Question

有一段代码用node.js查询来自mongodb的数据：

var MongoClient = require('mongodb').MongoClient
    , assert = require('assert');


//a query that returns all the documents
var findDocuments = function(db, callback) {
  // Get the documents collection
  var collection = db.collection('documents');
  // Find some documents
  collection.find({}).toArray(function(err, docs) {
    assert.equal(err, null);
    console.log("Found the following records");
    console.log(docs)
    callback(docs);
  });
}


var MongoClient = require('mongodb').MongoClient
  , assert = require('assert');

// Connection URL
var url = 'mongodb://localhost:27017/myproject';
// Use connect method to connect to the server
MongoClient.connect(url, function(err, db) {
  assert.equal(null, err);
  console.log("Connected correctly to server");

  var result = findDocuments(db, function() {
      db.close();
  });
});


//export the query result,but it doesn't work because it is a local variable in `MongoClient.connect()`
export {result}

问题：
我想导出变量result，但它不起作用，因为它是MongoClient.connect()中的局部变量。我该怎么办？

Answer 1

在函数之外定义它，如

var result

Answer 2

您需要在文档中全局定义它。

var MongoClient = require('mongodb').MongoClient
    , assert = require('assert');


var result; //define it out here to be able to use it on anywhere

//a query that returns all the documents
var findDocuments = function(db, callback) {
  // Get the documents collection
  var collection = db.collection('documents');
  // Find some documents
  collection.find({}).toArray(function(err, docs) {
    assert.equal(err, null);
    console.log("Found the following records");
    console.log(docs)
    callback(docs);
  });
}


var MongoClient = require('mongodb').MongoClient
  , assert = require('assert');

// Connection URL
var url = 'mongodb://localhost:27017/myproject';
// Use connect method to connect to the server
MongoClient.connect(url, function(err, db) {
  assert.equal(null, err);
  console.log("Connected correctly to server");

  result = findDocuments(db, function() {   //give it its value
      db.close();
  });
});


//export the query result,but it doesn't work because it is a local variable in `MongoClient.connect()`
export {result}

Answer 3

如果您使用的是ES6，我建议您使用关键字let/const代替var。

现在关于你的问题为什么要以这种方式导出结果？

您应该创建一个函数connect()并将其导出。然后在你的控制器中，调用这个函数并处理结果。

这里的连接是完全异步和不受控制的，这是非常糟糕的行为。

做什么的例子：

档案mongodb.es6

export default class Mongodb {

  connect() {
    ...
  }

  ...
}

档案main.es6

 const mongodbObject = new Mongodb();

 mongodbObject.connect()
   .then(() => {
     // I am connected so I can do whatever I want
   })
   .catch((err) => {
     // I have an error and do something about it
   });

Answer 4

不要这样做！

执行文件时，仅查询数据。我没有理由这样做。

而是从此文件中公开一个函数，该函数将连接到数据库，查询并返回结果对象，每次调用它。

export {
   getResult: function(query, callback) {
     // Connection URL
     var url = 'mongodb://localhost:27017/myproject';
     // Use connect method to connect to the server
     MongoClient.connect(url, function(err, db) {

        if (err) return callback(err);
        console.log("Connected correctly to server");

        result = findDocuments(db, function() {
           db.close();
        });
        callback(null, result);
     });
   }
}

ES6：如何导出node.js的这个变量？

4 个答案: