How do you become expert javascript?

JavaScript

Just like learning any other programming language, you must first grasp the concept or the gem of the language.

For C++ it was the improvement over C by bringing in the OOP(object oriented programming) functionality. For JavaScript it is, in my opinion, the expandability. You must keep in mind that also the application of these languages are drastically different. JavaScript is a scripting language aimed and applied on the Web, C and other compiled languages are mostly geared at Desktop Applications.

Once you grasp the gem, comes the syntax and the pre-built functions. Once you are feeling comfortable with those concept and you grasp the OOP ideal - you can move onto the more JavaScript specific realm.

Things like prototypes... At that point you can pat yourself on the back and test yourself. If you can program almost anything you need in JavaScript good job.

A good try would be to write your own JavaScript library (like jQuery for example).