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Knockout.js: n00b can't calculate

Hello, everyone.

I'm just now starting to learn Knockout, and I'm apparently unable to learn how to calculate values in Knockout. I've tried this several different ways, but my current code (pseudo-code) is:

Code:

In another <span data-bind="text: myAge"></span> years, I'll be <span data-bind="text: newAge"></span>

<input type="text" data-bind="value: myAge">

Code:

function AppViewModel(){

  this.myAge = ko.observable(personalInfo.age);

  this.newAge = ko.computed(function(){return this.myAge*2;});

}

ko.applyBindings(new AppViewModel());

text: newAge is resulting as NaN. If I alert this.myAge inside the AppViewModel(), it comes back as undefined. Didn't I just define it on the previous line?? In the SPAN tag of the sentence, it's appearing as 44

What am I doing incorrectly?

Thank you,

Has _anyone_ successfully implemented calculations using Knockout.js?

Code:

function AppViewModel(){

   this.myAge = ko.observable(personalInfo.age);

   this.newAge = ko.computed(function(){return this.myAge*2;},this);

 } 

ko.applyBindings(new AppViewModel());

The above fixed it.