使用module.exports与回调函数导出对象

Question

I am trying to learn how to write my own modules for nodejs, specifically to implement various objects, with which I could then work throughout the app.

I want to use result something like this:

//assuming i have database:
Person_table(ID int A_I, NAME varchar, AGE int)
//code:
var p = new Person("John", 22);
p.writeToDatabase();
//the object is now written in database

I have tried the following, but for some reason beyond my understanding it doesn't work. I have declared three files: db.js , person.js , app.js

db.js

var mysql = require('mysql');
var conn = mysql.createPool({
    host : 'localhost',
    database: 'db_name',
    user: 'user_name',
    password : 'pass',
    multipleStatement: true,
    connectionLimit : 10
});

conn.getConnection(function(err){
    if(err) throw err;
});

module.exports = conn;

person.js

var db = require('./db.js');

function Person(n, a) {
    this.name = n;
    this.age = a;
}

Person.prototype.writeToDatabase = function (callback) {
    db.query("INSERT INTO Person_table(NAME, AGE) VALUES(?,?)", [this.name, this.age], function (err, rows) {
        if (err) return callback(err);
        else return callback(null, rows);
    });
}

module.exports = Person;

app.js

var Person = require('./person.js')

var p = new Person("John", 22);

p.writeToDatabase(function(err, rows){
    if(err) console.log(err);
    else console.log("written to DB");
});

I would appreciate the help on what is wrong with the code. As a bonus, I would like to ask any good literature on subject of module exporting and object prototypes to create layered nodejs app.

Answer 1

The first advice I'll give is the following, you can use your callback like this in your person.js :

Person.prototype.writeToDatabase = function (callback) {
    db.query("INSERT INTO Person_table(NAME, AGE) VALUES(?,?)",
      [this.name, this.age],
      callback // Since you do nothing with `err` or `rows`
    );
}

Why ? Because you handle the callback in your app.js file :).

Next, I think it doesn't work because you don't wait for your function conn.getConnection to end, and moreover, you don't use the return of this function ! Check the documentation here .

You should reorganize your code like:

db.js

var mysql = require('mysql');

module.exports = mysql.createPool({
    host : 'localhost',
    database: 'db_name',
    user: 'user_name',
    password : 'pass',
    multipleStatement: true,
    connectionLimit : 10
});

person.js

module.exports = function (connection) { // You need the connection to query your database.

    function Person(n, a) {
        this.name = n;
        this.age = a;
    }

    Person.prototype.writeToDatabase = function (callback) {
        // The connection is used here.
        connection.query("INSERT INTO Person_table(NAME, AGE) VALUES(?,?)",
            [this.name, this.age],
            callback
        );
    }

    return Person;
};

app.js

var db = require('./db');

db.getConnection(function (err, connection) { // Once you're connected to the database, you can start using your app.

    var Person = require('./person')(connection); // Since we export a function with a parameter, we need to require the person.js like this.

    var p = new Person("John", 22);

    p.writeToDatabase(function(err, rows) {
        if(err) console.log(err);
        else console.log("written to DB");
    });
});

If I'm not clear, you can ask for more details :). If the pattern I show you doesn't fit with your expectation you can do the job in other ways ;)

Answer 2

I figured out the problem. My example is working perfectly, I had just one totally not related problem.

At the end of my short program i had process.exit() so that my app would end when it would finish the task. However I haven't taken into account the asynchronous nature of node.js so, my process.exit() executed before my write to database was completed, therefore it failed.

Thanks for help anyway!