Why should you learn JS?
JS is mostly used for frontend development, a.k.a. client-side scripting. Meaning, building interactive websites with modern UI elements using JS in conjunction with HTML and CSS is where it’s most used. So, it is not wrong to say that JS is primarily a web development language. But, it will be an overstatement that it can only be used for web development.
Courses, Websites, & Books
To set sails on a self-paced path with little guidance you can get paid courses which are also available on Udemy and Coursera. Free courses, of course, will have less guidance and direction while paid courses will provide you better guidance.
Bootcamps are even better!
Coding Boot Camps
Finally you cannot become a programmer by just learning. Once the basics are clear you should start coding to work on projects you like. Start with easy ones like building simple websites and calculator apps and continue increasing the difficulty.