It's the only language that enjoys native support in web browsers.
It's the only universal language that allows
executing the same code (e.g., for constraint validation) on the back-end and the front-end.
It's the only language that allows dynamic distribution, that is, executing the same code (e.g., for business computations) either in the back-end or the front-end, depending on run-time conditions such as the available front-end resources.
It combines object-oriented with functional programming.
Its dynamism allows various forms of meta-programming, which means it enables developers to program their own programming concepts, like classes and enumerations.