Type Checking for JavaScript

JavaScript is a powerful imperative object based language made popular by its use in web pages. It supports flexible program development by allowing dynamic addition of members to objects. Code is dynamically typed: a runtime access to a non-existing member causes an error. We suggest two static type systems for JavaScript that will detect such runtime type errors. Therefore, programmers can have the benefit of flexible programming offered by JavaScript with the safety offered by a static type system. We demonstrate our type systems with a formalism of JavaScript, JS0. Both type systems are based on structural recursive types. Members of a type are classified into definite and potential. The type of an object can evolve through assignment to potential members which turns them to definite. We first present a type system with integer and object types, and then we extend it adding also type variables to have polymorphism à la ML. We prove that our type systems are sound.