knockout.subscribeChanged.js

/**
 * Knockout.js extension that gives both new and old value to
 * subscription functions.
 *
 * Credit: http://stackoverflow.com/a/18184016/204610 
 * 
 * Changed JBeagle's code to return a disposable subscription
 * object so it conforms to subscribe()
 */
ko.subscribable.fn.subscribeChanged = function (callback) {
    var oldValue;
    var oldSubscription;
    var newSubscription;

    oldSubscription = this.subscribe(function (_oldValue) {
        oldValue = _oldValue;
    }, this, 'beforeChange');

    newSubscription = this.subscribe(function (newValue) {
        callback(newValue, oldValue);
    });

    return {
      oldSubscription: oldSubscription,
      newSubscription: newSubscription,
      dispose: function () {
        this.oldSubscription.dispose()
        this.newSubscription.dispose()
      }
    }
};

Shorter version by @mbest not relying on beforeChange

ko.subscribable.fn.subscribeChanged = function (callback) {
    var savedValue = this.peek();
    return this.subscribe(function (latestValue) {
        var oldValue = savedValue;
        savedValue = latestValue;
        callback(latestValue, oldValue);
    });
};