Why does Java code need to be compiled but JavaScript code does not

Question

How come code written in Java needs to be compiled in byte-code that is interpreted by the JVM, but code written in a language like JavaScript does not need to be compiled and can run directly in a browser?

Is there an easy way to understand this?

What is the fundamental difference between the way these two languages are written, that may help to understand this behavior?

I am not a CS student, so please excuse the naivete of the question.

Answer 1

Historically, JavaScript was an interpreted language. Which means an interpreter accepts the source code and executes it all in one step. The advantage here is simplicity and flexibility, but interpreters are very slow. Compilers convert the high level language into a lower level language that either the native processor or a VM (in this case, the Java VM) can execute directly. This is much faster.

JavaScript in modern browsers is now compiled on the fly. So when the script is loaded, the first thing the JavaScript engine does is compile it into a bytecode and then execute it. The reason the entire compilation step is missing from the end user's perspective is because browser developers have (thankfully) maintained the requirement that JavaScript is not explicitly compiled.

Java was from the getgo a language that always had an explicit compile step. But in many cases that's not true anymore. IDE's like IntelliJ or Eclipse can compile Java on the fly and in many cases remove the explicit compilation step.

Answer 2

JavaScript and Java are not the same thing. They might share a similar name, but I refer you to the JS guru - Douglas Crockford to help clear up the fact that they are really not related at all.

The reality is that there is nothing stopping Java being an interpreted language, and there's equally nothing stopping JavaScript being a compiled language (Chrome's javascript engine does do compilation to improve speed, and does a very good job of it).

In the context of the browser, Java runs in the same way as Flash or Silverlight - a plugin is required and the browser acts as a host to that plugin; which hosts a Java runtime environment.

Javascript was designed to be a scripting language for the browser, and that's why the browser can understand it natively. How the browser actually achieves the running of that code, however, is entirely up to the browser. That is - it can operate purely at a script level, assuming zero-knowledge of the next line of code and running a purely software-based stack; or it can perform some JIT to get the code closer to the hardware and (hopefully) improve speed.