javascript - What's the proper way to apply ko.computeds (Knockout computeds) to a Durandal viewmodel? -

i'm new durandal , haven't had luck applying ko.computed methods inside of viewmodel. can tell me correct syntax or pattern is?

you can see entire project @ https://github.com/robksawyer/durandal-todo/blob/master/views/todos.html.

each of computeds apply gets following error during bind.

error("cannot write value ko.computed unless specify 'write' option. if wish read current value, don't pass parameters.") 

compose method requesting viewmodel , view:

<!--ko compose: {          model: router.activeitem, //wiring router         aftercompose: router.aftercompose, //wiring router         cacheviews: false, //telling composition keep views in dom, , reuse them (only idea singleton view models)         transition: 'fadein'     }--><!--/ko--> 

viewmodel:

// count of completed todos     var completedcount = ko.computed(function () {         return ko.utils.arrayfilter(todos(), function (todo) {             return todo.completed();         }).length;     }); 

view https://github.com/robksawyer/durandal-todo/blob/master/views/todos.html

error screenshot

defining vm singleton first , adding ko.computed methods take care of error message.

var vm = {    current : current,    todos: todos,    ... // remove ko.computeds singleton };  vm.completedcount = ko.computed(function () {     return ko.utils.arrayfilter(todos(), function (todo) {         return todo.completed();     }).length; }, vm);  // add other ko.computeds  return vm;