Using 'this' in an object in Node?

Question

I'm using Electron to create a little desktop app and exporting with module.exports. On the 'server' side this works fine. However, when I'm using module.exports on the front end, as per Electron docs, I get this error.

Uncaught TypeError: this.showProgressbar is not a function"

var ViewController = {
    getPageCount: function (res) {
        this.total = res;
        this.showProgressbar(res);
    },

    showProgressBar: function (num) {
        $('.progress-container').addClass('show');
        $('.progress-bar').style('width', '0%');
    }
};

module.exports = ViewController;

On the client side, this is how I'm accessing this file.

var view = require(__dirname + '/client/ViewController.js');

ipc.on('page_count', view.getPageCount);

How should I be accessing internal methods in this instance?

this.showProgressBar is looking for showProgressbar on getPageCount not in the ViewController object. — jsuser, Jul 06 '15 at 17:59
What if you use this? `ipc.on('page_count', view.getPageCount.bind(view))` — Mike Cluck, Jul 06 '15 at 18:00
You need to call your function with the correct case too: `this.showProgressBar(res);`. — David Sherret, Jul 06 '15 at 18:05

score 2 · Answer 1 · answered Jul 06 '15 at 18:05

2

That's because when the callback is being called, it's being called in the wrong context. To bind the context, use Function.bind.

ipc.on('page_count', view.getPageCount.bind(view));

answered Jul 06 '15 at 18:05

Mike Cluck

31,869
13
80
91

score 2 · Accepted Answer · answered Jul 06 '15 at 18:36

ViewController is neither a 'Class' nor an instance, its a plain javascript object with two properties.

If you want it to behave like a class and be able to acccess to other properties from a method when creating an instance this is how you should do:

var ViewController = function(ipc){
        this.ipc=ipc;
        this.ipc.on('page_count', this.getPageCount);
};

ViewController.prototype.getPageCount: function (res) {
        this.total = res;
        this.showProgressbar(res);
},

ViewController.prototype.showProgressBar: function (num) {
    $('.progress-container').addClass('show');
    $('.progress-bar').style('width', '0%');
}
module.exports = ViewController;

You still need to instantiante ViewController :

var ViewController = require(__dirname + '/client/ViewController.js');

var controller = new ViewController(ipc);

Using 'this' in an object in Node?

2 Answers2