Easy to learn
Front-end & Back-end
JS also gives you entry to the world of backend programming. Node.js has taken JS to the next level. This essentially means JS can be used to create APIs, read and write from databases, create microservices, etc. The important part is that once you have learnt it, you can serve both front-end and back-end applications. This I believe is a significant win.
Desktop and Handheld devices
As I mentioned before progressive web apps are a reality. Some would know that Twitter already has a PWA and so does Firebase. This means that developers who can code in JS can now create Apps for mobile without having to rely on specific languages and technologies. While I agree PWAs are still in the nascent stage I also believe their emergence is imminent.
Flavours of engineering concepts
Go serverless — Lambdas / Cloud Functions
The concept of first-class functions in JS makes it a very good contender for writing Lambdas. AWS and GCP the leading cloud platforms have JS as a language to write serverless code. This gives the engineers who have good experience with JS a unique advantage. Not only can they code Front-end and Back-end applications, but they also have the ability to utilize cloud platforms to go serverless.
Data Science, AI, ML
JS has one of the most active open source community. This means you will not hit the wall very easily and this indeed is coming from my personal experience. Also, the community is continuously innovating and libraries such ad Ramda, TypeScript, RxJS, Flow, Immutable.js, prettier, etc are a testament to this.
While these are the reasons I have discovered over the many years, there I am sure are many more. But the fact of the matter is that JS is a language that was, is and will be in our lives for the future to come. You can look down upon it, rant about it, hate it even, but will not get rid of it. Not only JS has a strong footing in its own domain, but it is also expanding its influence very fast and this gives the engineers who are comfortable with JS and its echo-system a unique advantage.