Cobalt Web Extension Support

Cobalt provides a facility for extending the JavaScript Web API. This allows custom web apps running on Cobalt to make calls through a custom API to C++ Cobalt code defined per Starboard platform. This can allow for closer integration between the web app hosted by Cobalt and the system on which that web app is running.

The Cobalt Web Extension support will allow you to attach an instance of a custom class to the JavaScript window global object so that it can be referenced from a web app in JavaScript as in the following example:


Build-level modifications

In order to extend the interface, one should add the following lines to the variables section of their platform's gyp_configuration.gypi file:

  1. cobalt_webapi_extension_source_idl_files
    This should be a list of IDL files
    that define the collection of new interfaces introduced by your extensions. One of these new interfaces can be selected to be injected into the window element (see 3. cobalt_webapi_extension_gyp_target for information on how to do this). Each IDL file listed here simultaneously defines a JavaScript and a C++ interface. For each IDL file, you will be expected to also provide a header file in the same directory that re-declares (in C++) the interface declared in the IDL file, as well as an implementation of all the methods within it (either inline in the header file or in a corresponding source file).
  2. cobalt_webapi_extension_generated_header_idl_files
    This is a list of all files that may result in automatic header file generation that might be referenced from other C++ code. An example of this is the definition of enums that may then be referenced as types in a file from 1. cobalt_webapi_extension_source_idl_files.
  3. cobalt_webapi_extension_gyp_target
    This is the gyp target that will provide the IDL interface implementations, as well as any necessary auxiliary code. It will be added as a dependency of browser/cobalt.gyp:cobalt. It is expected that this target will implement the interface defined in browser/webapi_extension.h, which let you name the injected window property, and provide a function to instantiate it (i.e. to let you select which IDL object is the “entry point”).

The first two lists get included by cobalt/browser/browser_bindings_gen.gyp, where you can look to see many examples of existing Cobalt IDL files that define the Web API available through Cobalt. For each of these, you can also examine their corresponding .h files and in most cases their .cc files as well.

An example configuration for these variables is available at starboard/shared/test_webapi_extension/test_webapi_extension.gypi, which contains the following variable definitions:

'cobalt_webapi_extension_source_idl_files': [
'cobalt_webapi_extension_generated_header_idl_files': [

Implementing the webapi_extension.h interface

As discussed above in 3. cobalt_webapi_extension_gyp_target, you must provide an implementation of the two functions declared in browser/webapi_extension.h.


You should implement GetWebAPIExtensionObjectPropertyName() to return the name of the injected window property. For example, in the example from the beginning of this document, window.myInterface.RunMyFunction(), we would have the function return std::string("myInterface"). If you return nullopt from this function, it is assumed that you do not wish to extend the web interface.

Note that you should NOT name your window property the same as your class name as described in the IDL file, it will result in a name collision in the JavaScript environment.


This function should instantiate and return the object to be accessed from window. The object must be defined by an IDL file.


You may find the Cobalt debug console to be particularly useful for debugging IDL additions and changes. In it, you can enter arbitrary JavaScript and then hit enter to execute it. You can toggle it open by hitting either CTRL+O or F1, and you may have to hit the key twice to skip past the HUD mode.

Here is an example of an example interface being exercised through the debug console:

Debug console web extension example