When’s the last time you read the ECMAScript spec? Marja Hölttä on the V8 dev blog:

In this article, we take a simple function in the spec and try to understand the notation. … Even if you know JavaScript, reading its language specification, ECMAScript Language specification, or the ECMAScript spec for short, can be pretty daunting. At least that’s how I felt when I started reading it for the first time.

Also check out How to read the ECMAScript specification which covers most of the material from this post, but from a different angle.