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.
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.