In this appendix, we learned a lot about the Closure Tools. It is always important to understand how things work to be able to track why your code is not working. Without this review, when using, for example, the ADVANCED mode, you will stay stuck and end up keeping the default file for production. If you do so, you are losing partly the OpenLayers 3 philosophy: this version is a rewrite to have a better API and to get performances.
In the first part of the appendix, we learned why performance is important. Next, we saw how to reuse Closure Library or how to overload default OpenLayers 3 components. We then inspected how to practically use Closure Compiler itself in the OpenLayers development context.
Finally, we introduced you to some tools to improve your overall code quality. In this case, only remember this citation from idiomatic.js:
"All code in any code-base should look like a single person typed it, no matter how many people contributed."