JSTyper: Type inference for JavaScript

As with all untyped languages, JavaScript programs can contain type-mismatch-style errors. These mistakes do not necessarily cause runtime errors and can lead to subtle faults in deployed web apps. Type checking key portions of JavaScript applications can identify these mistakes early. Previous efforts to augment JavaScript with a type system have led to syntax extensions and compulsory type annotations; these increase the difficulty of integrating type-checked code into a significant code base. In JSTyper, we provide a type system which supports a broad range of idiomatic JavaScript programming, including dynamic property addition and object prototyping, and does not require type annotations. We use gradual typing-style checks to integrate with untyped JavaScript. To accomplish this, we separate a program into typed and untyped portions, checking the former and using dynamic checks to provide safe interaction with the latter. This in turn provides a low-effort migration path to checked JavaScript programs. We present our type system, a proof of soundness, and a practical implementation written in JavaScript that can be run in any modern browser.