Towards a Type System for Analyzing JavaScript Programs

JavaScript is a popular language for client-side web scripting. It has a dubious reputation among programmers for two reasons. First, many JavaScript programs are written against a rapidly evolving API whose implementations are sometimes contradictory and idiosyncratic. Second, the language is only weakly typed and comes virtually without development tools. The present work is a first attempt to address the second point. It does so by defining a type system that tracks the possible traits of an object and flags suspicious type conversions. Because JavaScript is a classless, object-based language with first-class functions, the type system must include singleton types, subtyping, and first class record labels. The type system covers a representative subset of the language and there is a type soundness proof with respect to an operational semantics.