How to access instance variables anywhere in the class for node.js

Question

How to access instance variables anywhere in the class for node.js

I am working on a node.js application with postgresql, using the express framework. I am trying to follow MVC as much as possible.

I want to generate query results in a model class and then pass them to a controller class. That controller class is actually defined in the routes, so that controller class can take the results and pass them as http response.

This is my database helper class, i.e. the model class. My problem is at the listener at the very end of the class.

exports.DatabaseHelper = function()
{
    var allVenues;
    var client;
    var customEventEmitter;

    this.init = function()
    {
        this.customEventEmitter = new events.EventEmitter();


        client = new pg.Client(
        {
            host:'localhost',
            port:5432,
            database:'postgres',
            user:'postgres',
            password:'password'
        });
    }



    this.getVenuesWithEvents = function(searchParams)
    {
        allVenues = new Array();

        var query_for_venues;

        this.init();

        client.connect();   

        client.addListener("error",function()
        {
            sys.puts("postgresql interface error");
        });

        query_for_venues = client.query("select id, name, location, latitude, longitude, category_generalized from venues");


        query_for_venues.addListener("row",function(row)
        {
            //some code
        });

        query_for_venues.addListener("end",function(result)
        {
                this.customEventEmitter.emit("data",allVenues);
//////////////////////////////////////////////////////////                  
//this line shows error....'this' refers to the query object so customEventEmitter is undefined
//customEventEmitter is my idea of sharing the query results to my controller class.
//but I cannot do this becasue of this error

                console.log("after emission");
        });
    }

}

How can I access the customEventEmitter instance variable from within the listener?

node.js
express

Answer 1

Just remove this from your init function:

this.customEventEmitter = new events.EventEmitter();

So you'll have:

customEventEmitter = new events.EventEmitter();

And in your listener just emit the emitter without this as follows:

query_for_venues.addListener("end",function(result){
            customEventEmitter.emit("data",allVenues);

            console.log("after emission");
 });

Answer 2

let me show you a nice trick.

you could change custom

var customEventEmitter;

to

this.customEventEmitter =null;

at the top of the function. then you can call

var self = this;

outside of the query function. then inside the query function you reference the outer "this" with self.

as in:

self.customEventEmitter.emit()

the methodology I just described is standard.