如何在 Node.js 中设置和访问类属性

How to set up and access class properties in Node.js?

本文关键字:访问 属性 设置 Node js      更新时间:2023-09-26

我在访问我创建的对象中的数据时遇到问题。我对JS和node很陌生,我认为我的问题是如何初始化变量,但我不知道。

这是我的初始化:

var http = require('http');
var MongoClient = require('mongodb').MongoClient;
var async = require('async');
var currentBoatList = [];
var BoatObjectList = [];

我有一个类来创建船只的当前信息(取自数据库):

function CurrentBoatInfo(boatName) {
    var name,MMSI,callSign,currentDate,positionJSON,status,speed,course;
    database.collection('Vessels').find({"BoatName":boatName},{"sort":{DateTime:-1}}).toArray(function(error1,vessel) {
            name = vessel[0].BoatName;
            MMSI = vessel[0].MMSI;
            callSign = vessel[0].VesselCallSign;
            console.log(name); ''logs the boats name, so the variable is there
        });
    });
}

我有我的 db 函数,可以拉取最近的船只,将它们的名称放在一个列表中,然后在另一个列表中为列表中的每个船名创建对象:

编辑:我看到我不必要地多次连接到mongoDB,使用代码来解决这个问题,并清除"db"变量名称。

var createBoats = function() {
    MongoClient.connect('mongodb://localhost:27017/tracks', function(err,database){
        if (err) {return console.dir(err); }
        else {console.log("connect to db");}
        database.collection('Vessels').find({"MostRecentContact": { "$gte": (new Date((new Date()).getTime() - (365*24*60*60*1000)))}}).toArray(function(error,docs) { //within a year
                docs.forEach(function(entry, index, array) {
                    currentBoatList.push(entry.BoatName); //create list of boats
                    BoatObjectList.push(new CurrentBoatInfo(entry.BoatName,database));
                });
                server();
            });
        });
};

最后,我的服务器代码只是创建一个服务器,并且应该从上面创建的每个对象中记录一些信息,但由于某种原因没有(输出如下):

var server = function() {
    http.createServer(function handler(req, res) {
        res.writeHead(200, {'Content-Type': 'text/plain'});
        console.log(BoatObjectList); //array of CurrentBoatInfo objects, prints [CurrentBoatInfo {}, CurrentBoatInfo {}, CurrentBoatInfo {}]
        console.log(BoatObjectList[0].name); //prints undefined
        BoatObjectList.forEach(function(entry) {
            var count = 0;
            for(var propertyName in entry.CurrentBoatInfo) { //nothing from here prints
                console.log(JSON.stringify(propertyName));
                count++;
                console.log(count);
            }
        });
        res.end();
    }).listen(1337, '127.0.0.1');
};

我看到的输出是这样的:

connect to db
[ 'DOCK HOLIDAY', 'BOATY MCBOATFACE', 'PIER PRESSURE' ] //list of boats
DOCK HOLIDAY  //boat names as they're being instantiated
BOATY MCBOATFACE
PIER PRESSURE
[ CurrentBoatInfo {}, CurrentBoatInfo {}, CurrentBoatInfo {} ] //list of boat objects
undefined //the name of the first boat in the object list
[ CurrentBoatInfo {}, CurrentBoatInfo {}, CurrentBoatInfo {} ]
undefined

仔细想想,我现在认为我的问题是 createServer 代码运行,但不记录,然后当我访问 127.0.0.1:1337 时,它会记录名称(实例化时未定义)......但是如何让创建服务器等待对象被实例化?

我能发现的明显问题是这段代码:

docs.forEach(function(entry, index, array) {
  currentBoatList.push(entry.BoatName); //create list of boats
  if (currentBoatList.length === array.length) {
    console.log(currentBoatList);
    async.eachOf(currentBoatList, function(entry,index,array) {     
      BoatObjectList[index] = new CurrentBoatInfo(currentBoatList[index]); //create object of the boat's info
    }, server()); //create the server after creating the objects
  }
});

这里的问题是 async.eachOf 是在同步 docs.forEach 函数中运行的异步函数。此外,async.eachOf 的第二个参数应该有一个回调,必须为数组中的每个项目调用该回调,如下所示:

    async.eachOf(array, function(item, index, callback) {     
      doSomethingAsync(function(err, result) {
         if (err) return callback(err);
         // do something with result
         callback();
      });
    }, function(err) {
      // this is called when all items of the array are processed
      // or if any of them had error, err here will contain the error
      if(err) {
        console.log("something went wrong", err);
      } else {
        console.log("success");
      }
  });

如您所见,将 server() 作为回调运行看起来不正确,因为您应该确保没有错误先传递给最终回调,然后再继续。

对于您的情况,我不明白为什么您使用async.eachOf而不是简单的currentBoatList.forEach 因为你没有在循环中执行任何异步操作,所以你只是填充了 BoatObjectList。

UPD 您的问题是,在假设变量已准备就绪之前,您没有等待异步操作完成。我希望以下内容能让你了解你应该如何实现你需要的东西(请注意,我重命名了一些变量和函数,只是为了让它们更清晰和易于理解):

database.collection('Vessels').find(query).toArray(function(err, vessels) {
  if (err) {
    // process error
    return;
  }
  async.eachOf(vessels, function(vessel, index, next) {
    currentBoatList.push(vessel.BoatName);
    getVesselInfoByName(vessel.BoatName, function(err, info) {
      if (err) {
        return next(err);
      }
      vesselsInfoList.push(info);
      next();
    });
  }, function(err) {
    if (err) {
      // process error
      return;
    }
    // all vessels are processed without errors, vesselsInfoList is ready to be used
    server();
  });
});
function getVesselInfoByName(vesselName, callback) {
  // do everything you need to get vessel info
  // once you have received the vesselInfo call callback
  // if any error happens return callback(err);
  // otherwise return callback(null, vesselInfo);
}

一般来说,我建议您了解有关node.js异步函数如何工作的更多信息,然后仔细查看异步库文档。