javascript oop with fixed this scope?

Question

I'm trying to implement some basic object orientation in javascript like so:

function Cat(name) {
  this.name = name;
}

Cat.prototype.sayName = function() {
  console.log(this.name);
};

Cat.prototype.fetchCat = function() {
  someAsyncCall()
      .success(this.sayName)
      .error(someOtherMethod);
};

when someAsyncCall is finished sayName gets called but the this scope is not what I expect. I can solve it by binding:

Cat.prototype.fetchCat = function() {
  someAsyncCall()
      .success(this.sayName.bind(this))
      .error(someOtherMethod.bind(this));
};

Is there some syntactically "nicer" solution to this than having to remember to .bind all the time when in callbacks?

Maybe not the best solution, but I usually use a local variable `var self = this;` in the function so that `self` can be used in the callbacks to be sure the correct `this` is being used. — jonhopkins, Dec 02 '13 at 13:26

Flavien Volken · Answer 1 · 2013-12-02T14:43:52.953

1

Cat.prototype.fetchCat = function() {
var self = this;
  someAsyncCall()
      .success(function(){self.sayName})
      .error(someOtherMethod);
};

edited Dec 02 '13 at 14:43

answered Dec 02 '13 at 13:31

Flavien Volken

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I think you need `.success(function(){self.sayName();})` or it'll be the same as passing this.sayname – HMR Dec 02 '13 at 13:56
Actually both will be working, **but** using mine assume we are using `self` everywhere in the other functions instead of `this`, yours will allow using `this` in the right context which is actually better. I updated my answer. – Flavien Volken Dec 02 '13 at 14:43

MIdhun Krishna · Answer 2 · 2013-12-02T13:59:38.237

With reference to javascript guide lines from airbnb,

// bad
function() {
  var self = this;
  return function() {
    console.log(self);
  };
}

// bad
function() {
  var that = this;
  return function() {
    console.log(that);
  };
}

// good
function() {
  var _this = this;
  return function() {
    console.log(_this);
  };
}

ecma-262 does not give any special treatment for underscore and says:

The dollar sign ($) and the underscore (_) are permitted anywhere in an IdentifierName.

but since javascript as a language does not support encapsulation out of the box, the '_' convention is meant to indicate that the variable is 'private' or shouldn't be used externally.

Possible reason here: [link](http://stackoverflow.com/questions/4484424/underscore-prefix-for-property-and-method-names-in-javascript), still I strongly prefer "self" — Flavien Volken, Dec 02 '13 at 13:33

javascript oop with fixed this scope?

2 Answers2