How to add a computed observable via prototype

by StrandedPirate

HTML

<script src="http://knockoutjs.com/js/knockout-2.0.0.js"></script>
<div data-bind="text: fullDomainName">test</div>

JavaScript

function SiteModel(rootUrl, data) {
    var self = this;
    self.rootUrl = rootUrl;
    self.DomainName = ko.observable(data.DomainName);
    self.IsSubDomain = ko.observable(data.IsSubDomain);
}

SiteModel.prototype.fullDomainName = ko.computed(function () {
    if (self.IsSubDomain() && self.DomainName()) { // bombs out here with "self.IsSubDomain is not a function"
        return self.DomainName() + ".myCompanyWebsite.com";
    }
    else {
        return self.DomainName();
   }
}); 
 
var temp = new SiteModel("someurl", { DomainName: "extraCool" });