I know it's present in at least IE 6 and later, Firefox 1 and later, and Dottoro reports that it is supported by all major browsers. However, it is not part of any DOM specification and therefore is not guaranteed to be available in or properly implemented by all browsers (for instance, mobile browsers with limited DOM implementations).

As some have discovered, IE returns true for navigator.cookieEnabled even if cookies are blocked for the current site. This means that you cannot currently rely on the property at all and you should avoid it completely.

For a complete cross browser cookie support check, you might want to go with something like this:

var cookies = (cookie in document && (document.cookie.length > 0 ||

        (document.cookie = test).indexOf.call(document.cookie, test) > -1));

Demo: http://codetester.org/31011785

This will return false in browsers that have cookies disabled or don't support the DOM level 2 property document.cookie, which is about as far as you can go in JS.