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. The authors define a type system that allows explicit extension of objects through add operation and implicit extension through method calls. They prove that a program is typable if and only if they can infer its types. They also extend the type system to allow strong updates and unrestricted extensions to new objects.