Concrete Types for TypeScript

Typescript extends JavaScript with optional type annotations that are, by design, unsound and, that the Typescript compiler discards as it emits code. This design point preserves programming idioms developers are familiar with, and allows them to leave their legacy code unchanged, while offering a measure of static error checking in parts of the program that have type annotations. We present an alternative design for TypeScript, one where it is possible to support the same degree of dynamism, but where types can be strengthened to provide hard guarantees. We report on an implementation, called StrongScript, that improves runtime performance of typed programs when run on a modified version of the V8 JavaScript engine.