title: How to extend an existing view model in knockout js
body: <p>For example, I create a master view model in global.js, which contains some functionalities that are used in all pages. Then, in pageA, I want to extend master view model. But I already called <code>ko.applybindings(masterViewModel)</code> in global.js. I can't change master view model in pageA.js then re-apply it, since it will cause error. <br>
  What should I do; what's the proper way to extend an existing view model?</p>