Knockout ko.mappings.fromJS not working

I am trying to use knockout mapping, but it isn't working as I expected. Here I created simpliest fiddle i can and it's not working.

Am I missing something?

https://jsfiddle.net/p48d11j5/1/

function Model(){
    var self = this;

    self.Id = ko.observable(0);
    self.Name = ko.observable("Default");
    self.Visible = ko.observable(false);
    self.Items = ko.observableArray([]);
}

function ModelItem(){
    var self = this;

    self.Id = ko.observable(0);
    self.Name = ko.observable("Default item name")
}

var m = new Model();

ko.mapping.fromJS({
    Id:1,
    Name: "Test",
    Visible: true,
    Items: [
    {
        Id:1,
        Name:"First"
    },
    {
        Id:2,
        Name:"Second"
    }
  ]
}, m);

ko.applyBindings(m);

edit: I am working with nested arrays, so I added array

edit2: I want have models "typed" to use functions or ko.computed properties of them

If you call ko.mapping.fromJS with two arguments :
ko.mapping.fromJS(data, mappedObject) the second argument is a mappedObject which is already created.Then the second argument will be taken as a viewModel not options.

All you have to do is: ko.mapping.fromJS(data, {}, viewModel) - this one puts your data in your model.

ko.mapping.fromJS({
    Id:1,
    Name: "Test",
    Visible: true,
    Items: [{Id: 1, Name: "First"}, {Id: 2, Name: "Second"}]
  }, {} ,m);  // pass the second argument as an empty object.