Imagine perfect vew...

basic.html

<!-- view -->
<div id='userTemplate' data-model='Domain.User'>
  <div><b>First Name:</b> <%= firstName %></div>
  <div><b>Last Name:</b> <%= lastName %></div>
  <div><b>Age:</b> <%= age %></div>
</div>

<script>
// view model
View.UserView = inherit(BaseView, {
   model : Domain.User,
   view : 'userTemplate'
});

// model
Domain.User = inherit(BaseModel, {
   firstName : '',
   lastName : '',
   age : 0
}

var johnSmith = new Domain.User({ firstName: 'John', lastName : 'Smith', age: 28 }),
    view = new View.UserView({ model: johnSmith });

view.renderTo('#usersList');

// Now if you change model it is automatically updated on the view:
johnSmith.set({ age : 27 });
</script>

formatNumber_viewModel.js

// now let's say we want to format number, before we render it:

View.UserViewFormatNumber = inherit(BaseView, {
   model : Domain.User,
   view : 'userTemplate'
   initialize : function() {
     this.model.on('change:age', this.formatAge);
   },
   formatAge : function() {
     // setting age on view model automatically 
     // tells the view to use viewmodels.age rather
     // than model.age
     this.age = utils.formatNumber(model.age);
   }
});