Mongodb find() 返回 undefined (node.js)

Question

我一直在 node.js 中使用 mongodb。我已经用一些数据做了一个基本的集合（我知道它在那里我检查过）。当我尝试在集合上运行 find() 时，它返回未定义。我不知道这是为什么。代码如下：

function get_accounts(){
    var MongoClient = mongodb.MongoClient;
    var url = "url";

    MongoClient.connect(url, function (err, db) {
      if (err) {
        console.log('Unable to connect to the mongoDB server. Error:', err);
      } else {
        //HURRAY!! We are connected. :)
        console.log('Connection established to database');
        var collection = db.collection('accounts');
        collection.find().toArray(function(err, docs) {
          console.log("Printing docs from Array")
          docs.forEach(function(doc) {
            console.log("Doc from Array ");
            console.dir(doc);
          });
        });
        console.log("mission complete");
        }
        db.close();
    }
  );
}

如果你知道为什么会这样，我想听听你的想法。谢谢！如果有任何区别，该数据库是 mongolab 托管的数据库。

Answer 1

由于 node.js 的异步性质，您得到了一个未定义的值，在您的代码中没有任何逻辑告诉 console.log 语句等待find()语句完成，然后再打印出文档。您必须了解Node.js中回调的概念。但是，这里有一些问题可以解决。很多刚开始使用 node 的人都倾向于嵌套大量匿名函数，从而造成可怕的“末日金字塔”或回调地狱。通过拆分一些功能并命名它们，您可以使其更清晰，更易于遵循：

var MongoClient = require("mongodb").MongoClient

// move connecting to mongo logic into a function to avoid the "pyramid of doom"
function getConnection(cb) {  
    MongoClient.connect("your-mongo-url", function(err, db) {
        if (err) return cb(err);
        var accounts = db.collection("accounts");
        cb(null, accounts);
    })
}    
// list all of the documents by passing an empty selector.
// This returns a 'cursor' which allows you to walk through the documents
function readAll(collection, cb) {  
   collection.find({}, cb);
}

function printAccount(account) {  
    // make sure you found your account!
    if (!account) {
        console.log("Couldn't find the account you asked for!");
    }
    console.log("Account from Array "+ account);
}

// the each method allows you to walk through the result set, 
// notice the callback, as every time the callback
// is called, there is another chance of an error
function printAccounts(accounts, cb) {  
    accounts.each(function(err, account) {
        if (err) return cb(err);
        printAccount(account);
    });
}

function get_accounts(cb) {  
    getConnection(function(err, collection) {
        if (err) return cb(err);    
        // need to make sure to close the database, otherwise the process
        // won't stop
        function processAccounts(err, accounts) {
            if (err) return cb(err);
            // the callback to each is called for every result, 
            // once it returns a null, you know
            // the result set is done
            accounts.each(function(err, account) {
                if (err) return cb(err)  
                if (hero) {  
                    printAccount(account);
                } else {
                    collection.db.close();
                    cb();
                }
            })
        }
        readAll(collection, processAccounts);        
    })
}

// Call the get_accounts function
get_accounts(function(err) {  
     if (err) {
         console.log("had an error!", err);
         process.exit(1);
     }
});