http://jsfiddle.net/uedatakuya/9hS5n/

gistfile1.js

var go = {};

function extend(_super, sub) {
    for (var k in _super) {
        if (_super.hasOwnProperty(k)) {
            sub[k] = _super[k];
        }
    }
}

// subscribable
(function () {
    var subscriptions = [];
    var subscribable = {
        subscribe: function (callback, callbackTarget) {
            var subscription = {
                callback: callback.bind(callbackTarget),
                dispose: function () {
                    for (var i = 0; i < subscriptions.length; i++) {
                        if (subscriptions[i] === this) {
                            subscriptions.splice(i, 1);
                            break;
                        }
                    }
                }
            };
            subscriptions.push(subscription);
            return subscription;
        },
        notifySubscripters: function (valueToNotify) {
            go.dependencyDetection.begin();
            var _subscriptions = subscriptions.slice(0);
            for (var i = 0; i < _subscriptions.length; i++) {
                var subscription = _subscriptions[i];
                subscription.callback(valueToNotify);
            }
            go.dependencyDetection.end();
        }
    };
    go.subscribable = subscribable;
})();

// observable
(function () {
    var observable = function (initialValue) {
        var lastValue = initialValue;

        function observable() {
            if (arguments.length > 0) {
                // write
                var newValue = arguments[0];
                if (newValue !== lastValue) {
                    lastValue = newValue;
                    observable.notifySubscripters(lastValue);
                }
            } else {
                // read
                go.dependencyDetection.registerDependency(this);
                return lastValue;
            }
        }
        extend(go.subscribable, observable);
        return observable;
    };
    go.observable = observable;
})();

// dependencyDetection
(function () {
    var frames = [];
    var dependencyDetection = {
        begin: function (callback) {
            frames.push(callback && {
                callback: callback,
                distinctDependencies: []
            });
        },
        end: function () {
            frames.pop();
        },
        registerDependency: function (subscribable) {
            if (frames.length > 0) {
                var topFrame = frames[frames.length - 1];
                if (topFrame || topFrame.distinctDependencies.indexOf(subscribable) >= 0) {
                    return;
                }
                topFrame.distinctDependencies.push(subscribable);
                topFrame.callback(subscribable);
            }
        }
    };
    go.dependencyDetection = dependencyDetection;
})();

// dependentObservable
(function () {
    var dependentObservable = function (readFunction) {
        var lastValue;
        var isBeingEvaluated;
        var subscriptionsToDependencies = [];

        function evaluateImmediate() {
            if (isBeingEvaluated) {
                return;
            }
            isBeingEvaluated = true;
            for (var i = 0; i < subscriptionsToDependencies.length; i++) {
                subscriptionsToDependencies[i].dispose();
            }
            subscriptionsToDependencies = [];
            go.dependencyDetection.begin(function (subscribable) {                
                subscriptionsToDependencies.push(subscribable.subscribe(evaluateImmediate));
            });
            var newValue = readFunction();
            if (newValue !== lastValue) {
                lastValue = newValue;
                dependentObservable.notifySubscripters(lastValue);
            }
            isBeingEvaluated = false;
        }

        function dependentObservable() {
            if (arguments.length > 0) {
                // write
                var newValue = arguments[0];
                if (newValue !== lastValue) {
                    lastValue = newValue;
                    observable.notifySubscripters(lastValue);
                }
            } else {
                // read
                evaluateImmediate();
                go.dependencyDetection.registerDependency(this);
                return lastValue;
            }
        }

        extend(go.subscribable, dependentObservable);
        evaluateImmediate();
        return dependentObservable;
    };
    go.dependentObservable = dependentObservable;
    go.computed = dependentObservable;
})();

var a = go.observable(100);
var b = go.observable(200);
var c = go.computed(function() {
    return a() + b();
});
var d = go.computed(function() {
    return c() * 2;
});
console.log(a(), b(), c(), d());
a(200);
console.log(a(), b(), c(), d());
b(300);
console.log(a(), b(), c(), d());