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 statically known layout at the allocation time, which in turn can en-able an ahead-of-time (AOT) compiler to generate efficient code. The main technical challenge we address is to ensure fixed object layout, while supporting popular language features such as objects with proto-type inheritance, structural subtyping, and method updates, with the additional constraint that SJS programs can run on any available stan-dard JavaScript engine, with no deviation from JavaScript’s standard operational semantics. The core difficulty arises from the way standard JavaScript semantics implements 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.