javascript - 对象未在 Javascript 中返回所需的对象

Question

我不明白为什么 fetch 函数没有返回我想要的信息:

(function(){
    var root = this;
    var Database = root.Database = {};

    var Db = require('mongodb').Db,
    Connection = require('mongodb').Connection,
    Server = require('mongodb').Server;

    var host = process.env['MONGO_NODE_DRIVER_HOST'] != null ? process.env['MONGO_NODE_DRIVER_HOST'] : 'localhost';
    var port = process.env['MONGO_NODE_DRIVER_PORT'] != null ? process.env['MONGO_NODE_DRIVER_PORT'] : Connection.DEFAULT_PORT;

    // Require Underscore, if we're on the server, and it's not already present.
    var _ = root._;
    if (!_ && (typeof require !== 'undefined')) _ = require('./underscore');



    Database.ActiveRecord = function(attributes, collection){
        this.collection = collection;
        this.cid = _.uniqueId('c');
        attributes || (attributes = {});
        this.attributes = attributes;
    };

    // Connecting to database and setting the connection as a shared variable among ActiveRecords
    console.log("Connecting to " + host + ":" + port);

    Database.ActiveRecord.prototype.db = new Db('node-mongo-eslip', new Server(host, port, {}));

    _.extend(Database.ActiveRecord.prototype, {

        initialize: function(){},

        // Returns `true` if the attribute contains a value that is not null
        // or undefined.
        has: function(attr) {
            return this.attributes[attr] != null;
        },

        // Sets attributes
        set: function(key, value){
            var attrs, attr, val;
            if(_.isObject(key) || key == null){
                throw Error("The key should not be an object or null");
            }else{
                attrs = {};
                attrs[key] = value;
            }

            if (!attrs) return this;

            var now = this.attributes;

            for(attr in attrs){
                val = attrs[attr];
                if(!_.isEqual(now[attr], val)) now[attr] = val;
            }
        },

        unset: function(key){
            return this.set(attr, null);
        },

        toJSON: function() {
            return _.clone(this.attributes);
        },

        fetch: function(query, fields, options){
            var record = this;
            record.db.open(function(err, db){
                if(!(record.collection||(typeof record.collection === 'string'))) throw Error('You should define a name attribute, which represents the collection of the Database');
                db.collection(record.collection, function(err, collection){
                    console.log('Fetching...');
                    collection.find(query, fields, options).toArray(function(err, docs) {
                        return docs;
                    });
                });
            });
        },

        save: function(){
            var record = this;
            record.db.open(function(err, db){
                if(!(record.collection||(typeof record.collection === 'string'))) throw Error('You should define a name attribute, which represents the collection of the Database');
                db.collection(record.collection, function(err, collection){
                    collection.insert(_.clone(record.attributes), {safe:true},
                    function(err, objects) {
                        if (err) console.warn(err.message);
                        if (err && err.message.indexOf('E11000 ') !== -1) {
                            console.log('This id has already been inserted into the database');
                        }
                    });
                });
                console.log('Saved!');
            });
        }
    });
}());

我花了很多时间试图弄清楚缺少什么，但没有成功，也许有人会更有可能弄清楚。

Answer 1

当您尝试从中运行 return docs 的回调函数正在执行时，外部 fetch 函数已经返回——正如@SLaks 所建议的，欢迎来到异步编程的精彩世界。

Promises 是处理异步代码的绝佳方式——它们在 jQuery、Dojo 和其他库和工具包中可用。您的 fetch 方法可以返回一个 promise，调用 fetch 方法的代码可以在返回的 promise 被“解决”时使用react。这是 Dojo 的样子:

    fetch: function(query, fields, options){
        var record = this, dfd = new dojo.Deferred;

        record.db.open(function(err, db){
            if(!(record.collection||(typeof record.collection === 'string'))) throw Error('You should define a name attribute, which represents the collection of the Database');
            db.collection(record.collection, function(err, collection){
                console.log('Fetching...');
                collection.find(query, fields, options).toArray(function(err, docs) {
                    dfd.resolve(docs);
                });
            });
        });

        return dfd.promise;
    },

然后，调用 fetch 的代码可能如下所示:

myDB.fetch().then(function(docs) {
  // do whatever you need with the docs
});