Gradual Typing for a More Pure JavaScript

Abstract: Dynamically typed languages have surged in popularity in recent years, owing to their flexibility and ease of use. However, for projects of a certain size dynamic typing can cause problems of maintainability as refactoring becomes increasingly difficult. One proposed solution is the use of gradual type systems, where static type annotations are optional. This results in providing the best of both worlds. The purpose of this project is to create a gradual type system on top of JavaScript. Another goal is to explore the possibility of making guarantees about function purity and immutability using the type system. The types and their relations are defined and a basic type checker is implemented to confirm the ideas. Extending type systems to be aware of side effects makes it easier to write safer software. It is concluded that all of this is possible and reasonable to do in JavaScript.