This report describes a project about designing and implementing a usable applicationprogramming interface [API] for an existing application that was being ported to a webplatform.Today we experience a shift in how we humans work with computers in the sense thateverything becomes more and more web oriented. To stay usable, software has to adaptto this and this project is an example on how such adaption may be performed.The application, called Algodoo is an interactive 2D physics simulator written in C++and as this was ported to the web using Native Client [NaCl] , it required new functionalityto allow web page elements to interact with it. In other words, an API. It is thedesign and implementation of this API that is the focus of this report, but within it, thebackground for the project, will also be explained.