By default knockout notifies subscribers of a change when an object value is written, even if it is identical to the old one

knockout-observables-with-object-values.js

// spec/observableBehaviors.js
it('Should notify subscribers of a change when an object value is written, even if it is identical to the old value', function() {
        // Because we can't tell whether something further down the object graph has changed, we regard
        // all objects as new values. To override this, set an "equalityComparer" callback
        var constantObject = {};
        var instance = new ko.observable(constantObject);
        var notifiedValues = [];
        instance.subscribe(notifiedValues.push, notifiedValues);
        instance(constantObject);
        expect(notifiedValues).toEqual([constantObject]);
    });

// src/subscribables/observable.js
ko.observable['fn'] = {
    "equalityComparer": function valuesArePrimitiveAndEqual(a, b) {
        var oldValueIsPrimitive = (a === null) || (typeof(a) in primitiveTypes);
        return oldValueIsPrimitive ? (a === b) : false;
    }
};