie?
A. Every browser that supports
max-age will ignore the
expires regardless of it’s value, and likewise, Internet Explorer will ignore the
max-age and just use
Q. What if I set just max-age in a cookie?
A. Every browser—except Internet Explorer—uses it properly. In Internet Explorer it will be a session cookie (it will be deleted when you close your browser).
Q. What if I set just expires in a cookie?
A. Every browser uses and persists it properly, just remember to set it in GMT time as seen in the example above.
Q. Where did you get these facts from?
A. I wrote a cookie persistence test page and tested it out on IE6, IE7, IE8, FF2, FF3, Safari 4, Google Chrome, and Opera 9.6. Let me know if you try it out on any other browsers or see anything contradictory.
Q. What’s the moral of this story?
A. If you care about your cookies functioning properly for a huge percentage of web users (65.66%), don’t persist your cookies “the right way” according to spec (
max-age), persist them the way that works (