Knockout subscribeChanged

gistfile1.js

ko.subscribable.fn.subscribeChanged = function(callback) {
    if (!this.previousValueSubscription) {
        this.previousValueSubscription = this.subscribe(function(_previousValue) {
            this.previousValue = _previousValue;
        }, this, 'beforeChange');
    }
    return this.subscribe(function(latestValue) {
        callback(latestValue, this.previousValue);
    }, this);
};

ko.observableArray.fn.subscribeChanged = function(callback) {
    if (!this.previousValueSubscription) {
        this.previousValueSubscription = this.subscribe(function(_previousValue) {
            this.previousValue = _previousValue.slice(0);
        }, this, 'beforeChange');
    }
    return this.subscribe(function(latestValue) {
        callback(latestValue, this.previousValue);
    }, this);
};