You can get a job as a front-end and back-end developer, which is basically full-stack development. If you specialise in JavaScript, then you can also become a Game Developer or Mobile Developer.
Yes, you can learn JavaScript from scratch, as there are many beginner courses that are designed to introduce programming concepts gradually. They start with the basics and practice through hands-on exercises.
It's important to learn HTML and CSS to understand JavaScript because a website's structure comes from HTML, its look from CSS, and its interaction from JavaScript.