Question

I want add some methods. And this directive can’t help me.

var viewModel = function () {
    firstName = ko.observable("Mike");
    lastName = ko.observable("Rassel")
    //fullName = ko.computed(function () { 
         //return firstName() + " " + lastName(); }, viewModel);
}

viewModel.fullName = ko.computed(function () { 
    return this.firstName() + " " + this.lastName(); }, viewModel);

DEMO: http://jsfiddle.net/gJUqK/

On local with 2.1.0 its work.

Answer 1

The firstName, lastName and fullName should be properties of the myViewModel instance.

This will work, as explained in the documentation. The self is added, because inside the fullName observable callback function this would point to something else.

function myViewModel()
{
    var self = this;

    this.firstName = ko.observable("Mike");
    this.lastName = ko.observable("Rassel");

    this.fullName = ko.computed(function() {
        return self.firstName() + ' ' + self.lastName();
    })
}

ko.applyBindings(new myViewModel());

Working here: http://jsfiddle.net/gJUqK/1/