How can I scope this "public" method correctly?

Question

I have this code (JSFiddle)

var OBJ = function(){
    var privateVar = 23;
    var self = this;

    return {
        thePrivateVar : function() {
          return privateVar;
        },  

        thePrivateVarTimeout : function() {
            setTimeout(function() { alert(self.thePrivateVar()); } , 10);
        }
    }

}();

alert(OBJ.thePrivateVar());

OBJ.thePrivateVarTimeout();

This is an abstraction of a real problem I'm having.

So - I would expect the call to OBJ.thePrivateVarTimeout() to wait 10 and then alert with 23 (which I want it to access through the other exposed method).

However self doesn't seem to be setting correctly. When I am setting self = this it appears that this isn't a reference to the function but a reference to the global object. Why is this?

How can I make the public method thePrivateVarTimeout call the other public method thePrivateVar?

Answer 1

var OBJ = (function(){
    var privateVar = 23;
    var self = {
        thePrivateVar : function() {
          return privateVar;
        },  

        thePrivateVarTimeout : function() {
            setTimeout(function() { alert(self.thePrivateVar); } , 10);
        }
    };

    return self;

}());

this === global || undefined inside an invoked function. In ES5 it's whatever the global environment is, in ES5 strict it is undefined.

More common patterns would involve using the var that = this as a local value in a function

var obj = (function() {
  var obj = {
    property: "foobar",
    timeout: function _timeout() {
      var that = this;
      setTimeout(alertData, 10);

      function alertData() {
        alert(that.property);
      }
    }
  }

  return obj;
}());

or using a .bindAll method

var obj = (function() {
  var obj = {
    alertData: function _alertData() {
      alert(this.property);
    }
    property: "foobar",
    timeout: function _timeout() {
      setTimeout(this.alertData, 10);
    }
  }

  bindAll(obj)

  return obj;
}());


/*
    bindAll binds all methods to have their context set to the object

    @param Object obj - the object to bind methods on
    @param Array methods - optional whitelist of methods to bind

    @return Object - the bound object
*/
function bindAll(obj, whitelist) {
    var keys = Object.keys(obj).filter(stripNonMethods);

    (whitelist || keys).forEach(bindMethod);

    function stripNonMethods(name) {
        return typeof obj[name] === "function";
    }

    function bindMethod(name) {
        obj[name] = obj[name].bind(obj);
    }

    return obj;
}