JavaScript. The Core: 2nd Edition
As many useful things as there are in ES2015 (and beyond!), it's probably worth pointing out a few "corporate" environmental constraints that might limit one to ES 5.1:* IE11 isn't dead yet.  MS still supports it.* The "Nashorn" engine inside (Oracle) Java 8 only supports 5.1.Scoff at them if you will, but some of us have to live with these things, and aren't using a transpiler.