1: Application frameworks make your life easier: Building an app is easier than ever now with the help of tools like Angular and Ember. While many of these have steep learning curves that take time and dedication to master, the benefits of doing so are well worth the effort. That being said, brace yourself for the absolutely HUGE number of frameworks being released every year. Knowing which ones are worth learning, and which ones will fizzle out can be a challenge.
2: It's found in more than just browsers: We are living in the future. Well, technically it's the present but you get I'm trying to say. We now have all kinds of "smart" things. Our cars have computers, our watches can show social media updates, and even our refrigerators are getting in the game with projects like ChillHub. Using programming languages like JS are now opening doors to much more than just your computer. I personally can't wait until I can use an app to do my dishes for me!
3: It's a widely used language with a very big community: Have you ever looked at the amount of jQuery plugins or Angular modules, and just sat staring at your computer in awe of how many people use JS? Sadly, I have. That's a good thing though. It means that when you have an idea or need help there's a huge community just waiting to help you out. It also means that there's more people inventing new JS things everyday for you to use in your next project.
4: It's getting more popular as a backend language: More and more projects everyday are using a full JS stack. Projects like the MEAN stack are replacing more traditional server-side languages like PHP with 100% JS. Proof that JS has been underutilized for too long. The days of JS being just a "client-side" language are over.