`this.some_property` becomes undefined inside anonymous callback function

Question

So I can't quite figure out why the variable this.tasks becomes undefined inside of the add event listener I have inside of my goal object. I have a feeling it might have something to do with asynchronous programming(which I still don't fully understand). Sorry I'm a bit of a JS noob, but if you guys could explain to me what I'm doing wrong and what might be a better solution that would be awesome! Thanks.

function Goal(name) {
        this.gDiv =  document.createElement('div');
        this.name = name || "goal";
        this.tasks = document.createElement('ul');
        //Sets the styling and content and adds it to the parent element
        this.initialize = function() {
            this.gDiv.className = "default";
            this.gDiv.setAttribute("id", this.name);
            this.gDiv.innerHTML = this.name;
            elem.appendChild(this.gDiv);

            this.gDiv.parentNode.insertBefore(this.tasks, this.gDiv.nextSibling);
            this.tasks.style.display = "none";


        };  
        //Creates a list underneath the a dive associated with the Goal object
        this.addTask = function(task) {
            var newLi = document.createElement('li');
                newLi.innerHTML = task;
                this.tasks.appendChild(newLi);
        };

        this.gDiv.addEventListener('click', function(){
            alert(this.tasks);              
        });

    }

Thank you guys! You all answered my question! I'd been scratching my head at this for a while. Kudos to you all!

Answer 1

Here is a comparison of some methods (including your problem), to give you a taster, and to try and explain things a little.

// This is the problem that you have,
// where `this` inside the anonymous function
// is a different scope to it's parent
function Test1(something) {
  // `this` here refers to Test1's scope
  this.something = something;
  setTimeout(function() {
    // `this` here refers to the anonymous function's scope
    // `this.something` is `undefined` here
    console.log(this.something);
  }, 1000);
};

new Test1('Hello');

// This solution captures the parent `this` as `test2This`,
// which can then be used inside the anonymous function
function Test2(something) {
  var test2This = this;

  this.something = something;
  setTimeout(function() {
    console.log(test2This.something);
  }, 1000);
}

new Test2('World');

// This solution captures `this` as `test3This` in an `IIFE closure`
// which can then be used in the anonymous function
// but is not available outside of the `IIFE closure` scope
function Test3(something) {
  this.something = something;
  (function(test3This) {
    setTimeout(function() {
      console.log(test3This.something);
    }, 1000);
  }(this));
}

new Test3('Goodbye');

// This method requires that you load an external library: jQuery
// and then use it's `$.proxy` method to achieve the basics of 
// Test3 but instead of being referred to as `test3This` the
// outer scope `this` becomes the inner scope `this`
// Ahh, that's much clearer?
function Test4(something) {
  this.something = something;
  setTimeout($.proxy(function() {
    console.log(this.something);
  }, this), 1000);
}

new Test4('Mum');

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js"></script>

// This is approximately what jQuery's `$.proxy` does
// but without having to load the whole library
function Test5(something) {
  this.something = something;
  setTimeout((function(func, context) {
    return function() {
      func.call(context);
    };
  }(function() {
    console.log(this.something);
  }, this)), 1000);
}

new Test5('Dad');

// Lets create the proxy method as a reuseable
function proxy(func, context) {
  var args = Array.prototype.slice.call(arguments, 2);

  return function() {
    return func.apply(
      context, 
      args.concat(Array.prototype.slice.call(arguments))
    );
  };
}

// and now using it
function Test6(something) {
  this.something = something;
  setTimeout(proxy(function() {
    console.log(this.something);
  }, this), 1000);
}

new Test6('Me want cookies');

Then we have Function#bind

function Test7(something) {
  this.something = something;
  setTimeout(function() {
    // `this` was bound to the parent's `this` using bind
    console.log(this.something);
  }.bind(this), 1000);
};

new Test7('Num num');

<script src="https://cdnjs.cloudflare.com/ajax/libs/es5-shim/4.5.9/es5-shim.min.js"></script>

And most recently ES2015 Arrow functions

function Test8(something) {
  this.something = something;
  setTimeout(() => console.log(this.something), 1000);
};

new Test8('Whoop');

Answer 2

The scope changes when you enter that anonymous closure and 'this' changes. You can hack around it by doing

var self = this;

And then using self in place of this (eg):

function Goal(name) {
    var self = this;

    /* ... */

    this.gDiv.addEventListener('click', function(){
        alert(self.tasks);              
    });

If you're using jQuery you could do something nicer:

this.gDiv.addEventListener('click', $.proxy(function() {
    alert(this.tasks);
 }, this));

Either way works just fine.

EDIT: In ES6, arrow functions can be used instead as they don't bind their own "this", so it becomes even simpler:

this.gDiv.addEventListener('click', () => {
    alert(this.tasks);
 });