As a developer you should always be looking at other languages and striving to be a polyglot programmer. Learning a second language will make you a better developer in your primary language. This seems like a strange statement but what happens is that you see a strange construct or pattern being applied in the second language you say "hey, that's not available in my primary language." On further investigation, something like that or a clever workaround is usually available and that then gets added to your arsenal of tricks in your primary language.
No computer language is more important to learn today that JavaScript. Becoming an expert at JavaScript paves your way for future success.
1. C-based
JavaScript's syntax is influenced by the language C. Most developers have used a C-based language in the past (Java, C, C++, C# etc.) so this immediately gives you a good chunk of the syntax and keywords used in JavaScript. Almost all developers who have worked with a C-based language and then look at JavaScript think that they know JavaScript. This is a mistake. Unlike traditional C-based languages JavaScript is prototype-base, dynamic, and weakly typed.
2. Programming styles
JavaScript supports object-oriented, imperative, and functional programming styles giving the developer the flexibility to experiment with different programming styles in the same language.
3. Full stack
With Node.js it is now possible to use one language to develop web applications on both the client and the server. If there's one language to rule them all then this is it.
4. They all compile to JavaScript
On the client side you have other options than JavaScript to develop in. For example, CoffeeScript, TypeScript, and Google's Dart. However, at the end of the day they all compile down to JavaScript. If they stop maintaining that transpiler then you need to get stuck into the JavaScript code.
In 2006 Google Web Toolkit (GWT) created a way to compile Java to JavaScript
In 2007 we had pyjamas to transpile Python into JavaScript.
List of languages that compile to JavaScript.
5. JavaScript is the only standards-based language that runs in all web browsers
There is no other language that will run in all browser and that has a committee defining and developing standards for it.
6. More JavaScript code is being created each day than any other language
Okay, I just made this one up. However, it might be true. I have no idea how you would measure this but I bet that if it's not true today it will be one day soon.
7. Expert JavaScript developers are tough to find
Want to bullet proof your prospects of finding a job? Then become an expert JavaScript developer. I've been interviewing developers for a number of years and most recently been looking for expert JavaScript developers and I can tell you that they are the hardest type of developer to find. (Drop me a note if you're an expert JavaScript developer and you want to work at Go Daddy.)
8. Legacy JavaScript is going to be with us for decades
Just like COBOL programmers make comebacks every now and then because there is so much legacy written in COBOL, JavaScript programmers will be in demand for a very long time because of the amount of JS that's being written today.