document.cookie=c_name + "=" + c_value;这样写。

下面有Cookies的综合使用。
/* COOKIES OBJECT */
var Cookies = {
        // Initialize by splitting the array of Cookies
    init: function () {
        var allCookies = document.cookie.split('; ');
        for (var i=0;i<allCookies.length;i++) {
            var cookiePair = allCookies[i].split('=');
            this[cookiePair[0]] = cookiePair[1];
        }
    },
        // Create Function: Pass name of cookie, value, and days to expire
    create: function (name,value,days) {
        if (days) {
            var date = new Date();
            date.setTime(date.getTime()+(days*24*60*60*1000));
            var expires = "; expires="+date.toGMTString();
        }
        else var expires = "";
        document.cookie = name+"="+value+expires+"; path=/";
        this[name] = value;
    },
        // Erase cookie by name
    erase: function (name) {
        this.create(name,'',-1);
        this[name] = undefined;
    }
};
Cookies.init();