SJS: A Type System for JavaScript with Fixed Object Layout

Abstract. We propose a static type system for a significant subset of JavaScript, dubbed SJS, with the goal of ensuring that objects have a sta-tically known layout at the allocation time, which in turn can enable an ahead-of-time (AOT) compiler to generate efficient code.The main tech-nical challenge we address is to ensure fixed object layout, while support-ing popular language features such as objects with prototype inheritance, structural subtyping, and method updates, with the additional constraint that SJS programs can run on any available standard JavaScript engine, with no deviation from JavaScript’s standard operational semantics. The core difficulty arises from the way standard JavaScript semantics imple-ments object attribute update with prototype-based inheritance. To our knowledge, combining a fixed object layout property with prototype inheritance and subtyping has not been achieved previously.