Type inference for scripting languages with implicit extension

This paper presents a constraint-based type inference algorithm for a subset of the JavaScript language. The goal is to prevent accessing undefined members of objects. We define a type system that allows explicit extension of objects through add operation and implicit extension through method calls. We prove that a program is typable if and only if we can infer its types. We also extend the type system to allow strong updates and unrestricted extensions to new objects. 1