SJS: A Type System for JavaScript with Fixed Object Layout

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 enable 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 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 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.