Access object property within a callback

Question

I wrote the following code:

var Request = require('./request');

function Treasure(id) {
    Treasure.prototype.valid = false;
    Treasure.prototype.id = id;
    Treasure.prototype.contentLength = 0;
    Treasure.prototype.title = null;
    Treasure.prototype.seller = null;
    Treasure.prototype.buyer = null;
    Treasure.prototype.cost = 0;
}

Treasure.prototype.loadData = function() {
    EpvpRequest.treasureRequest(Treasure.prototype.id, function(data) {
        if (data.valid) {
            Treasure.prototype.valid = data.valid;
            Treasure.prototype.contentLength = data.contentLength;
            Treasure.prototype.title = data.title;
            Treasure.prototype.seller = data.seller;
            Treasure.prototype.buyer = data.buyer;
            Treasure.prototype.cost = data.cost;
        }
    });
}

module.exports = Treasure;

Please don't hit me, I just started learning javascript. I want ot access the properties of "Treasure"; but I can't use this, because I have a callback in the loadData function and this would refer to the function which called the callback - is that correct?

But it seems that I can't access the properties the way I tried with Treasure.prototype.property.

What is the correct way to to this?

*"and this would refer to the function which called the callback - is that correct?"* No. `this` almost never refers to a function. Have a look at http://stackoverflow.com/questions/11922383/access-process-nested-objects-arrays-or-json — Felix Kling, Apr 21 '15 at 03:23

score 4 · Accepted Answer · answered Apr 21 '15 at 02:59

First of all, you should be assigning instance variables in the constructor instead of assiginng to the prototype. The prototype is for methods and other things that will be shared by all Treasure instances.

function Treasure(id) {
    this.valid = false;
    this.id = id;
    this.contentLength = 0;
    this.title = null;
    this.seller = null;
    this.buyer = null;
    this.cost = 0;
}

As for your problem with this inside callbacks, the usual workaround is to store the value of this in a regular variable and then use that variable inside the callback.

Treasure.prototype.loadData = function() {
    // Nothing special about "that"
    // Its just a regular variable.
    var that = this;
    EpvpRequest.treasureRequest(that.id, function(data) {
        if (data.valid) {
            that.valid = data.valid;
            that.contentLength = data.contentLength;
            that.title = data.title;
            that.seller = data.seller;
            that.buyer = data.buyer;
            that.cost = data.cost;
        }
    });
}

Since this pattern comes up very often, some people choose to always use the same name for the "this-storage" variable. Some of the more popular names are self and that.

Wow, why didn't I think of that. Thank you very much. – Michael Pittino Apr 21 '15 at 03:02 — Michael Pittino, Apr 21 '15 at 03:02

Access object property within a callback

1 Answers1