变量始终记录为NaN

Question

有人能给我解释一下为什么每次我用console.log调用我的var clients时，它总是返回NaN吗？

//Imports
var inherits = require('util').inherits,
  MultiServer = require('../classes/servers/multi_server.js');
var clients = 0;

function PingServer(servers) {
  MultiServer.call(this, servers); //Inheritance

  //Basic server setup
  this.on('listen', function(params) {
    console.log("TCP server listening at", params['TCP'].ip + ":" + params['TCP'].port);
    console.log("Socket.IO server listening at", params['Socket.IO'].ip + ":" + params['Socket.IO'].port);
    console.log("All systems operational.");
  });

  this.on('error', function(err, source) {
    console.log("Error in " + source + " server:", err.message);
  });

  //Server behaviour
  this.on('connect', function(client) {
    console.log("A client has connected.");
    var clients = clients + 1;
    console.log("Client = " + clients);

    //Basic client setup
    client.on('disconnect', function() {
      console.log("A client has disconnected.");
      var clients = clients - 1;
      console.log("Clients = " + clients);
    });

    client.on('error', function(err) {
      console.log("A client has experienced an error:", err.message);
    });

    //Main server behaviour
    client.on('message', function(msg) {
      client.send(msg); //Ping reply
      console.log("Received message:", msg);
    });
  });
}
inherits(PingServer, MultiServer); //Inheritance
//Exports
module.exports = PingServer;

Answer 1

用var声明的变量是函数范围的，并在JavaScript中提升。每次输入var clients时，它都会在当前函数的作用域中创建一个变量。

这句话：

var clients = 0;

首先声明变量clients，然后为其赋值0。

然后你就在写

(function(){
  var clients = clients + 1;
  console.log("Client = " + clients);
})

然而，这又做了同样的事情:它声明了一个新的变量clients，并将它的作用域限定为函数。这个新的clients变量与初始变量不同。在此函数中，它被遮蔽(即无法访问)。实际上，函数中的clients是undefined。

提升意味着变量声明会自动移到函数的顶部。

这意味着语句

var clients = clients + 1;

函数中的等同于

var clients = undefined + 1;

这就是

var clients = NaN;

但是，如果删除了var关键字，则不会创建新变量，而是使用旧变量

var clients = 0;

(function(){
  clients = clients + 1; // clients = 0 + 1 === 1;
  console.log("Client = " + clients); // "Client = 1"
})

而且，将clients = clients + 1写成clients++可能更好。

Javascript function scoping and hoisting

• An example of variable shadowing in JavaScript

上的

• More

Answer 2

clients变量有多个声明。

改变这一点

var clients = clients + 1;

至

clients = clients + 1;

此文件(&T)

var clients = clients - 1;

至

clients = clients - 1;