Abstract. TypeScript is a typed extension of JavaScript that has become widely used. More than 2 000 JavaScript libraries now have publicly available TypeScript declaration files, which allows the libraries to be used when programming TypeScript applications. Such declaration files are written manually, however, and they are often lagging behind the continuous development of the libraries, thereby hindering their usability. The existing tool tscheck is capable of detecting mismatches between the libraries and their declaration files, but it is less suitable when creating and evolving declaration files. In this work we present the tools tsinfer and tsevolve that are designed to assist the construction of new TypeScript declaration files and support the co-evolution of the declaration files as the underlying JavaScript libraries evolve. Our experimental results involving major libraries demonstrate that tsinfer and tsevolve are superior to tscheck regarding these tasks and that the tools are sufficiently fast and precise for practical use.

TypeScript applications often use untyped JavaScript libraries. To support static type checking of such applications, the typed APIs of the libraries are expressed as separate declaration files. This raises the challenge of checking that the declaration files are correct with respect to the library implementations. Previous work has shown that mismatches are frequent and cause TypeScript's type checker to misguide the programmers by rejecting correct applications and accepting incorrect ones. This paper shows how feedback-directed random testing, which is an automated testing technique that has mostly been used for testing Java libraries, can be adapted to effectively detect such type mismatches. Given a JavaScript library with a TypeScript declaration file, our tool TStest generates a type test script, which is an application that interacts with the library and tests that it behaves according to the type declarations. Compared to alternative solutions that involve static analysis, this approach finds significantly more mismatches in a large collection of real-world JavaScript libraries with TypeScript declaration files, and with fewer false positives. It also has the advantage that reported mismatches are easily reproducible with concrete executions, which aids diagnosis and debugging. CCS Concepts: • Software and its engineering → Software testing and debugging;

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