Nodejs變量聲明

Question

我是Nodejs和JavaScript的新手。我已經開始了一個使用快遞發電機的項目，並且已經安裝了貓鼬皮卡。

的路線/ index.js文件，我有這樣的代碼

var express = require('express'); 
var router = express.Router(); 
var mongoose = require('mongoose'); 
mongoose.connect('mongodb://localhost/test'); 
var db = mongoose.connection; 
db.on('error', console.error.bind(console, 'connection error:')); 
db.once('open', function (callback) { 
    var toSchema = mongoose.Schema({ 
     name: String, 
     state : Boolean 
    }); 
    var listing = mongoose.model('todoSchema', toSchema) 
}); 

/* GET home page. */ 
router.get('/', function(req, res) { 
    var silence = new listing({ name: 'Silence', state:true }) 
    console.log(silence.name) // 'Silence' 
    res.render('index', { title: silence.name }); 
}); 
module.exports = router; 

但是當我啓動服務器，我得到錯誤說

listing is not defined 

ReferenceError: listing is not defined 
at module.exports (D:\xampp\htdocs\nodeproject\myapp\routes\index.js:18:20) 
at Layer.handle [as handle_request] 
    D:\xampp\htdocs\nodeproject\myapp\node_modules\express\lib\router\layer.js:82:5) 
    at next (D:\xampp\htdocs\nodeproject\myapp\node_modules\express\lib\router\route.js:100:13) 
    at Route.dispatch (D:\xampp\htdocs\nodeproject\myapp\node_modules\express\lib\router\route.js:81:3) 
at Layer.handle [as handle_request] (D:\xampp\htdocs\nodeproject\myapp\node_modules\express\lib\router\layer.js:82:5) 
at D:\xampp\htdocs\nodeproject\myapp\node_modules\express\lib\router\index.js:235:24 
at Function.proto.process_params (D:\xampp\htdocs\nodeproject\myapp\node_modules\express\lib\router\index.js:313:12) 
at D:\xampp\htdocs\nodeproject\myapp\node_modules\express\lib\router\index.js:229:12 
at Function.match_layer (D:\xampp\htdocs\nodeproject\myapp\node_modules\express\lib\router\index.js:296:3) 
at next (D:\xampp\htdocs\nodeproject\myapp\node_modules\express\lib\router\index.js:190:10) 

你能告訴我什麼我做錯了，變量聲明如何工作？

Answer 1

這是因爲您的列表變量是從您想要使用的scope中定義的。嘗試在回調函數外部聲明該變量：

var listing; 
db.once('open', function (callback) { 
    var toSchema = mongoose.Schema({ 
     name: String, 
     state : Boolean 
    }); 
    listing = mongoose.model('todoSchema', toSchema); 
}); 

// you can use it anywhere as soon as it is initialized in time. 

Answer 2

混淆來自JavaScript沒有塊範圍但具有函數範圍的事實。您在函數中定義了listing，因此該變量將僅存在於該函數中。嘗試定義的功能範圍之外的變量：

var express = require('express'); 
var router = express.Router(); 
var mongoose = require('mongoose'); 
var listing; 

mongoose.connect('mongodb://localhost/test'); 

var db = mongoose.connection; 
db.on('error', console.error.bind(console, 'connection error:')); 
db.once('open', function (callback) { 
    var toSchema = mongoose.Schema({ 
     name: String, 
     state : Boolean 
    }); 
    listing = mongoose.model('todoSchema', toSchema) 
}); 

/* GET home page. */ 
router.get('/', function(req, res) { 
    var silence = new listing({ name: 'Silence', state:true }) 
    console.log(silence.name) // 'Silence' 
    res.render('index', { title: silence.name }); 
}); 
module.exports = router;