OP, this is the best answer to your question. Lots of other answers on this thread are either ignoring the part where you said you wanted to really master the language and recommending introductory JS materials, or, bizarrely, telling you to learn something else instead.

I don’t think you can master a language in one go. It’s not how most people’s brains work. I think you do it in stages. Get yourself a good introductory JS resource. The eloquent javascript book others posted looks good although I haven’t read it myself. Then read You Don’t Know JS. And then study the spec. Almost by definition the spec is the only thing that can give you a perfect and complete understanding the language. But don’t start there as a complete beginner is not its intended audience.

Be sure to apply what you learn from each book by working on projects. Don’t just read the books. The material won’t stick that way.