knockout mapping JSON update existing view model

Question

I have this JSON data coming from the server:

{"HaveNotification":false,"IsError":false,"Title":null,"Description":null}

and am trying to populate this view model via ko.mapping:

var notifyVM = {
  HaveNotification: ko.observable(true),
  IsError: ko.observable(false), 
  Title: ko.observable('Title goes here'), 
  Description: ko.observable('Description goes here'), 
}

with this code, which is called on a polling interval:

function pollNotifications() {

  $.getJSON('@Url.Action("GetNotifications", "Home")', function (data) {

    ko.mapping.fromJSON(data, notifyVM);

    setTimeout(pollNotifications, 10000);
  });
}

and this is page load code:

$(function () {

  ko.applyBindings(notifyVM);

  setTimeout(pollNotifications, 10000);
});

but its not working. If I inspect the view model after the fromJSON call the observables are not updated, they are still at their initial values.

UPDATE: Some more info... if I do this in the pollNotifications function

var newVM = ko.mapping.fromJSON(data);

I notice that the view model it creates is not the same as my one, it consists of a single observable function, whereas mine is an object with a set of observable properties.

Answer 1

Try:

ko.mapping.fromJS(data, {}, notifyVM);

The two parameter form expects a view-model that was previously created with ko.mapping.fromJS. If it doesn't find one (based on looking for a specific property that fromJS adds), it assumes you're calling the fromJS(data, options) form.

The 3 parameter form removes this ambiguity.