Eloquent JavaScript by by Marijn Haverbeke.
From the webpage:
This is a book about JavaScript, programming, and the wonders of the digital. You can read it online here, or get your own paperback copy of the book.
Embarrassing that authors post free content for the betterment of others, but wealthy governments play access games.
This book is also available in Български (Bulgarian), Português (Portuguese), and Русский (Russian).
Enjoy!