On Mar 16, 2014, at 09:18, Evgeny Nepomnyashchiy
<[email protected]> wrote:
> I'm learning programming on NodeJS, it's my first back-end experience.
Welcome!
> I have a small problem. I created class (module) Database, which manages
> connection and queries to a database, here is implementation:
>
> var mysql = require('mysql');
>
> function Database() {
> this.connection = null;
> this.connected = false;
> }
>
> Database.prototype.connect = function (config, callback) {
> console.log('[Database] Connecting...');
>
> this.connection = mysql.createConnection(config);
> this.connection.connect(function (err) {
> if (err == null) {
> this.connected = true;
> }
>
> console.log('connect: this.connected = ' + this.connected);
> console.log('[Database] Connected');
>
> callback(err);
> });
> }
>
> Database.prototype.makeQuery = function (query, callback) {
> console.log('makeQuery: this.connection = ' + this.connection);
> console.log('makeQuery: this.connected = ' + this.connected);
>
> if (this.connected == false) {
> return callback(new Error('[Database] You cannot make queries before
> connection'));
> }
> console.log('[Database] Making query: %s', query.sql());
>
> // making query to the database
> }
>
> exports.Database = Database;
>
> As you can see, here I store connected property. If I try to make query
> before successful connection I will get error [Database] You cannot make
> queries before connection. But it doesn't work, it shows me this error
> always, because connected property becomes false, here is example of usage
> this module outside:
>
> var config = require('./config.json');
> var query = require('./modules/database/query');
>
> console.log('Server Started');
>
> var database = new (require('./modules/database')).Database();
> database.connect(config.database, function (err) {
> console.log("connect handler");
>
> if (err) {
> throw err;
> }
>
> var query = new query.Query();
> // some actions with query, doesn't matter
>
> database.makeQuery(categoriesNameQuery, function (err, result) {
> if (err) {
> throw err;
> }
>
> // working with the result
> });
> });
>
> In console I get the following:
>
> /usr/local/bin/node database_test.js
> Server Started
> [Database] Connecting...
> connect: this.connected = true
> [Database] Connected
> connect handler
> makeQuery: this.connection = [object Object]
> makeQuery: this.connected = false
>
> Error: [Database] You cannot make queries before connection
> at Database.makeQuery (/modules/database/index.js:35:25)
> at /database_test.js:30:14
> ...
>
> I can't understand, why property connected is true after connection, and
> false when I make a query. But property connection works fine! Maybe
> enclosures in NodeJS work by other way.
You’re running into a fun quirk of “this”. This is not unique to node; it’s
just how the JavaScript language works, and you’d have the some problem in a
browser. The problem is that at the point where you’re setting this.connected =
true, “this” isn’t what you think it is, because the code in question is in a
function that’s called back later (whenever this.connection.connect() has
finished connecting); at that later time, “this” no longer points to the
instance of the Database class, but instead to the global object.
The pattern usually used to combat this quirk is to save the value of “this” in
a new variable, often called “that” or “self”, and then use that instead of
“this”:
Database.prototype.connect = function (config, callback) {
console.log('[Database] Connecting…’);
var that = this;
// Here you could use “this” or “that”; it makes no difference:
this.connection = mysql.createConnection(config);
this.connection.connect(function (err) {
// In here, you have to use “that”; “this” points to the global object,
not the class instance:
if (err == null) {
that.connected = true;
}
console.log('connect: that.connected = ' + that.connected);
console.log('[Database] Connected');
callback(err);
});
}
Some more reading if you’re interested:
http://stackoverflow.com/questions/4886632/what-does-var-that-this-mean-in-javascript
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