node.js, setTimeout callback method and “this”

Question

I am trying to write a simple polling application using NodeJS. I'm wanting to write an EventEmitter that performs an action on a timer and emits events based on the result of that periodic action.

I'm starting by creating my own object and inheriting from EventEmitter . I kick off a timer using setInterval , and specify the method to call after the timer elapses. Within the timer callback method, I want to reference a variable of the object that I've created, but this does not appear to refer to the object.

How can I reference my variable in this method? Here is my code:

var util = require('util'),
    events = require('events'),
    timers = require('timers'),
    redis = require('redis');

// define worker object
var JobPoller = function () {
    // inherit event emitter
    events.EventEmitter.call(this);

    // save reference to database
    this.db = redis.createClient();

    // start main loop
    this.interval_id = timers.setTimeout(this.check, 1000);
};

JobPoller.prototype.check = function () {
    // pop a job off the queue if possible
    this.db.rpop('pdf-job-queue', function (err, result) {  
        if (err != null)
            this.emit('error', err);

        if (result != null)
            this.emit('job', JSON.parse(result));

        // check for more jobs
        this.interval_id = timers.setTimeout(this.check, 1000);
    });
};

// inherit from event emitter
util.inherits(JobPoller, events.EventEmitter);

// export the poller instance
module.exports = new JobPoller;

Answer 1

this.check is just a simple function, the value of this inside that function will be determined when the function is called.

Node should support bind so you can bind the function to your desired this like this:

this.interval_id = timers.setTimeout(this.check.bind(this), 1000);

Alternatively, you can use a closure to manually force this to be what you want:

var self = this;
this.interval_id = timers.setTimeout(function() { self.check() }, 1000);