Optional typing in ES4

ES4 takes a very interesting approach to typing.  They've added static typing but made it completely optional.   Variable declarations can optionally be annotated with a type declaration.  However, the variable declarations don't change the run-time semantics of the language.  The only effect of the declarations is that if you run the program in strict mode, then the program will be verified before execution and rejected if type errors are found.  Implementations don't have to support strict mode.  You can still have simple, small footprint implementations that do all checks dynamically.  Users who don't want to be bothered with types can write programs without having to learn anything about the type system.

There's a good paper by Gilad Bracha on Pluggable Type Systems that explains why type systems should be optional not mandatory.   I think he's right. The dichotomy between statically and dynamically typed languages is false: an optional type system allows you to have the benefits of both. The paper goes further and argues that type systems should be not merely optional but pluggable.  I'm not convinced on this.  Pluggable type systems are a great idea if you are a language designer who wants to experiment with type systems; but for a production language, I think it's a fundamental responsibility of the language designer to choose a single type system.

Anyway, it's great to see optional typing being adopted by a mainstream language.