JavaScript is one of those languages I thought I knew.
For way too long, writing JavaScript was done solely to manipulate HTML pages. My background is C/C++, and I have dabbled in Java and numerous scripting languages.
Recent opportunities transpired which allowed me to spend some time actually learning JavaScript.
Here is what I recommend:
Read JavaScript: The Definitive Guide by David Flanagan. Luckily I had purchased the 5th edition many many years ago, and like all books, I did not read it. This is the only book recommended by Douglas Crockford.
I read it cover to cover, digesting all the the language offered.
The next step is to know which bits to ignore, which is done by reading JavaScript: The Good Parts by Douglas Crockford himself. You can find a PDF version of the book via Google.
Watch any of the Douglas Crockford JavaScript videos.
The easiest way to play with JavaScript is to download node.js, which allows you to write pure JavaScript outside of the confines of the browser.