Knockout extender: observe

observe.js

ko.extenders.observe = function (target, source) {
    target(source);
    var result = ko.computed({
        read: function () {
            return target()();
        },
        write: function (value) {
            target()(value);
        }
    });
    result.observe = function (source) {
        target(source);
    };
    return result;
};

var x = ko.observable(10);
var y = ko.observable(20);
var z = ko.observable().extend({ observe: x });
console.log(z());
x(11);
console.log(z());
y(21);
console.log(z());
z.observe(y);
console.log(z());
x(12);
console.log(z());
y(22);
console.log(z());
z(30);
console.log(z());
console.log(y());

output.txt

10
11
11
21
21
22
30
30