Link to "this" from javascript callback

Question

Something always bothered me about how I do object oriented coding in Javascript. When there is a callback, I often want to refer to an object that was originally called a function, which forces me to do something like this:

MyClass.prototype.doSomething = function(obj, callback) { var me = this; // ugh obj.loadSomething(function(err, result) { me.data = result; // ugh callback(null, me); }); } 

First, creating an extra variable has always seemed ... excessive for me. Also, I have to wonder if this can cause problems (circular links? Un-GCd objects?) By passing the variable "me" back to the callback.

Is there a better way to do this? Is this approach evil?

Answer 1

This is what Function.bind() for:

 MyClass.prototype.doSomething = function(obj, callback) { obj.loadSomething((function(err, result) { this.data = result; callback(null, this); }).bind(this)); } 

Answer 2

AFAIK, what you do is an accepted template for this kind of thing and does not cause any problems. Many people use either "I" or "that", because the stored link - "I" can be more intuitive if you come from the background in python.

Answer 3

This is normal behavior for JavaScript. The context for this has changed for the loadSomething object, and the reason for having a callback is to capture closure references, such as your me variable.

Answer 4

You can associate this with an inner function area

 MyClass.prototype.doSomething = function(obj, callback) { obj.loadSomething(function(err, result) { this.data = result; callback(null, this); }.bind( this )); } 

Answer 5

This is the most common way I've come across this; I usually use var self = this ;, but its just a name. Despite the fact that this is an additional variable, given the overhead of javascript and the fact that it does not duplicate the object, this does not really affect performance.