UAX: Measuring the Usability of TypeScript APIs

Application Programming Interfaces (APIs) have become an asset used everyday by developers looking for opportunities of code reuse. However, this ubiquity has led to numerous complexities, making the construction of good APIs challenging. Developers in fact often find APIs hard to learn and use. Therefore, usability has become a fundamental attribute for APIs in the last years, but it is often neglected by API designers. In reality, traditional user studies are frequently deemed expensive and difficult to interpret which stimulated the search for objective, and less expensive ways to measure API usability like metrics. The studies of metrics revealed good ways to measure API structurally, but the availability of publicly tools are either lacking or not being maintained. In this paper, we present UAX, a public tool that implements API usability metrics initially for TypeScript (a superset of JavaScript). We discuss its implementation and show its application to three reactive programming (RP) APIs with the help of a dashboard we also produced to help interpret the results. The evaluation showed that the RP APIs presented a high level of usability but with areas of improvements, like parameter consistency and API documentation. This demonstrates that the tool could support API designers to better plan the API construction and software engineering process, especially before the APIs get even released.