Here are some differences between the two languages:
- Java is a statically typed language; JavaScript is dynamic.
- Java is class-based; JavaScript is prototype-based.
- Java constructors are special functions that can only be called at object creation; JavaScript "constructors" are just standard functions.
- Java requires all non-block statements to end with a semicolon; JavaScript inserts semicolons at the ends of certain lines.
- Java uses block-based scoping; JavaScript uses function-based scoping.
- Java has an implicit
this scope for non-static methods, and implicit class scope; JavaScript has implicit global scope.
Here are some features that I think are particular strengths of JavaScript:
- JavaScript supports closures; Java can simulate sort-of "closures" using anonymous classes. (Real closures may be supported in a future version of Java.)
- All JavaScript functions are variadic; Java functions are only variadic if explicitly marked.
- JavaScript prototypes can be redefined at runtime, and has immediate effect for all referring objects. Java classes cannot be redefined in a way that affects any existing object instances.
- JavaScript allows methods in an object to be redefined independently of its prototype (think eigenclasses in Ruby, but on steroids); methods in a Java object are tied to its class, and cannot be redefined at runtime.
