sessionStorage polyfill

sessionStorage.js

/*
 * Based on: http://www.quirksmode.org/js/cookies.html
 * and https://github.com/wojodesign/local-storage-js/blob/master/storage.js
 * and https://gist.github.com/350433
 * License: http://www.opensource.org/licenses/MIT
 */

(function(window) {
    'use strict';
    window.sessionStorage = window.sessionStorage || {
        length: 0,
        setItem: function(key, value) {
            document.cookie = key + '=' + value + '; path=/';
            this.length++;
        },
        getItem: function(key) {
            var keyEQ = key + '=';
            var ca = document.cookie.split(';');
            for (var i = 0, len = ca.length; i < len; i++) {
                var c = ca[i];
                while (c.charAt(0) === ' ') c = c.substring(1, c.length);
                if (c.indexOf(keyEQ) === 0) return c.substring(keyEQ.length, c.length);
            }
            return null;
        },
        removeItem: function(key) {
            this.setItem(key, '', -1);
            this.length--;
        },
        clear: function() {
            // Caution: will clear all persistent cookies as well
            var ca = document.cookie.split(';');
            for (var i = 0, len = ca.length; i < len; i++) {
                var c = ca[i];
                while (c.charAt(0) === ' ') c = c.substring(1, c.length);
                var key = c.substring(0, c.indexOf('='));
                this.removeItem(key);
            }
            this.length = 0;
        },
        key: function(n) {
            var ca = document.cookie.split(';');
            if (n >= ca.length || isNaN(parseFloat(n)) || !isFinite(n)) return null;
            var c = ca[n];
            while (c.charAt(0) === ' ') c = c.substring(1, c.length);
            return c.substring(0, c.indexOf('='));
        }
    };
})(this);

Does it stick around after the session has ended?

Not in my testing. They're cookies without an expiry date, so they (should) die when the browser closes.
For persistent storage, this localStorage polyfill seems to work well:
https://github.com/wojodesign/local-storage-js/blob/master/storage.js

Nice!

Firefox 14 tells me "TypeError: setting a property that has only a getter, key: function(n) {" when loading the script. Any ideas?