Pro Runs on both the browser and the server
Pro Massive ecosystem
Pro Prototype based Object Oriented System
For example, in a prototypical language, you think of a rectangle, and define it. You now have a rectangle. Let's say you want a red rectangle, you copy the rectangle and give it the property red. Want a blue one? Copy rectangle again give it a blue. Big blue rectangle? Copy blue rectangle and make it big. In a class-based language, first you describe a rectangle, describe a red rectangle as a type of rectangle, describe blue and big blue rectangles, and now that you have described them, you must create instances of them, and only then do you have the rectangles. Essentially: Classes are factories for objects, prototypes are objects themselves, and the latter method was created to be more intuitive, which is particularly advantageous to beginners.
Pro Several Platforms to use the web stack and JS to create multi-platform apps
Opens the door to native application development as well as just websites. Use with React Native, Weex or Quasar (Vue), PhoneGap or Cordova, NativeScript... (etc) to build native apps. Use mostly the same code base for multi-platform and web.
Pro Easy to build an application
By using the UI capabilities in HTML and CSS you can develop substantial applications with graphical interfaces more quickly and with less effort than in other languages which would require you to learn a windowing library.
Building a useful application is one of the best ways to learn a new language and because of the low learning curve for creating applications you can create more substantial programs and learn more practical programming principle faster.
Pro No installation required
Con The `null` and `undefined` objects aren't really objects
Therefore, attempts to call methods on them will fail. This is especially frustrating since these are often returned instead of throwing exceptions. So a failure may appear far away from the actual cause, which makes bugs very hard to find.