In this chapter, the authors augment javaScript's ContractRegistry to handle argument validation, showing how to check whether a function's incoming arguments meet expectations. The chapter shows how to create libraries of contracts, import them into our application, and attach them to our functions in aspects. The multipleFulfills method will be the workhorse of the argument validator. All of the testing for multipleFulfills has laid a very solid foundation for multipleAssert, which is a test‐and‐throw wrapper for multipleFulfills. The ContractRegistry in this chapter takes a dynamic, function‐oriented approach to type validation. It compares the ContractRegistry with TypeScript, a popular compile‐time type‐checker. The open‐source library TypeScript offers a static, declarative approach that makes JavaScript more like a compiled language, and one may wish to consider it as well.