Really JavaScript has been gaining a lot of space in recent years, mainly due to changes in the web development scenario. With the advent and popularity of libraries like AngularJS, it was possible to further reduce dependence on a server-side language (such as Java, C #, PHP, etc.) in web projects.
In addition, the rise of Node.js allowed the development of full web applications using only JavaScript as a programming language (of course, HTML and CSS in the layout part).
So, it is possible to use only JavaScript, but this choice will depend very much on the situation and the programmer. I, for example, work with C # and PHP quite well and do not see the need to use Node.js in the back end of my applications, but it is good to know that this is possible and if at some point this is a requirement or advantage to a project, I will use it without problems. For example, Microsft Azure supports the development of cloud services with Node.js, but also (obviously) supports C #, so for now I'm getting C #.
JavaScript, however, will always be present on the front end of applications, working on the validations, Ajax, animations, etc.
I hope I have helped. A hug.