Validated observables in KnockousJS

knockout.validatedObservable.js

ko.validatedObservable = function (initialValue, validationFunction) {
    var latestValue = initialValue;

    function observable() {
        if (arguments.length > 0) {
            // Write

            // Ignore writes if the value hasn't changed
            if (((!observable['equalityComparer']) || !observable['equalityComparer'](latestValue, arguments[0]))) {
                if (validationFunction.call(null, arguments[0])) {
                    latestValue = arguments[0];
                    observable.notifySubscribers(latestValue);
                } else {
                    console.log('invalid');
                }
            }
            return this; // Permits chained assignments
        }
        else {
            // Read
            ko.dependencyDetection.registerDependency(observable); // The caller only needs to be notified of changes if they did a "read" operation
            return latestValue;
        }
    }
    ko.subscribable.call(observable);
    observable.valueHasMutated = function () {
        observable.notifySubscribers(latestValue);
    };
    ko.utils.extend(observable, ko.observable['fn']);

    ko.exportProperty(observable, "valueHasMutated", observable.valueHasMutated);

    return observable;
};

usage.js

var viewModel = {
    numbersOnly: new ko.validatedObservable(1, function(x) { return !x || /\d+/g.test(x); }),
    positive: new ko.validatedObservable(1, fuction(x) { return x && x > 0; }),
    helloWorld: new ko.validatedObservable('hello world', function(x) { return x === 'hello world'; })
};