If you want to target browsers or Node.js, JavaScript is the only language natively supported by these environments. This is different to VM-based environments like the .NET runtime and the JVM, which support multiple languages.

The .NET runtime supports C#, F#, VB.NET, and others. The JVM supports Java, Scala, Clojure, and others. These languages work by compiling down to an assembly language for the environment's VM. This is the Common Intermediate Language in .NET or Java bytecode in the case of the JVM.

There is a reason why programmers don't all write CIL or Java bytecode though. These are low-level machine languages and much less human-friendly than C#, Java, and so on. In general, higher-level languages can support better productivity, as well as safety (for example, through type systems and memory management).

There is also a reason why .NET programmers don't always use C# and JVM programmers don't always use Java. A range of languages can serve different use...