Node.js：模块系统，对象引用

Question

// connections.js
...
module.exports = {
    conn: {
        mongodb: {
            connect: function() {throw ...},
            disconnect: function() {throw ...},
            getState: function() {throw...}
        },
        mysql: {
            connect: function() {throw ...},
            disconnect: function() {throw ...},
            getState: function() {throw ...}
        }
    },
    drivers: {
        mongoose: mongoose,
        mysql: mysql
    },
    states: connectionStates,

    setup: function(config, cb) {
        // provides concrete implementations of connect(), discconnect(),
        // getState(), sets up listeners to relay connection events 
        this.conn.mongodb = setupMongo(config.mongodb); 
        this.conn.mysql = setupSql(config.mysql);
        ...
        cb();
    }
};

Now if I include this as:

// main.js

var connections = require(__dirname + '/connections'),   
    conn = connections.conn,
    db = conn.mongodb;

// connectionections.setup() not been called yet
exports.foo = function() {
    // connections.setup() already been called before this point
    db.connect(...);            // fails - error thrown - using the abstract function
    conn.mongodb.connect(...);  // works
}

Why does the first one fail? The db var should contain a reference to connections.conn.mongodb ? At very least, I'd expect both to either work, or not work. What is the difference that allows the first to fail and second to succeed? Thank you

Answer 1

Its failing in the first case because setup() was called in a different scope and db/conn.mongodb diverged (with a copy on write) when setup was called. If you compare db and conn.mongodb in the exports.foo function, you should see that conn.mongodb has been initialized with the setupMongo and db still has the uninitialized versions. Not sure what the code looks like that is calling connections.setup, but from the looks of this, db !=== conn.mongodb.