watchers to keep an eye on your data. Watchers are methods that are invoked when the specified attribute changes. They are useful when you want to perform asynchronous operations in response to changing data.
Great video. simple to follow and to the point.
When trying to follow along, I noticed after following the course from the 4th video and then doing the 5th video my example in codepen didn't work. The examples assumed you have underscore as a dependency. I added that and everything was good :)