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);

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