In this day and age it is pretty common to design a large-scale web application as a collection of modular and reusable components. These components need to have a reasonable set of APIs (application programming interfaces) which deliver against the stipulated engineering criteria. In addition this API set needs to retain ease of use. The success of a module is thus reflected in the way it is being used in other modules. This talk is intended to initiate the discussion on finding the principles for designing a good JavaScript API by observing the interactions between modules, particularly on the influence of an interface on the code patterns. Some good and bad examples found in real-world libraries will be shown, along with a collection of language tools designed to detect and analyze those patterns.