It's a lot harder to add types to a general purpose programming language. Your types have to match actual programming idioms, and if you care about them being s

It's a lot harder to add types to a general purpose programming language. Your types have to match actual programming idioms, and if you care about them being safe (which, to be fair, recent languages like Dart and TypeScript don't), you have to consider every possible loophole that could lead to a dynamic error -- and the legacy language fights you, because all of its dynamism was designed back when nobody was thinking about respecting some not-yet-existent type system. The type system for asm.js is a far more restricted problem, which is why we were able to come up with a solution so quickly (we only started this project in late 2012). The type hierarchy is extremely spartan, and it's just designed to map a low-level language (intended for compilers to be writing in) to the low-level machine model of modern computers.