Gradual typing embedded securely in JavaScript, ... We present TS# , a gradual type system and source-to-source compiler for JavaScript. In contrast to prior gradual type systems, TS# features full runtime reflection over three kinds of types: (1) simple types for higher-order functions, recursive datatypes and dictionary-based extensible records; (2) the type any, for dynamically type-safe TS# expressions; and (3) the type un, for untrusted, potentially malicious JavaScript contexts in which TS# is embedded. After type-checking, the compiler instruments the program with various checks to ensure the type safety of TS# despite its interactions with arbitrary JavaScript contexts, which are free to use eval, stack walks, prototype customizations, and other offensive features. The proof of our main theorem employs a form of type-preserving compilation, wherein we prove all the runtime invariants of the translation of TS# to JavaScript by showing that translated programs are well-typed in JS# , a previously proposed dependently typed language for proving functional correctness of JavaScript programs. We describe a prototype compiler, a secure runtime, and sample applications for TS#. Our examples illustrate how web security patterns that developers currently program in JavaScript (with much difficulty and still with dubious results) can instead be programmed naturally in TS#, retaining a flavor of idiomatic JavaScript, while providing strong safety guarantees by virtue of typing.

This software is also peer reviewed by journal TOMS.