Generating WebIDL definitions from WebExtensions API JSONSchema

In toolkit/components/extensions/webidl-api, a python script named helps to generation of the WebIDL definitions for the WebExtensions API namespaces based on the existing JSONSchema data.

Diagram of the script data flow

Example: how to execute

As an example, the following shell command generates (or regenerates if one exists) the webidl bindings for the runtime API namespace:

$ export SCRIPT_DIR="toolkit/components/extensions/webidl-api"
$ mach python $SCRIPT_DIR/ -- runtime

this command will generates a .webdil file named dom/webidl/ExtensionRuntime.webidl.


This python script uses some python libraries part of mozilla-central mach command and so it has to be executed using mach python and any command line options that has to the passed to the script should be passed after the -- one that ends mach python own command line options.

  • If a webidl file with the same name already exist, the python script will ask confirmation and offer to print a diff of the changes (or just continue without changing the existing webidl file if the content is exactly the same):

$ mach python $SCRIPT_DIR/ -- runtime

Generating webidl definition for 'runtime' => dom/webidl/ExtensionRuntime.webidl
Found existing dom/webidl/ExtensionRuntime.webidl.

(Run again with --overwrite-existing to allow overwriting it automatically)

Overwrite dom/webidl/ExtensionRuntime.webidl? (Yes/No/Diff)
--- ExtensionRuntime.webidl--existing
+++ ExtensionRuntime.webidl--updated
@@ -24,6 +24,9 @@
 [Exposed=(ServiceWorker), LegacyNoInterfaceObject]
 interface ExtensionRuntime {
   // API methods.
+  [Throws, WebExtensionStub="Async"]
+  any myNewMethod(boolean aBoolParam, optional Function callback);

   [Throws, WebExtensionStub="Async"]
   any openOptionsPage(optional Function callback);

Overwrite dom/webidl/ExtensionRuntime.webidl? (Yes/No/Diff)
  • By convention each WebExtensions API WebIDL binding is expected to be paired with C++ files named ExtensionMyNamespace.h and ExtensionMyNamespace.cpp and located in toolkit/components/extensions/webidl-api:

    • if no files with the expected names is found the python script will generate an initial boilerplate files and will store them in the expected mozilla-central directory.

    • The Firefox developers are responsible to fill this initial boilerplate as needed and to apply the necessary changes (if any) when the webidl definitions are updated because of changes to the WebExtensions APIs JSONSchema.

ExtensionWebIDL.conf config file


  • mention the role of the “webidl generation” script config file in handling special cases (e.g. mapping types and method stubs)

  • notes on desktop-only APIs and API namespaces only partially available on Android

WebExtensionStub WebIDL extended attribute


  • mention the special webidl extended attribute used in the WebIDL definitions