cookie-parser相信使用过Express的人肯定使用过，cookie-parser是一个Express解析cookie的中间件，其中关于signed cookie的疑问可以在 What are “signed” cookies in connect/expressjs?这里找到。（等我智商涨一点再来看了）。
req本质是inComingMessage，cookie-parser所做的工作是将cookie从header中转移到req中，并且转化为json对象。

大神片段：

var secrets = !secret || Array.isArray(secret)
            ? (secret || [])
            : [secret]

作用是：将字符串、空值、数组都转化为数组

/*!
 * cookie-parser
 * Copyright(c) 2014 TJ Holowaychuk
 * Copyright(c) 2015 Douglas Christopher Wilson
 * MIT Licensed
 */

'use strict'

/**
 * Module dependencies.
 * @private
 */

var cookie = require('cookie')
var signature = require('cookie-signature')

/**
 * Module exports.
 * @public
 */

module.exports = cookieParser
module.exports.JSONCookie = JSONCookie
module.exports.JSONCookies = JSONCookies
module.exports.signedCookie = signedCookie
module.exports.signedCookies = signedCookies

/**
 * Parse Cookie header and populate `req.cookies`
 * with an object keyed by the cookie names.
 *
 * @param {string|array} [secret] A string (or array of strings) representing cookie signing secret(s).
 * @param {Object} [options]
 * @return {Function}
 * @public
 */

function cookieParser (secret, options) {
    return function cookieParser (req, res, next) {
        // 如果cookies已经存储在req对象中，则直接跳过
        if (req.cookies) {
            return next()
        }

        // req本质是inComingMessage，cookie存在headers中

        var cookies = req.headers.cookie
        // secret可以为空，字符串，或者数组，全部转化为数组
        var secrets = !secret || Array.isArray(secret)
            ? (secret || [])
            : [secret]

        req.secret = secrets[0]
        // 普通cookie对象
        req.cookies = Object.create(null)
        // signed cookie对象
        req.signedCookies = Object.create(null)

        // no cookies
        if (!cookies) {
            return next()
        }

        // 利用cookie模块将其解析为json对象
        req.cookies = cookie.parse(cookies, options)

        // parse signed cookies
        if (secrets.length !== 0) {
            req.signedCookies = signedCookies(req.cookies, secrets)
            req.signedCookies = JSONCookies(req.signedCookies)
        }

        // parse JSON cookies
        req.cookies = JSONCookies(req.cookies)

        next()
    }
}

/**
 * Parse JSON cookie string.
 *
 * @param {String} str
 * @return {Object} Parsed object or undefined if not json cookie
 * @public
 */

function JSONCookie (str) {
    if (typeof str !== 'string' || str.substr(0, 2) !== 'j:') {
        return undefined
    }

    try {
        return JSON.parse(str.slice(2))
    } catch (err) {
        return undefined
    }
}

/**
 * Parse JSON cookies.
 *
 * @param {Object} obj
 * @return {Object}
 * @public
 */

function JSONCookies (obj) {
    var cookies = Object.keys(obj)
    var key
    var val

    for (var i = 0; i < cookies.length; i++) {
        key = cookies[i]
        val = JSONCookie(obj[key])

        if (val) {
            obj[key] = val
        }
    }

    return obj
}

/**
 * Parse a signed cookie string, return the decoded value.
 *
 * @param {String} str signed cookie string
 * @param {string|array} secret
 * @return {String} decoded value
 * @public
 */

function signedCookie (str, secret) {
    if (typeof str !== 'string') {
        return undefined
    }

    if (str.substr(0, 2) !== 's:') {
        return str
    }

    var secrets = !secret || Array.isArray(secret)
        ? (secret || [])
        : [secret]

    for (var i = 0; i < secrets.length; i++) {
        var val = signature.unsign(str.slice(2), secrets[i])

        if (val !== false) {
            return val
        }
    }

    return false
}

/**
 * Parse signed cookies, returning an object containing the decoded key/value
 * pairs, while removing the signed key from obj.
 *
 * @param {Object} obj
 * @param {string|array} secret
 * @return {Object}
 * @public
 */

function signedCookies (obj, secret) {
    // 获取cookie的所有键名为数组
    var cookies = Object.keys(obj)
    var dec
    var key
    var ret = Object.create(null)
    var val

    for (var i = 0; i < cookies.length; i++) {
        // 键
        key = cookies[i]
        // 值
        val = obj[key]
        dec = signedCookie(val, secret)

        if (val !== dec) {
            ret[key] = dec
            delete obj[key]
        }
    }

    return ret
}