[ice109]

which language's run time cares about types?

[nrinaudo]

Any dynamic language - by definition, they check types at runtime.

Or, unfortunately, a lot of static languages. Any static language that allows type casting, for instance - that's the only way they can check whether a cast from a type to one of its descendants is valid (eg, in Java, casting an Object to a, say, URL).

[Chyzwar]

Type information is used in JS engines to compile into native code. If type information is too vague or change you can have deoptimization[0]

[0]http://jayconrod.com/posts/54/a-tour-of-v8-crankshaft-the-op...