This book is for people who have had some experience with Java­Script, and want to have a better, deeper understanding of how it works and how to use it well. It is also for experienced pro­gram­mers who are looking to understand the workings of another lan­guage. This book is not for beginners. I hope to someday write a book for beginners. This is not that book. This is not a light book. If you skim it, you will likely get nothing from of it. This book is not about JavaScript engines or virtual machines. It is about the lan­guage itself and the things every pro­gram­mer should know about it. This book is a radical reappraisal of Java­Script, how it works, how it could be made better, and how it can be better used. It is about how to think about Java­Script and how to think in Java­Script. I am going to pretend that the current version of the lan­guage is the only version. I am not going to waste your time by showing how things worked in ES1 or ES3 or ES5. That does not matter. The focus is on how Java­Script works for us now. This book is not comprehensive. There are large, complex chunks of the lan­guage that will be dismissed without a word. If I fail to mention your most favorite fea­ture, that is most likely because that fea­ture is crap. I will not be paying much attention to syntax. I am assuming that you already know how to write an if statement.