cookie的用途：用于在客户端存储会话消息
相关API：doument.cookie;
创建cookie:document.cookie = newCookie;
newCookie是一个键值对形成的字符串，由以下几部分组成：
  • 名称(name=value)：一个唯一确定的cookie名称
  • 域(;domain=domain) :对于哪个域是有效的，如果没有定义，默认为当前文档位置的路径的域名部分
  • 有效时间( expires=date-in-GMTString-format) 如果没有定义，cookie会在对话结束时过期 或（ ;max-age=max-age-in-seconds ）
  • 路径（;path=path）默认为当前文档位置的路径
  • 安全标志(;secure)设置以后，cookie只能通过https协议传输
获取cookie:alert(document.cookie);
cookie的值字符串可以用encodeURIComponent()来保证它不包含任何逗号、分号或空格
 以下是一个支持cookie读写的工具库源码
/*\
|*|
|*|  :: cookies.js ::
|*|
|*|  A complete cookies reader/writer framework with full unicode support.
|*|
|*|  https://developer.mozilla.org/en-US/docs/DOM/document.cookie
|*|
|*|  This framework is released under the GNU Public License, version 3 or later.
|*|  http://www.gnu.org/licenses/gpl-3.0-standalone.html
|*|
|*|  Syntaxes:
|*|
|*|  * docCookies.setItem(name, value[, end[, path[, domain[, secure]]]])
|*|  * docCookies.getItem(name)
|*|  * docCookies.removeItem(name[, path], domain)
|*|  * docCookies.hasItem(name)
|*|  * docCookies.keys()
|*|
\*/

var docCookies = {
    getItem: function (sKey) {
        return decodeURIComponent(document.cookie.replace(
            new RegExp("(?:(?:^|.*;)\\s*" + encodeURIComponent(sKey).replace(/[\-\.\+\*]/g, "\\$&") + "\\s*\\=\\s*([^;]*).*$)|^.*$"), "$1"))
            || null;
    },
    setItem: function (sKey, sValue, vEnd, sPath, sDomain, bSecure) {
        if (!sKey || /^(?:expires|max\-age|path|domain|secure)$/i.test(sKey)) {
            return false;
        }
        var sExpires = "";
        if (vEnd) {
            switch (vEnd.constructor) {
                case Number:
                    sExpires = vEnd === Infinity ? "; expires=Fri, 31 Dec 9999 23:59:59 GMT" : "; max-age=" + vEnd;
                    break;
                case String:
                    sExpires = "; expires=" + vEnd;
                    break;
                case Date:
                    sExpires = "; expires=" + vEnd.toUTCString();
                    break;
            }
        }
        document.cookie = encodeURIComponent(sKey) +
            "=" + encodeURIComponent(sValue) +
            sExpires +
            (sDomain ? "; domain=" + sDomain : "") +
            (sPath ? "; path=" + sPath : "") +
            (bSecure ? "; secure" : "");
        return true;
    },
    removeItem: function (sKey, sPath, sDomain) {
        if (!sKey || !this.hasItem(sKey)) {
            return false;
        }
        document.cookie = encodeURIComponent(sKey) +
            "=; expires=Thu, 01 Jan 1970 00:00:00 GMT" +
            ( sDomain ? "; domain=" + sDomain : "") +
            ( sPath ? "; path=" + sPath : "");
        return true;
    },
    hasItem: function (sKey) {
        return (new RegExp("(?:^|;\\s*)" + encodeURIComponent(sKey).replace(/[\-\.\+\*]/g, "\\$&") + "\\s*\\=")).test(document.cookie);
    },
    /* optional method: you can safely remove it! */
    keys: function () {
        var aKeys = document.cookie.replace(/((?:^|\s*;)[^\=]+)(?=;|$)|^\s*|\s*(?:\=[^;]*)?(?:\1|$)/g, "").split(/\s*(?:\=[^;]*)?;\s*/);
        for (var nIdx = 0; nIdx < aKeys.length; nIdx++) {
            aKeys[nIdx] = decodeURIComponent(aKeys[nIdx]);
        }
        return aKeys;
    }
};