node/node_modules/mongoose/node_modules/mongodb/lib/server.js:242

        process.nextTick(function() { throw err; })

                                      ^

Error: getaddrinfo ENOTFOUND localhost localhost:27017

    at errnoException (dns.js:26:10)

    at GetAddrInfoReqWrap.onlookup [as oncomplete] (dns.js:77:26)

进行到在app.js里 添加mongoose.connect()之后，启动命令行报错，求解