This specification describes technologies relating to software execution. A computing device includes a processor. An operating system includes an execution environment in which applications can execute computer-specific commands. A web-browser application includes a scripting environment for interpreting scripted modules. The web-browser application further includes a native environment in which native modules can execute computer-specific commands. The web-browser application further includes an interface between the scripting environment and the native environment. The interface includes functions to asynchronously pass data objects by value, from one of the scripting environment and the native environment, to the other of the scripting environment and the native environment.