Web IDL bindings

Note

Need to document the setup for indexed and named setters/creators/deleters.

The Web IDL bindings are generated at build time based on two things: the actual Web IDL file and a configuration file that lists some metadata about how the Web IDL should be reflected into Gecko-internal code.

All Web IDL files should be placed in dom/webidl and added to the list in the moz.build file in that directory.

Note that if you’re adding new interfaces, then the test at dom/tests/mochitest/general/test_interfaces.html will most likely fail. This is a signal that you need to get a review from a DOM peer. Resist the urge to just add your interfaces to the moz.build list without the review; it will just annoy the DOM peers and they’ll make you get the review anyway.

The configuration file, dom/bindings/Bindings.conf, is basically a Python dict that maps interface names to information about the interface, called a descriptor. There are all sorts of possible options here that handle various edge cases, but most descriptors can be very simple.

All the generated code is placed in the mozilla::dom namespace. For each interface, a namespace whose name is the name of the interface with Binding appended is created, and all the things pertaining to that interface’s binding go in that namespace.

There are various helper objects and utility methods in dom/bindings that are also all in the mozilla::dom namespace and whose headers are all exported into mozilla/dom (placed in $OBJDIR/dist/include by the build process).

Adding Web IDL bindings to a class

To add a Web IDL binding for interface MyInterface to a class mozilla::dom::MyInterface that’s supposed to implement that interface, you need to do the following:

1. If your interface doesn’t inherit from any other interfaces, inherit from nsWrapperCache and hook up the class to the cycle collector so it will trace the wrapper cache properly. Note that you may not need to do this if your objects can only be created, never gotten from other objects. If you also inherit from nsISupports, make sure the nsISupports comes before the nsWrapperCache in your list of parent classes. If your interface does inherit from another interface, just inherit from the C++ type that the other interface corresponds to.

   If you do need to hook up cycle collection, it will look like this in the common case of also inheriting from nsISupports:

   // Add strong pointers your class holds here. If you do, change to using
   // NS_IMPL_CYCLE_COLLECTION_WRAPPERCACHE.
   NS_IMPL_CYCLE_COLLECTION_WRAPPERCACHE_0(MyClass)
   NS_IMPL_CYCLE_COLLECTING_ADDREF(MyClass)
   NS_IMPL_CYCLE_COLLECTING_RELEASE(MyClass)
   NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(MyClass)
     NS_WRAPPERCACHE_INTERFACE_MAP_ENTRY
     NS_INTERFACE_MAP_ENTRY(nsISupports)
   NS_INTERFACE_MAP_END
   

2. If your class doesn’t inherit from a class that implements GetParentObject, then add a function of that name that, for a given instance of your class, returns the same object every time (unless you write explicit code that handles your parent object changing by reparenting JS wrappers, as nodes do). The idea is that walking the GetParentObject chain will eventually get you to a Window, so that every Web IDL object is associated with a particular Window. For example, nsINode::GetParentObject returns the node’s owner document. The return type of GetParentObject doesn’t matter other than it must either singly-inherit from nsISupports or have a corresponding ToSupports method that can produce an nsISupports from it. (This allows the return value to be implicitly converted to a ParentObject instance by the compiler via one of that class’s non-explicit constructors.) If many instances of MyInterface are expected to be created quickly, the return value of GetParentObject should itself inherit from nsWrapperCache for optimal performance. Returning null from GetParentObject is allowed in situations in which it’s OK to associate the resulting object with a random global object for security purposes; this is not usually ok for things that are exposed to web content. Again, if you do not need wrapper caching you don’t need to do this. The actual type returned from GetParentObject must be defined in a header included from your implementation header, so that this type’s definition is visible to the binding code.

3. Add the Web IDL for MyInterface in dom/webidl and to the list in dom/webidl/moz.build.

4. Add an entry to dom/bindings/Bindings.conf that sets some basic information about the implementation of the interface. If the C++ type is not mozilla::dom::MyInterface, you need to set the 'nativeType' to the right type. If the type is not in the header file one gets by replacing ‘::’ with ‘/’ and appending ‘.h’, then add a corresponding 'headerFile' annotation (or HeaderFile annotation to the .webidl file). If you don’t have to set any annotations, then you don’t need to add an entry either and the code generator will simply assume the defaults here. Note that using a 'headerFile' annotation is generally not recommended. If you do use it, you will need to make sure your header includes all the headers needed for your Func annotations.

5. Add external interface entries to Bindings.conf for whatever non-Web IDL interfaces your new interface has as arguments or return values.

6. Implement a WrapObject override on mozilla::dom::MyInterface that just calls through to mozilla::dom::MyInterface_Binding::Wrap. Note that if your C++ type is implementing multiple distinct Web IDL interfaces, you need to choose which mozilla::dom::MyInterface_Binding::Wrap to call here. See AudioContext::WrapObject, for example.

7. Expose whatever methods the interface needs on mozilla::dom::MyInterface. These can be inline, virtual, have any calling convention, and so forth, as long as they have the right argument types and return types. You can see an example of what the function declarations should look like by running mach webidl-example MyInterface. This will produce two files in dom/bindings in your objdir: MyInterface-example.h and MyInterface-example.cpp, which show a basic implementation of the interface using a class that inherits from nsISupports and has a wrapper cache.

See this sample patch that migrates window.performance.* to Web IDL bindings.

Note

If your object can only be reflected into JS by creating it, not by retrieving it from somewhere, you can skip steps 1 and 2 above and instead add 'wrapperCache': False to your descriptor. You will need to flag the functions that return your object as [NewObject] in the Web IDL. If your object is not refcounted then the return value of functions that return it should return a UniquePtr.

C++ reflections of Web IDL constructs

C++ reflections of Web IDL operations (methods)

A Web IDL operation is turned into a method call on the underlying C++ object. The return type and argument types are determined as described below. In addition to those, all methods that are allowed to throw will get an ErrorResult& argument appended to their argument list. Non-static methods that use certain Web IDL types like any or object will get a JSContext* argument prepended to the argument list. Static methods will be passed a const GlobalObject& for the relevant global and can get a JSContext* by calling Context() on it.

The name of the C++ method is simply the name of the Web IDL operation with the first letter converted to uppercase.

Web IDL overloads are turned into C++ overloads: they simply call C++ methods with the same name and different signatures.

For example, this Web IDL:

interface MyInterface
{
  undefined doSomething(long number);
  double doSomething(MyInterface? otherInstance);

  [Throws]
  MyInterface doSomethingElse(optional long maybeNumber);
  [Throws]
  undefined doSomethingElse(MyInterface otherInstance);

  undefined doTheOther(any something);

  undefined doYetAnotherThing(optional boolean actuallyDoIt = false);

  static undefined staticOperation(any arg);
};

will require these method declarations:

class MyClass
{
  void DoSomething(int32_t a number);
  double DoSomething(MyClass* aOtherInstance);

  already_AddRefed<MyInterface> DoSomethingElse(Optional<int32_t> aMaybeNumber,
                                                ErrorResult& rv);
  void DoSomethingElse(MyClass& aOtherInstance, ErrorResult& rv);

  void DoTheOther(JSContext* cx, JS::Handle<JS::Value> aSomething);

  void DoYetAnotherThing(bool aActuallyDoIt);

  static void StaticOperation(const GlobalObject& aGlobal, JS::Handle<JS::Value> aSomething);
}

C++ reflections of Web IDL attributes

A Web IDL attribute is turned into a pair of method calls for the getter and setter on the underlying C++ object. A readonly attribute only has a getter and no setter.

The getter’s name is the name of the attribute with the first letter converted to uppercase. This has Get prepended to it if any of these conditions hold:

1. The type of the attribute is nullable.

2. The getter can throw.

3. The return value of the attribute is returned via an out parameter in the C++.

The method signature for the getter looks just like an operation with no arguments and the attribute’s type as the return type.

The setter’s name is Set followed by the name of the attribute with the first letter converted to uppercase. The method signature looks just like an operation with an undefined return value and a single argument whose type is the attribute’s type.

C++ reflections of Web IDL constructors

A Web IDL constructor is turned into a static class method named Constructor. The arguments of this method will be the arguments of the Web IDL constructor, with a const GlobalObject& for the relevant global prepended. For the non-worker case, the global is typically the inner window for the DOM Window the constructor function is attached to. If a JSContext* is also needed due to some of the argument types, it will come after the global. The return value of the constructor for MyInterface is exactly the same as that of a method returning an instance of MyInterface. Constructors are always allowed to throw.

For example, this IDL:

interface MyInterface {
  constructor();
  constructor(unsigned long someNumber);
};

will require the following declarations in MyClass:

class MyClass {
  // Various nsISupports stuff or whatnot
  static
  already_AddRefed<MyClass> Constructor(const GlobalObject& aGlobal,
                                        ErrorResult& rv);
  static
  already_AddRefed<MyClass> Constructor(const GlobalObject& aGlobal,
                                        uint32_t aSomeNumber,
                                        ErrorResult& rv);
};

C++ reflections of Web IDL types

The exact C++ representation for Web IDL types can depend on the precise way that they’re being used (e.g., return values, arguments, and sequence or dictionary members might all have different representations).

Unless stated otherwise, a type only has one representation. Also, unless stated otherwise, nullable types are represented by wrapping Nullable<> around the base type.

In all cases, optional arguments which do not have a default value are represented by wrapping const Optional<>& around the representation of the argument type. If the argument type is a C++ reference, it will also become a NonNull<> around the actual type of the object in the process. Optional arguments which do have a default value are just represented by the argument type itself, set to the default value if the argument was not in fact passed in.

Variadic Web IDL arguments are treated as a const Sequence<>& around the actual argument type.

Here’s a table, see the specific sections below for more details and explanations.

Web IDL Type	Argument Type	Return Type	Dictionary/Member Type
any	JS::Handle<JS::Value>	JS::MutableHandle<JS::Value>	JS::Value
boolean	bool	bool	bool
byte	int8_t	int8_t	int8_t
ByteString	const nsACString&	nsCString& (outparam) nsACString& (outparam)	nsCString
Date			mozilla::dom::Date
DOMString	const nsAString&	mozilla::dom::DOMString& (outparam) nsAString& (outparam) nsString& (outparam)	nsString
UTF8String	const nsACString& (outparam)	nsACString&	nsCString
double	double	double	double
float	float	float	float
interface: non-nullable	Foo&	already_addRefed<Foo>	OwningNonNull<Foo>
interface: nullable	Foo*	already_addRefed<Foo> Foo*	RefPtr<Foo>
long	int32_t	int32_t	int32_t
long long	int64_t	int64_t	int64_t
object	JS::Handle<JSObject*>	JS::MutableHandle<JSObject*>	JSObject*
octet	uint8_t	uint8_t	uint8_t
sequence	const Sequence<T>&	nsTArray<T>& (outparam)
short	int16_t	int16_t	int16_t
unrestricted double	double	double	double
unrestricted float	float	float	float
unsigned long	uint32_t	uint32_t	uint32_t
unsigned long long	uint64_t	uint64_t	uint64_t
unsigned short	uint16_t	uint16_t	uint16_t
USVString	const nsAString&	mozilla::dom::DOMString& (outparam) nsAString& (outparam) nsString& (outparam)	nsString

any

any is represented in three different ways, depending on use:

• any arguments become JS::Handle<JS::Value>. They will be in the compartment of the passed-in JSContext.

• any return values become a JS::MutableHandle<JS::Value> out param appended to the argument list. This comes after all IDL arguments, but before the ErrorResult&, if any, for the method. The return value is allowed to be in any compartment; bindings will wrap it into the context compartment as needed.

• any dictionary members and sequence elements become JS::Value. The dictionary members and sequence elements are guaranteed to be marked by whomever puts the sequence or dictionary on the stack, using SequenceRooter and DictionaryRooter.

Methods using any always get a JSContext* argument.

For example, this Web IDL:

interface Test {
  attribute any myAttr;
  any myMethod(any arg1, sequence<any> arg2, optional any arg3);
};

will correspond to these C++ function declarations:

void MyAttr(JSContext* cx, JS::MutableHandle<JS::Value> retval);
void SetMyAttr(JSContext* cx, JS::Handle<JS::Value> value);
void MyMethod(JSContext* cx, JS::Handle<JS::Value> arg1,
              const Sequence<JS::Value>& arg2,
              const Optional<JS::Handle<JS::Value>>& arg3,
              JS::MutableHandle<JS::Value> retval);

boolean

The boolean Web IDL type is represented as a C++ bool.

For example, this Web IDL:

interface Test {
  attribute boolean myAttr;
  boolean myMethod(optional boolean arg);
};

will correspond to these C++ function declarations:

bool MyAttr();
void SetMyAttr(bool value);
bool MyMethod(const Optional<bool>& arg);

Integer types

Integer Web IDL types are mapped to the corresponding C99 stdint types.

For example, this Web IDL:

interface Test {
  attribute short myAttr;
  long long myMethod(unsigned long? arg);
};

will correspond to these C++ function declarations:

int16_t MyAttr();
void SetMyAttr(int16_t value);
int64_t MyMethod(const Nullable<uint32_t>& arg);

Floating point types

Floating point Web IDL types are mapped to the C++ type of the same name. So float and unrestricted float become a C++ float, while double and unrestricted double become a C++ double.

For example, this Web IDL:

interface Test {
  float myAttr;
  double myMethod(unrestricted double? arg);
};

will correspond to these C++ function declarations:

float MyAttr();
void SetMyAttr(float value);
double MyMethod(const Nullable<double>& arg);

DOMString

Strings are reflected in three different ways, depending on use:

• String arguments become const nsAString&.

• String return values become a mozilla::dom::DOMString& out param appended to the argument list. This comes after all IDL arguments, but before the ErrorResult&, if any, for the method. Note that this allows callees to declare their methods as taking an nsAString& or nsString& if desired.

• Strings in sequences, dictionaries, owning unions, and variadic arguments become nsString.

Nullable strings are represented by the same types as non-nullable ones, but the string will return true for DOMStringIsNull(). Returning null as a string value can be done using SetDOMStringToNull on the out param if it’s an nsAString or calling SetNull() on a DOMString.

For example, this Web IDL:

interface Test {
  DOMString myAttr;
  [Throws]
  DOMString myMethod(sequence<DOMString> arg1, DOMString? arg2, optional DOMString arg3);
};

will correspond to these C++ function declarations:

void GetMyAttr(nsString& retval);
void SetMyAttr(const nsAString& value);
void MyMethod(const Sequence<nsString>& arg1, const nsAString& arg2,
              const Optional<nsAString>& arg3, nsString& retval, ErrorResult& rv);

USVString

USVString is reflected just like DOMString.

UTF8String

UTF8String is a string with guaranteed-valid UTF-8 contents. It is not a standard in the Web IDL spec, but its observables are the same as those of USVString.

It is a good fit for when the specification allows a USVString, but you want to process the string as UTF-8 rather than UTF-16.

It is reflected in three different ways, depending on use:

• Arguments become const nsACString&.

• Return values become an nsACString& out param appended to the argument list. This comes after all IDL arguments, but before the ErrorResult&, if any, for the method.

• In sequences, dictionaries owning unions, and variadic arguments it becomes nsCString.

Nullable UTF8Strings are represented by the same types as non-nullable ones, but the string will return true for IsVoid(). Returning null as a string value can be done using SetIsVoid() on the out param.

ByteString

ByteString is reflected in three different ways, depending on use:

• ByteString arguments become const nsACString&.

• ByteString return values become an nsCString& out param appended to the argument list. This comes after all IDL arguments, but before the ErrorResult&, if any, for the method.

• ByteString in sequences, dictionaries, owning unions, and variadic arguments becomes nsCString.

Nullable ByteString are represented by the same types as non-nullable ones, but the string will return true for IsVoid(). Returning null as a string value can be done using SetIsVoid() on the out param.

object

object is represented in three different ways, depending on use:

• object arguments become JS::Handle<JSObject*>. They will be in the compartment of the passed-in JSContext.

• object return values become a JS::MutableHandle<JSObject*> out param appended to the argument list. This comes after all IDL arguments, but before the ErrorResult&, if any, for the method. The return value is allowed to be in any compartment; bindings will wrap it into the context compartment as needed.

• object dictionary members and sequence elements become JSObject*. The dictionary members and sequence elements are guaranteed to be marked by whoever puts the sequence or dictionary on the stack, using SequenceRooter and DictionaryRooter.

Methods using object always get a JSContext* argument.

For example, this Web IDL:

interface Test {
  object myAttr;
  object myMethod(object arg1, object? arg2, sequence<object> arg3, optional object arg4,
                  optional object? arg5);
};

will correspond to these C++ function declarations:

void GetMyAttr(JSContext* cx, JS::MutableHandle<JSObject*> retval);
void SetMyAttr(JSContext* cx, JS::Handle<JSObject*> value);
void MyMethod(JSContext* cx, JS::Handle<JSObject*> arg1, JS::Handle<JSObject*> arg2,
              const Sequence<JSObject*>& arg3,
              const Optional<JS::Handle<JSObject*>>& arg4,
              const Optional<JS::Handle<JSObject*>>& arg5,
              JS::MutableHandle<JSObject*> retval);

Interface types

There are four kinds of interface types in the Web IDL bindings. Callback interfaces are used to represent script objects that browser code can call into. External interfaces are used to represent objects that have not been converted to the Web IDL bindings yet. Web IDL interfaces are used to represent Web IDL binding objects. “SpiderMonkey” interfaces are used to represent objects that are implemented natively by the JavaScript engine (e.g., typed arrays).

Callback interfaces

Callback interfaces are represented in C++ as objects inheriting from mozilla::dom::CallbackInterface, whose name, in the mozilla::dom namespace, matches the name of the callback interface in the Web IDL. The exact representation depends on how the type is being used.

• Nullable arguments become Foo*.

• Non-nullable arguments become Foo&.

• Return values become already_AddRefed<Foo> or Foo* as desired. The pointer form is preferred because it results in faster code, but it should only be used if the return value was not addrefed (and so it can only be used if the return value is kept alive by the callee until at least the binding method has returned).

• Web IDL callback interfaces in sequences, dictionaries, owning unions, and variadic arguments are represented by RefPtr<Foo> if nullable and OwningNonNull<Foo> otherwise.

If the interface is a single-operation interface, the object exposes two methods that both invoke the same underlying JS callable. The first of these methods allows the caller to pass in a this object, while the second defaults to undefined as the this value. In either case, the this value is only used if the callback interface is implemented by a JS callable. If it’s implemented by an object with a property whose name matches the operation, the object itself is always used as this.

If the interface is not a single-operation interface, it just exposes a single method for every IDL method/getter/setter.

The signatures of the methods correspond to the signatures for throwing IDL methods/getters/setters with an additional trailing mozilla::dom::CallbackObject::ExceptionHandling aExceptionHandling argument, defaulting to eReportExceptions. If aReportExceptions is set to eReportExceptions, the methods will report JS exceptions before returning. If aReportExceptions is set to eRethrowExceptions, JS exceptions will be stashed in the ErrorResult and will be reported when the stack unwinds to wherever the ErrorResult was set up.

For example, this Web IDL:

callback interface MyCallback {
  attribute long someNumber;
  short someMethod(DOMString someString);
};
callback interface MyOtherCallback {
  // single-operation interface
  short doSomething(Node someNode);
};
interface MyInterface {
  attribute MyCallback foo;
  attribute MyCallback? bar;
};

will lead to these C++ class declarations in the mozilla::dom namespace:

class MyCallback : public CallbackInterface
{
  int32_t GetSomeNumber(ErrorResult& rv, ExceptionHandling aExceptionHandling = eReportExceptions);
  void SetSomeNumber(int32_t arg, ErrorResult& rv,
                     ExceptionHandling aExceptionHandling = eReportExceptions);
  int16_t SomeMethod(const nsAString& someString, ErrorResult& rv,
                     ExceptionHandling aExceptionHandling = eReportExceptions);
};

class MyOtherCallback : public CallbackInterface
{
public:
  int16_t
  DoSomething(nsINode& someNode, ErrorResult& rv,
              ExceptionHandling aExceptionHandling = eReportExceptions);

  template<typename T>
  int16_t
  DoSomething(const T& thisObj, nsINode& someNode, ErrorResult& rv,
              ExceptionHandling aExceptionHandling = eReportExceptions);
};

and these C++ function declarations on the implementation of MyInterface:

already_AddRefed<MyCallback> GetFoo();
void SetFoo(MyCallback&);
already_AddRefed<MyCallback> GetBar();
void SetBar(MyCallback*);

A consumer of MyCallback would be able to use it like this:

void
SomeClass::DoSomethingWithCallback(MyCallback& aCallback)
{
  ErrorResult rv;
  int32_t number = aCallback.GetSomeNumber(rv);
  if (rv.Failed()) {
    // The error has already been reported to the JS console; you can handle
    // things however you want here.
    return;
  }

  // For some reason we want to catch and rethrow exceptions from SetSomeNumber, say.
  aCallback.SetSomeNumber(2*number, rv, eRethrowExceptions);
  if (rv.Failed()) {
    // The exception is now stored on rv. This code MUST report
    // it usefully; otherwise it will assert.
  }
}

External interfaces

External interfaces are represented in C++ as objects that XPConnect knows how to unwrap to. This can mean XPCOM interfaces (whether declared in XPIDL or not) or it can mean some type that there’s a castable native unwrapping function for. The C++ type to be used should be the nativeType listed for the external interface in the Bindings.conf file. The exact representation depends on how the type is being used.

• Arguments become nsIFoo*.

• Return values can be already_AddRefed<nsIFoo> or nsIFoo* as desired. The pointer form is preferred because it results in faster code, but it should only be used if the return value was not addrefed (and so it can only be used if the return value is kept alive by the callee until at least the binding method has returned).

• External interfaces in sequences, dictionaries, owning unions, and variadic arguments are represented by RefPtr<nsIFoo>.

Web IDL interfaces

Web IDL interfaces are represented in C++ as C++ classes. The class involved must either be refcounted or must be explicitly annotated in Bindings.conf as being directly owned by the JS object. If the class inherits from nsISupports, then the canonical nsISupports must be on the primary inheritance chain of the object. If the interface has a parent interface, the C++ class corresponding to the parent must be on the primary inheritance chain of the object. This guarantees that a void* can be stored in the JSObject which can then be reinterpret_cast to any of the classes that correspond to interfaces the object implements. The C++ type to be used should be the nativeType listed for the interface in the Bindings.conf file, or mozilla::dom::InterfaceName if none is listed. The exact representation depends on how the type is being used.

• Nullable arguments become Foo*.

• Non-nullable arguments become Foo&.

• Return values become already_AddRefed<Foo> or Foo* as desired. The pointer form is preferred because it results in faster code, but it should only be used if the return value was not addrefed (and so it can only be used if the return value is kept alive by the callee until at least the binding method has returned).

• Web IDL interfaces in sequences, dictionaries, owning unions, and variadic arguments are represented by RefPtr<Foo> if nullable and OwningNonNull<Foo> otherwise.

For example, this Web IDL:

interface MyInterface {
  attribute MyInterface myAttr;
  undefined passNullable(MyInterface? arg);
  MyInterface? doSomething(sequence<MyInterface> arg);
  MyInterface doTheOther(sequence<MyInterface?> arg);
  readonly attribute MyInterface? nullableAttr;
  readonly attribute MyInterface someOtherAttr;
  readonly attribute MyInterface someYetOtherAttr;
};

Would correspond to these C++ function declarations:

already_AddRefed<MyClass> MyAttr();
void SetMyAttr(MyClass& value);
void PassNullable(MyClass* arg);
already_AddRefed<MyClass> doSomething(const Sequence<OwningNonNull<MyClass>>& arg);
already_AddRefed<MyClass> doTheOther(const Sequence<RefPtr<MyClass>>& arg);
already_Addrefed<MyClass> GetNullableAttr();
MyClass* SomeOtherAttr();
MyClass* SomeYetOtherAttr(); // Don't have to return already_AddRefed!

“SpiderMonkey” interfaces

Typed array, array buffer, and array buffer view arguments are represented by the objects in TypedArray.h. For example, this Web IDL:

interface Test {
  undefined passTypedArrayBuffer(ArrayBuffer arg);
  undefined passTypedArray(ArrayBufferView arg);
  undefined passInt16Array(Int16Array? arg);
}

will correspond to these C++ function declarations:

void PassTypedArrayBuffer(const ArrayBuffer& arg);
void PassTypedArray(const ArrayBufferView& arg);
void PassInt16Array(const Nullable<Int16Array>& arg);

Typed array return values become a JS::MutableHandle<JSObject*> out param appended to the argument list. This comes after all IDL arguments, but before the ErrorResult&, if any, for the method. The return value is allowed to be in any compartment; bindings will wrap it into the context compartment as needed.

Typed arrays store a JSObject* and hence need to be rooted properly. On-stack typed arrays can be declared as RootedTypedArray<TypedArrayType> (e.g. RootedTypedArray<Int16Array>). Typed arrays on the heap need to be traced.

Dictionary types

A dictionary argument is represented by a const reference to a struct whose name is the dictionary name in the mozilla::dom namespace. The struct has one member for each of the dictionary’s members with the same name except the first letter uppercased and prefixed with “m”. The members that are required or have default values have types as described under the corresponding Web IDL type in this document. The members that are not required and don’t have default values have those types wrapped in Optional<>.

Dictionary return values are represented by an out parameter whose type is a non-const reference to the struct described above, with all the members that have default values preinitialized to those default values.

Note that optional dictionary arguments are always forced to have a default value of an empty dictionary by the IDL parser and code generator, so dictionary arguments are never wrapped in Optional<>.

If necessary, dictionaries can be directly initialized from a JS::Value in C++ code by invoking their Init() method. Consumers doing this should declare their dictionary as RootedDictionary<DictionaryName>. When this is done, passing in a null JSContext* is allowed if the passed-in JS::Value is JS::NullValue(). Likewise, a dictionary struct can be converted to a JS::Value in C++ by calling ToJSValue with the dictionary as the second argument. If Init() or ToJSValue() returns false, they will generally set a pending exception on the JSContext; reporting those is the responsibility of the caller.

For example, this Web IDL:

dictionary Dict {
  long foo = 5;
  DOMString bar;
};

interface Test {
  undefined initSomething(optional Dict arg = {});
};

will correspond to this C++ function declaration:

void InitSomething(const Dict& arg);

and the Dict struct will look like this:

struct Dict {
  bool Init(JSContext* aCx, JS::Handle<JS::Value> aVal, const char* aSourceDescription = "value");

  Optional<nsString> mBar;
  int32_t mFoo;
}

Note that the dictionary members are sorted in the struct in alphabetical order.

API for working with dictionaries

There are a few useful methods found on dictionaries and dictionary members that you can use to quickly determine useful things.

• member.WasPassed() - as the name suggests, was a particular member passed? (e.g., if (arg.foo.WasPassed() { /* do nice things!*/ })

• dictionary.IsAnyMemberPresent() - great for checking if you need to do anything. (e.g., if (!arg.IsAnyMemberPresent()) return; // nothing to do)

• member.Value() - getting the actual data/value of a member that was passed. (e.g., mBar.Assign(args.mBar.value()))

Example implementation using all of the above:

void
MyInterface::InitSomething(const Dict& aArg){
  if (!aArg.IsAnyMemberPresent()) {
    return; // nothing to do!
  }
  if (aArg.mBar.WasPassed() && !mBar.Equals(aArg.mBar.value())) {
    mBar.Assign(aArg.mBar.Value());
  }
}

Enumeration types

Web IDL enumeration types are represented as C++ enum classes. The values of the C++ enum are named by taking the strings in the Web IDL enumeration, replacing all non-alphanumerics with underscores, and uppercasing the first letter, with a special case for the empty string, which becomes the value _empty.

For a Web IDL enum named MyEnum, the C++ enum is named MyEnum and placed in the mozilla::dom namespace, while the values are placed in the mozilla::dom::MyEnum namespace.

The type of the enum class is automatically selected to be the smallest unsigned integer type that can hold all the values. In practice, this is always uint8_t, because Web IDL enums tend to not have more than 255 values.

For example, this Web IDL:

enum MyEnum {
  "something",
  "something-else",
  "",
  "another"
};

would lead to this C++ enum declaration:

enum class MyEnum : uint8_t {
  Something,
  Something_else,
  _empty,
  Another
};

mozilla::dom::GetEnumString is a templated helper function declared in BindingUtils.h and exported to mozilla/dom/BindingUtils.h that can be used to convert an enum value to its corresponding string value. It returns a const nsCString& containing the string value.

mozilla::dom::StringToEnum is a templated helper function in BindingUtils.h and exported to mozilla/dom/BindingUtils.h that can be used to convert a string to the corresponding enum value. It needs to be supplied with the enum class as a template argument, and returns a mozilla::Maybe<Enum>. If the string value passed to it as an argument is not one of the string values for the enum then it returns mozilla::Nothing(), else it returns the right enum value in the mozilla::Maybe.

mozilla::dom::WebIDLEnumSerializer is a templated alias in BindingIPCUtils.h exported to mozilla/dom/BindingIPCUtils.h to implement an IPC serializer with the right validation for WebIDL enums. It uses a mozilla::MaxContinuousEnumValue that is generated for every WebIDL enum to implement the validation.

mozilla::dom::MakeWebIDLEnumeratedRange is a templated helper function in BindingUtils.h and exported to mozilla/dom/BindingUtils.h that can be used to create a mozilla::EnumeratedRange for a WebIDL enum.

Callback function types

Callback functions are represented as an object, inheriting from mozilla::dom::CallbackFunction, whose name, in the mozilla::dom namespace, matches the name of the callback function in the Web IDL. If the type is nullable, a pointer is passed in; otherwise a reference is passed in.

The object exposes two Call methods, which both invoke the underlying JS callable. The first Call method has the same signature as a throwing method declared just like the callback function, with an additional trailing mozilla::dom::CallbackObject::ExceptionHandling aExceptionHandling argument, defaulting to eReportExceptions, and calling it will invoke the callable with undefined as the this value. The second Call method allows passing in an explicit this value as the first argument. This second call method is a template on the type of the first argument, so the this value can be passed in in whatever form is most convenient, as long as it’s either a type that can be wrapped by XPConnect or a Web IDL interface type.

If aReportExceptions is set to eReportExceptions, the Call methods will report JS exceptions before returning. If aReportExceptions is set to eRethrowExceptions, JS exceptions will be stashed in the ErrorResult and will be reported when the stack unwinds to wherever the ErrorResult was set up.

For example, this Web IDL:

callback MyCallback = long (MyInterface arg1, boolean arg2);
interface MyInterface {
  attribute MyCallback foo;
  attribute MyCallback? bar;
};

will lead to this C++ class declaration, in the mozilla::dom namespace:

class MyCallback : public CallbackFunction
{
public:
  int32_t
  Call(MyInterface& arg1, bool arg2, ErrorResult& rv,
       ExceptionHandling aExceptionHandling = eReportExceptions);

  template<typename T>
  int32_t
  Call(const T& thisObj, MyInterface& arg1, bool arg2, ErrorResult& rv,
       ExceptionHandling aExceptionHandling = eReportExceptions);
};

and these C++ function declarations in the MyInterface class:

already_AddRefed<MyCallback> GetFoo();
void SetFoo(MyCallback&);
already_AddRefed<MyCallback> GetBar();
void SetBar(MyCallback*);

A consumer of MyCallback would be able to use it like this:

void
SomeClass::DoSomethingWithCallback(MyCallback& aCallback, MyInterface& aInterfaceInstance)
{
  ErrorResult rv;
  int32_t number = aCallback.Call(aInterfaceInstance, false, rv);
  if (rv.Failed()) {
    // The error has already been reported to the JS console; you can handle
    // things however you want here.
    return;
  }

  // Now for some reason we want to catch and rethrow exceptions from the callback,
  // and use "this" as the this value for the call to JS.
  number = aCallback.Call(*this, true, rv, eRethrowExceptions);
  if (rv.Failed()) {
    // The exception is now stored on rv.  This code MUST report
    // it usefully; otherwise it will assert.
  }
}

Sequences

Sequence arguments are represented by const Sequence<T>&, where T depends on the type of elements in the Web IDL sequence.

Sequence return values are represented by an nsTArray<T> out param appended to the argument list, where T is the return type for the elements of the Web IDL sequence. This comes after all IDL arguments, but before the ErrorResult&, if any, for the method.

Arrays

IDL array objects are not supported yet. The spec on these is likely to change drastically anyway.

Union types

Union types are reflected as a struct in the mozilla::dom namespace. There are two kinds of union structs: one kind does not keep its members alive (is “non-owning”), and the other does (is “owning”). Const references to non-owning unions are used for plain arguments. Owning unions are used in dictionaries, sequences, and for variadic arguments. Union return values become a non-const owning union out param. The name of the struct is the concatenation of the names of the types in the union, with “Or” inserted between them, and for an owning struct “Owning” prepended. So for example, this IDL:

undefined passUnion((object or long) arg);
(object or long) receiveUnion();
undefined passSequenceOfUnions(sequence<(object or long)> arg);
undefined passOtherUnion((HTMLDivElement or ArrayBuffer or EventInit) arg);

would correspond to these C++ function declarations:

void PassUnion(const ObjectOrLong& aArg);
void ReceiveUnion(OwningObjectObjectOrLong& aArg);
void PassSequenceOfUnions(const Sequence<OwningObjectOrLong>& aArg);
void PassOtherUnion(const HTMLDivElementOrArrayBufferOrEventInit& aArg);

Union structs expose accessors to test whether they’re of a given type and to get hold of the data of that type. They also expose setters that set the union as being of a particular type and return a reference to the union’s internal storage where that type could be stored. The one exception is the object type, which uses a somewhat different form of setter where the JSObject* is passed in directly. For example, ObjectOrLong would have the following methods:

bool IsObject() const;
JSObject* GetAsObject() const;
void SetToObject(JSContext*, JSObject*);
bool IsLong() const;
int32_t GetAsLong() const;
int32_t& SetAsLong()

Owning unions used on the stack should be declared as a RootedUnion<UnionType>, for example, RootedUnion<OwningObjectOrLong>.

Date

Web IDL Date types are represented by a mozilla::dom::Date struct.

C++ reflections of Web IDL declarations

Web IDL declarations (maplike/setlike/iterable) are turned into a set of properties and functions on the interface they are declared on. Each has a different set of helper functions it comes with. In addition, for iterable, there are requirements for C++ function implementation by the interface developer.

Maplike

Example Interface:

interface StringToLongMap {
  maplike<DOMString, long>;
};

The bindings for this interface will generate the storage structure for the map, as well as helper functions for accessing that structure from C++. The generated C++ API will look as follows:

namespace StringToLongMapBinding {
namespace MaplikeHelpers {
void Clear(mozilla::dom::StringToLongMap* self, ErrorResult& aRv);
bool Delete(mozilla::dom::StringToLongMap* self, const nsAString& aKey, ErrorResult& aRv);
bool Has(mozilla::dom::StringToLongMap* self, const nsAString& aKey, ErrorResult& aRv);
void Set(mozilla::dom::StringToLongMap* self, const nsAString& aKey, int32_t aValue, ErrorResult& aRv);
} // namespace MaplikeHelpers
} // namespace StringToLongMapBindings

Setlike

Example Interface:

interface StringSet {
  setlike<DOMString>;
};

The bindings for this interface will generate the storage structure for the set, as well as helper functions for accessing that structure from c++. The generated C++ API will look as follows:

namespace StringSetBinding {
namespace SetlikeHelpers {
void Clear(mozilla::dom::StringSet* self, ErrorResult& aRv);
bool Delete(mozilla::dom::StringSet* self, const nsAString& aKey, ErrorResult& aRv);
bool Has(mozilla::dom::StringSet* self, const nsAString& aKey, ErrorResult& aRv);
void Add(mozilla::dom::StringSet* self, const nsAString& aKey, ErrorResult& aRv);
} // namespace SetlikeHelpers
}

Iterable

Unlike maplike and setlike, iterable does not have any C++ helpers, as the structure backing the iterable data for the interface is left up to the developer. With that in mind, the generated iterable bindings expect the wrapper object to provide certain methods for the interface to access.

Iterable interfaces have different requirements, based on if they are single or pair value iterators.

Example Interface for a single value iterator:

interface LongIterable {
  iterable<long>;
  getter long(unsigned long index);
  readonly attribute unsigned long length;
};

For single value iterator interfaces, we treat the interface as an indexed getter, as required by the spec. See the indexed getter implementation section for more information on building this kind of structure.

Example Interface for a pair value iterator:

interface StringAndLongIterable {
  iterable<DOMString, long>;
};

The bindings for this pair value iterator interface require the following methods be implemented in the C++ object:

class StringAndLongIterable {
public:
  // Returns the number of items in the iterable storage
  size_t GetIterableLength();
  // Returns key of pair at aIndex in iterable storage
  nsAString& GetKeyAtIndex(uint32_t aIndex);
  // Returns value of pair at aIndex in iterable storage
  uint32_t& GetValueAtIndex(uint32_t aIndex);
}

Stringifiers

Named stringifiers operations in Web IDL will just invoke the corresponding C++ method.

Anonymous stringifiers in Web IDL will invoke the C++ method called Stringify. So, for example, given this IDL:

interface FirstInterface {
  stringifier;
};

interface SecondInterface {
  stringifier DOMString getStringRepresentation();
};

the corresponding C++ would be:

class FirstInterface {
public:
  void Stringify(nsAString& aResult);
};

class SecondInterface {
public:
  void GetStringRepresentation(nsAString& aResult);
};

Legacy Callers

Only anonymous legacy callers are supported, and will invoke the C++ method called LegacyCall. This will be passed the JS “this” value as the first argument, then the arguments to the actual operation. A JSContext will be passed if any of the operation arguments need it. So for example, given this IDL:

interface InterfaceWithCall {
  legacycaller long (float arg);
};

the corresponding C++ would be:

class InterfaceWithCall {
public:
  int32_t LegacyCall(JS::Handle<JS::Value> aThisVal, float aArgument);
};

Named getters

If the interface has a named getter, the binding will expect several methods on the C++ implementation:

• A NamedGetter method. This takes a property name and returns whatever type the named getter is declared to return. It also has a boolean out param for whether a property with that name should exist at all.

• A NameIsEnumerable method. This takes a property name and returns a boolean that indicates whether the property is enumerable.

• A GetSupportedNames method. This takes an unsigned integer which corresponds to the flags passed to the iterate proxy trap and returns a list of property names. For implementations of this method, the important flags is JSITER_HIDDEN. If that flag is set, the call needs to return all supported property names. If it’s not set, the call needs to return only the enumerable ones.

The NameIsEnumerable and GetSupportedNames methods need to agree on which names are and are not enumerable. The NamedGetter and GetSupportedNames methods need to agree on which names are supported.

So for example, given this IDL:

interface InterfaceWithNamedGetter {
  getter long(DOMString arg);
};

the corresponding C++ would be:

class InterfaceWithNamedGetter
{
public:
  int32_t NamedGetter(const nsAString& aName, bool& aFound);
  bool NameIsEnumerable(const nsAString& aName);
  undefined GetSupportedNames(unsigned aFlags, nsTArray<nsString>& aNames);
};

Indexed getters

If the interface has a indexed getter, the binding will expect the following methods on the C++ implementation:

• A IndexedGetter method. This takes an integer index value and returns whatever type the indexed getter is declared to return. It also has a boolean out param for whether a property with that index should exist at all. The implementation must set this out param correctly. The return value is guaranteed to be ignored if the out param is set to false.

So for example, given this IDL:

interface InterfaceWithIndexedGetter {
  getter long(unsigned long index);
  readonly attribute unsigned long length;
};

the corresponding C++ would be:

class InterfaceWithIndexedGetter
{
public:
  uint32_t Length() const;
  int32_t IndexedGetter(uint32_t aIndex, bool& aFound) const;
};

Throwing exceptions from Web IDL methods, getters, and setters

Web IDL methods, getters, and setters that are explicitly marked as allowed to throw have an ErrorResult& argument as their last argument. To throw an exception, simply call Throw() on the ErrorResult& and return from your C++ back into the binding code.

In cases when the specification calls for throwing a TypeError, you should use ErrorResult::ThrowTypeError() instead of calling Throw().

Custom extended attributes

Our Web IDL parser and code generator recognize several extended attributes that are not present in the Web IDL spec.

[Alias=propName]

This extended attribute can be specified on a method and indicates that another property with the specified name will also appear on the interface prototype object and will have the same Function object value as the property for the method. For example:

interface MyInterface {
  [Alias=performSomething] undefined doSomething();
};

MyInterface.prototype.performSomething will have the same Function object value as MyInterface.prototype.doSomething.

Multiple [Alias] extended attribute can be used on the one method. [Alias] cannot be used on a static method, nor on methods on a global interface (such as Window).

Aside from regular property names, the name of an alias can be Symbol.iterator. This is specified by writing [Alias="@@iterator"].

[BindingAlias=propName]

This extended attribute can be specified on an attribute and indicates that another property with the specified name will also appear on the interface prototype object and will call the same underlying C++ implementation for the getter and setter. This is more efficient than using the same BinaryName for both attributes, because it shares the binding glue code between them. The properties still have separate getter/setter functions in JavaScript, so from the point of view of web consumers it’s as if you actually had two separate attribute declarations on your interface. For example:

interface MyInterface {
  [BindingAlias=otherAttr] readonly attribute boolean attr;
};

MyInterface.prototype.otherAttr and MyInterface.prototype.attr will both exist, have separate getter/setter functions, but call the same binding glue code and implementation function on the objects implementing MyInterface.

Multiple [BindingAlias] extended attributes can be used on a single attribute.

[BindingTemplate=(name, value)]

This extended attribute can be specified on an attribute, and causes the getter and setter for this attribute to forward to a common generated implementation, shared with all other attributes that have a [BindingTemplate] with the same value for the name argument. The TemplatedAttributes dictionary in Bindings.conf needs to contain a definition for the template with the name name. The value will be passed as an argument when calling the common generated implementation.

This is aimed at very specialized use cases where an interface has a large number of attributes that all have the same type, and for which we have a native implementation that’s common to all these attributes, and typically uses some id based on the attribute’s name in the implementation. All the attributes that use the same template need to mostly have the same extended attributes, except form a small number that are allowed to differ ([BindingTemplate], [BindingAlias], [Pure], [Pref] and [Func], and the annotations for whether the getter and setter throws exceptions).

[ChromeOnly]

This extended attribute can be specified on any method, attribute, or constant on an interface or on an interface as a whole. It can also be specified on dictionary members.

Interface members flagged as [ChromeOnly] are only exposed in chrome Windows (and in particular, are not exposed to webpages). From the point of view of web content, it’s as if the interface member were not there at all. These members are exposed to chrome script working with a content object via Xrays.

If specified on an interface as a whole, this functions like [Func] except that the binding code will automatically check whether the caller script has the system principal (is chrome or a worker started from a chrome page) instead of calling into the C++ implementation to determine whether to expose the interface object on the global. This means that accessing a content global via Xrays will show [ChromeOnly] interface objects on it.

If specified on a dictionary member, then the dictionary member will only appear to exist in system-privileged code.

This extended attribute can be specified together with [Func], and [Pref]. If more than one of these is specified, all conditions will need to test true for the interface or interface member to be exposed.

[Pref=prefname]

This extended attribute can be specified on any method, attribute, or constant on an interface or on an interface as a whole. It can also be specified on dictionary members. It takes a value, which must be the name of a boolean preference exposed from StaticPrefs. The StaticPrefs function that will be called is calculated from the value of the extended attribute, with dots replaced by underscores (StaticPrefs::my_pref_name() in the example below).

If specified on an interface member, the interface member involved is only exposed if the preference is set to true. An example of how this can be used:

interface MyInterface {
  attribute long alwaysHere;
  [Pref="my.pref.name"] attribute long onlyHereIfEnabled;
};

If specified on an interface as a whole, this functions like [Func] except that the binding will check the value of the preference directly without calling into the C++ implementation of the interface at all. This is useful when the enable check is simple and it’s desirable to keep the prefname with the Web IDL declaration.

If specified on a dictionary member, the web-observable behavior when the pref is set to false will be as if the dictionary did not have a member of that name defined. That means that on the JS side no observable get of the property will happen. On the C++ side, the behavior would be as if the passed-in object did not have a property with the relevant name: the dictionary member would either be !Passed() or have the default value if there is a default value.

An example of how this can be used:

[Pref="my.pref.name"]
interface MyConditionalInterface {
};

This extended attribute can be specified together with [ChromeOnly], and [Func]. If more than one of these is specified, all conditions will need to test true for the interface or interface member to be exposed.

[Func="funcname"]

This extended attribute can be specified on any method, attribute, or constant on an interface or on an interface as a whole. It can also be specified on dictionary members. It takes a value, which must be the name of a static function.

If specified on an interface member, the interface member involved is only exposed if the specified function returns true. An example of how this can be used:

interface MyInterface {
  attribute long alwaysHere;
  [Func="MyClass::StuffEnabled"] attribute long onlyHereIfEnabled;
};

The function is invoked with two arguments: the JSContext that the operation is happening on and the JSObject for the global of the object that the property will be defined on if the function returns true. In particular, in the Xray case the JSContext is in the caller compartment (typically chrome) but the JSObject is in the target compartment (typically content). This allows the method implementation to select which compartment it cares about in its checks.

The above IDL would also require the following C++:

class MyClass {
  static bool StuffEnabled(JSContext* cx, JSObject* obj);
};

If specified on an interface as a whole, then lookups for the interface object for this interface on a DOM Window will only find it if the specified function returns true. For objects that can only be created via a constructor, this allows disabling the functionality altogether and making it look like the feature is not implemented at all.

If specified on a dictionary member, the web-observable behavior when the function returns false will be as if the dictionary did not have a member of that name defined. That means that on the JS side no observable get of the property will happen. On the C++ side, the behavior would be as if the passed-in object did not have a property with the relevant name: the dictionary member would either be !Passed() or have the default value if there is a default value.

An example of how [Func] can be used:

[Func="MyClass::MyConditionalInterfaceEnabled"]
interface MyConditionalInterface {
};

In this case, the C++ function is passed a JS::Handle<JSObject*>. So the C++ in this case would look like this:

class MyClass {
  static bool MyConditionalInterfaceEnabled(JSContext* cx, JS::Handle<JSObject*> obj);
};

Just like in the interface member case, the JSContext is in the caller compartment but the JSObject is the actual object the property would be defined on. In the Xray case that means obj is in the target compartment (typically content) and cx is typically chrome.

This extended attribute can be specified together with [ChromeOnly], and [Pref]. If more than one of these is specified, all conditions will need to test true for the interface or interface member to be exposed.

Binding code will include the headers necessary for a [Func], unless the interface is using a non-default header file. If a non-default header file is used, that header file needs to do any header inclusions necessary for [Func] annotations.

[Throws], [GetterThrows], [SetterThrows]

Used to flag methods or attributes as allowing the C++ callee to throw. This causes the binding generator, and in many cases the JIT, to generate extra code to handle possible exceptions. Possibly-throwing methods and attributes get an ErrorResult& argument.

[Throws] applies to both methods and attributes; for attributes it means both the getter and the setter can throw. [GetterThrows] applies only to attributes. [SetterThrows] applies only to non-readonly attributes.

For interfaces flagged with [JSImplementation], all methods and properties are assumed to be able to throw and do not need to be flagged as throwing.

[DependsOn]

Used for a method or attribute to indicate what the return value depends on. Possible values are:

• Everything

  This value can’t actually be specified explicitly; this is the default value you get when [DependsOn] is not specified. This means we don’t know anything about the return value’s dependencies and hence can’t rearrange other code that might change values around the method or attribute.

• DOMState

  The return value depends on the state of the “DOM”, by which we mean all objects specified via Web IDL. The return value is guaranteed to not depend on the state of the JS heap or other JS engine data structures, and is guaranteed to not change unless some function with [Affects=Everything] is executed.

• DeviceState

  The return value depends on the state of the device we’re running on (e.g., the system clock). The return value is guaranteed to not be affected by any code running inside Gecko itself, but we might get a new value every time the method or getter is called even if no Gecko code ran between the calls.

• Nothing

  The return value is a constant that never changes. This value cannot be used on non-readonly attributes, since having a non-readonly attribute whose value never changes doesn’t make sense.

Values other than Everything, when used in combination with [Affects=Nothing], can used by the JIT to perform loop-hoisting and common subexpression elimination on the return values of IDL attributes and methods.

[Affects]

Used for a method or attribute getter to indicate what sorts of state can be affected when the function is called. Attribute setters are, for now, assumed to affect everything. Possible values are:

• Everything

  This value can’t actually be specified explicitly; this is the default value you get when [Affects] is not specified. This means that calling the method or getter might change any mutable state in the DOM or JS heap.

• Nothing

  Calling the method or getter will have no side-effects on either the DOM or the JS heap.

Methods and attribute getters with [Affects=Nothing] are allowed to throw exceptions, as long as they do so deterministically. In the case of methods, whether an exception is thrown is allowed to depend on the arguments, as long as calling the method with the same arguments will always either throw or not throw.

The Nothing value, when used with [DependsOn] values other than Everything, can used by the JIT to perform loop-hoisting and common subexpression elimination on the return values of IDL attributes and methods, as well as code motion past DOM methods that might depend on system state but have no side effects.

[Pure]

This is an alias for [Affects=Nothing, DependsOn=DOMState]. Attributes/methods flagged in this way promise that they will keep returning the same value as long as nothing that has [Affects=Everything] executes.

[Constant]

This is an alias for [Affects=Nothing, DependsOn=Nothing]. Used to flag readonly attributes or methods that could have been annotated with [Pure] and also always return the same value. This should only be used when it’s absolutely guaranteed that the return value of the attribute getter will always be the same from the JS engine’s point of view.

The spec’s [SameObject] extended attribute is an alias for [Constant], but can only be applied to things returning objects, whereas [Constant] can be used for any type of return value.

[NeedResolve]

Used to flag interfaces which have a custom resolve hook. This annotation will cause the DoResolve method to be called on the underlying C++ class when a property lookup happens on the object. The signature of this method is: bool DoResolve(JSContext*, JS::Handle<JSObject*>, JS::Handle<jsid>, JS::MutableHandle<JS::Value>). Here the passed-in object is the object the property lookup is happening on (which may be an Xray for the actual DOM object) and the jsid is the property name. The value that the property should have is returned in the MutableHandle<Value>, with UndefinedValue() indicating that the property does not exist.

If this extended attribute is used, then the underlying C++ class must also implement a method called GetOwnPropertyNames with the signature void GetOwnPropertyNames(JSContext* aCx, nsTArray<nsString>& aNames, ErrorResult& aRv). This method will be called by the JS engine’s enumerate hook and must provide a superset of all the property names that DoResolve might resolve. Providing names that DoResolve won’t actually resolve is OK.

[HeaderFile="path/to/headerfile.h"]

Indicates where the implementation can be found. Similar to the headerFile annotation in Bindings.conf. Just like headerFile in Bindings.conf, should be avoided.

[JSImplementation="@mozilla.org/some-contractid;1"]

Used on an interface to provide the contractid of the JavaScript component implementing the interface.

[StoreInSlot]

Used to flag attributes that can be gotten very quickly from the JS object by the JIT. Such attributes will have their getter called immediately when the JS wrapper for the DOM object is created, and the returned value will be stored directly on the JS object. Later gets of the attribute will not call the C++ getter and instead use the cached value. If the value returned by the attribute needs to change, the C++ code should call the ClearCachedFooValue method in the namespace of the relevant binding, where foo is the name of the attribute. This will immediately call the C++ getter and cache the value it returns, so it needs a JSContext to work on. This extended attribute can only be used on attributes whose getters are [Pure] or [Constant] and which are not [Throws] or [GetterThrows].

So for example, given this IDL:

interface MyInterface {
  [Pure, StoreInSlot] attribute long myAttribute;
};

the C++ implementation of MyInterface would clear the cached value by calling mozilla::dom::MyInterface_Binding::ClearCachedMyAttributeValue(cx, this). This function will return false on error and the caller is responsible for handling any JSAPI exception that is set by the failure.

If the attribute is not readonly, setting it will automatically clear the cached value and reget it again before the setter returns.

[Cached]

Used to flag attributes that, when their getter is called, will cache the returned value on the JS object. This can be used to implement attributes whose value is a sequence or dictionary (which would otherwise end up returning a new object each time and hence not be allowed in Web IDL).

Unlike [StoreInSlot] this does not cause the getter to be eagerly called at JS wrapper creation time; the caching is lazy. [Cached] attributes must be [Pure] or [Constant], because otherwise not calling the C++ getter would be observable, but are allowed to have throwing getters. Their cached value can be cleared by calling the ClearCachedFooValue method in the namespace of the relevant binding, where foo is the name of the attribute. Unlike [StoreInSlot] attributes, doing so will not immediately invoke the getter, so it does not need a JSContext.

So for example, given this IDL:

interface MyInterface {
  [Pure, Cached] attribute long myAttribute;
};

the C++ implementation of MyInterface would clear the cached value by calling mozilla::dom::MyInterface_Binding::ClearCachedMyAttributeValue(this). JS-implemented Web IDL can clear the cached value by calling this.__DOM_IMPL__._clearCachedMyAttributeValue().

If the attribute is not readonly, setting it will automatically clear the cached value.

[Frozen]

Used to flag attributes that, when their getter is called, will call Object.freeze on the return value before returning it. This extended attribute is only allowed on attributes that return sequences, dictionaries and MozMap, and corresponds to returning a frozen Array (for the sequence case) or Object (for the other two cases).

[BinaryName]

[BinaryName] can be specified on method or attribute to change the C++ function name that will be used for the method or attribute. It takes a single string argument, which is the name you wish the method or attribute had instead of the one it actually has.

For example, given this IDL:

interface InterfaceWithRenamedThings {
  [BinaryName="renamedMethod"]
  undefined someMethod();
  [BinaryName="renamedAttribute"]
  attribute long someAttribute;
};

the corresponding C++ would be:

class InterfaceWithRenamedThings
{
public:
  void RenamedMethod();
  int32_t RenamedAttribute();
  void SetRenamedAttribute(int32_t);
};

[Deprecated="tag"]

When deprecating an interface or method, the [Deprecated] annotation causes the Web IDL compiler to insert code that generates deprecation warnings. This annotation can be added to interface methods or interfaces. Adding this to an interface causes a warning to be issued the first time the object is constructed, or any static method on the object is invoked.

The complete list of valid deprecation tags is maintained in nsDeprecatedOperationList.h. Each new tag requires that a localized string be defined, containing the deprecation message to display.

[CrossOriginReadable]

Used to flag an attribute that, when read, will not have the same-origin constraint tested: it can be read from a context with a different origin.

[CrossOriginWrite]

Used to flag an attribute that, when written, will not have the same-origin constraint tested: it can be written from a context with a different origin.

[CrossOriginCallable]

Used to flag a method that, when called, will not have the same-origin constraint tested: it can be called from a context with a different origin.

[SecureContext]

We implement the standard extended attribute with a few details specific to Gecko:

• System principals are considered secure.

• An extension poking at non-secured DOM objects will see APIs marked with [SecureContext].

• XPConnect sandboxes don’t see [SecureContext] APIs, except if they’re created with isSecureContext: true.

[NeedsSubjectPrincipal], [GetterNeedsSubjectPrincipal], [SetterNeedsSubjectPrincipal]

Used to flag a method or an attribute that needs to know the subject principal. This principal will be passed as argument. The argument will be a nsIPrincipal& because a subject principal is always available.

[NeedsSubjectPrincipal] applies to both methods and attributes; for attributes it means both the getter and the setter need a subject principal. [GetterNeedsSubjectPrincipal] applies only to attributes. [SetterNeedsSubjectPrincipal] applies only to non-readonly attributes.

These attributes may also be constrained to non-system principals using [{Getter,Setter,}NeedsSubjectPrincipal=NonSystem]. This changes the argument type to nsIPrincipal*, and passes nullptr when called with a system principal.

[NeedsCallerType]

Used to flag a method or an attribute that needs to know the caller type, in the mozilla::dom::CallerType sense. This can be safely used for APIs exposed in workers; there it will indicate whether the worker involved is a ChromeWorker or not. At the moment the only possible caller types are System (representing system-principal callers) and NonSystem.

[GenerateInit]

When set on a dictionary it will add two Init methods to the generated C++ class with the following signatures:

bool Init(BindingCallContext& cx, JS::Handle<JS::Value> val, const char* sourceDescription="Value", bool passedToJSImpl=false);
bool Init(JSContext* cx_, JS::Handle<JS::Value> val, const char* sourceDescription="Value", bool passedToJSImpl=false);

These methods will initialize the dictionary from val by following WebIDL’s JavaScript type mapping.

[GenerateInitFromJSON]

When set on a dictionary it will add an Init method to the generated C++ class with the following signature:

bool Init(const nsAString& aJSON);

This extended attribute will only have an effect if all of the types of the dictionary’s members are representable in JSON (they are a string type, a primitive type that’s not an unrestricted float/double, a void type, or a sequence, union, dictionary or record containing these types).

The method is expected to be called with a JSON string as input. The JSON string will be parsed into a JavaScript value, and then the dictionary is initialized with this value by following WebIDL’s JavaScript type mapping.

Note: As a side-effect of how this is implemented it will also add the two Init methods that would be added by a [GenerateInit] extended attribute.

[GenerateToJSON]

When set on a dictionary it will add a ToJSON method to the generated C++ class with the following signature:

bool ToJSON(nsAString& aJSON);

The method will generate a JSON representation of the dictionary members’ values in aJSON by converting the dictionary to a JavaScript object by following WebIDL’s JavaScript type mapping and then converting that object to a JSON string.

The same restrictions on types applies as on [GenerateInitFromJSON].

Note: As a side-effect of how this is implemented it will also add the ToObjectInternal method that would be added by a [GenerateConversionToJS] extended attribute.

[GenerateConversionToJS]

When set on a dictionary it will add a ToObjectInternal method to the generated C++ class with the following signature:

bool ToObjectInternal(JSContext* cx, JS::MutableHandle<JS::Value> rval);

The method will create a JavaScript object by following WebIDL’s JavaScript type mapping.

[GenerateEqualityOperator]

When set on a dictionary it will add an equality operator to the generated C++ class.

This is only allowed on dictionaries who only have members (own or inherited) with string, primitive or enum types.

[Unsorted]

When set on a dictionary the dictionary’s members will not be sorted in lexicographic order (which is specified by WebIDL).

This should only ever be used on internal APIs that are not exposed to the Web!

[ReflectedHTMLAttributeReturningFrozenArray]

Used to flag a HTML reflected IDL attribute as having a FrozenArray<T>? type, where T is either Element or an interface that inherits from Element. This should only be used to implement the algorithms for that kind of reflected IDL attributes.

When this attribute’s getter is called, it will cache the JS reflection of the returned value on the JS object. The C++ getter will be passed an additional bool* argument before the result argument. If this argument is not null then the implementation is supposed to set the bool that this argument is pointing to to whether the attr-associated elements and the cached attr-associated elements are equal. If it is not null, and the getter sets the pointee to true then the cached JS value will be returned, and the result value from the C++ getter will be ignored (so there is no need to set the result’s value). If it is null, or the getter sets the pointee to false, then the cached value will be set to the JS reflection of the result value from the C++ getter, and that JS value will then be returned.

Note that this will not cause the JIT to directly get the cached value from the slot (as [StoreInSlot] or [Cached] would). The setter will also not clear the cached value from the slot.

For example, this IDL:

interface Element {
  [Frozen, ReflectedHTMLAttributeReturningFrozenArray]
  attribute sequence<Element>? reflectedHTMLAttribute;
};

will require the following declarations in Element:

class Element {
  // …
  void GetReflectedHTMLAttribute(
    bool* aUseCachedValue, Nullable<nsTArray<RefPtr<Element>>>& aResult);
  void SetReflectedHTMLAttribute(
    const Nullable<Sequence<OwningNonNull<Element>>>& aValue);
};

Helper objects

The C++ side of the bindings uses a number of helper objects.

Nullable<T>

Nullable<> is a struct declared in Nullable.h and exported to mozilla/dom/Nullable.h that is used to represent nullable values of types that don’t have a natural way to represent null.

Nullable<T> has an IsNull() getter that returns whether null is represented and a Value() getter that returns a const T& and can be used to get the value when it’s not null.

Nullable<T> has a SetNull() setter that sets it as representing null and two setters that can be used to set it to a value: void SetValue(T) (for setting it to a given value) and T& SetValue() for directly modifying the underlying T&.

Optional<T>

Optional<> is a struct declared in BindingDeclarations.h and exported to mozilla/dom/BindingDeclarations.h that is used to represent optional arguments and dictionary members, but only those that have no default value.

Optional<T> has a WasPassed() getter that returns true if a value is available. In that case, the Value() getter can be used to get a const T& for the value.

NonNull<T>

NonNull<T> is a struct declared in BindingUtils.h and exported to mozilla/dom/BindingUtils.h that is used to represent non-null C++ objects. It has a conversion operator that produces T&.

OwningNonNull<T>

OwningNonNull<T> is a struct declared in OwningNonNull.h and exported to mozilla/OwningNonNull.h that is used to represent non-null C++ objects and holds a strong reference to them. It has a conversion operator that produces T&.

Typed arrays, arraybuffers, array buffer views

TypedArray.h is exported to mozilla/dom/TypedArray.h and exposes structs that correspond to the various typed array types, as well as ArrayBuffer and ArrayBufferView, all in the mozilla::dom namespace. Each struct has a Data() method that returns a pointer to the relevant type (uint8_t for ArrayBuffer and ArrayBufferView) and a Length() method that returns the length in units of *Data(). So for example, Int32Array has a Data() returning int32_t* and a Length() that returns the number of 32-bit ints in the array.

Sequence<T>

Sequence<> is a type declared in BindingDeclarations.h and exported to mozilla/dom/BindingDeclarations.h that is used to represent sequence arguments. It’s some kind of typed array, but which exact kind is opaque to consumers. This allows the binding code to change the exact definition (e.g., to use auto arrays of different sizes and so forth) without having to update all the callees.

CallbackFunction

CallbackFunction is a type declared in CallbackFunction.h and exported to mozilla/dom/CallbackFunction.h that is used as a common base class for all the generated callback function representations. This class inherits from nsISupports, and consumers must make sure to cycle-collect it, since it keeps JS objects alive.

CallbackInterface

CallbackInterface is a type declared in CallbackInterface.h and exported to mozilla/dom/CallbackInterface.h that is used as a common base class for all the generated callback interface representations. This class inherits from nsISupports, and consumers must make sure to cycle-collect it, since it keeps JS objects alive.

DOMString

DOMString is a class declared in BindingDeclarations.h and exported to mozilla/dom/BindingDeclarations.h that is used for Web IDL DOMString return values. It has a conversion operator to nsString& so that it can be passed to methods that take that type or nsAString&, but callees that care about performance, have an StringBuffer available, and promise to hold on to the StringBuffer at least until the binding code comes off the stack can also take a DOMString directly for their string return value and call its SetStringBuffer method with the StringBuffer and its length. This allows the binding code to avoid extra reference-counting of the string buffer in many cases, and allows it to take a faster codepath even if it does end up having to addref the StringBuffer.

GlobalObject

GlobalObject is a class declared in BindingDeclarations.h and exported to mozilla/dom/BindingDeclarations.h that is used to represent the global object for static attributes and operations (including constructors). It has a Get() method that returns the JSObject* for the global and a GetAsSupports() method that returns an nsISupports* for the global on the main thread, if such is available. It also has a Context() method that returns the JSContext* the call is happening on. A caveat: the compartment of the JSContext may not match the compartment of the global!

Date

Date is a class declared in BindingDeclarations.h and exported to mozilla/dom/BindingDeclarations.h that is used to represent Web IDL Dates. It has a TimeStamp() method returning a double which represents a number of milliseconds since the epoch, as well as SetTimeStamp() methods that can be used to initialize it with a double timestamp or a JS Date object. It also has a ToDateObject() method that can be used to create a new JS Date.

ErrorResult

ErrorResult is a class declared in ErrorResult.h and exported to mozilla/ErrorResult.h that is used to represent exceptions in Web IDL bindings. This has the following methods:

• Throw: allows throwing an nsresult. The nsresult must be a failure code.

• ThrowTypeError: allows throwing a TypeError with the given error message. The list of allowed TypeErrors and corresponding messages is in dom/bindings/Errors.msg.

• ThrowJSException: allows throwing a preexisting JS exception value. However, the MightThrowJSException() method must be called before any such exceptions are thrown (even if no exception is thrown).

• Failed: checks whether an exception has been thrown on this ErrorResult.

• ErrorCode: returns a failure nsresult representing (perhaps incompletely) the state of this ErrorResult.

• operator=: takes an nsresult and acts like Throw if the result is an error code, and like a no-op otherwise (unless an exception has already been thrown, in which case it asserts). This should only be used for legacy code that has nsresult everywhere; we would like to get rid of this operator at some point.

Events

Simple Event interfaces can be automatically generated by adding the interface file to GENERATED_EVENTS_WEBIDL_FILES in the appropriate dom/webidl/moz.build file. You can also take a simple generated C++ file pair and use it to build a more complex event (i.e., one that has methods).

Event handler attributes

A lot of interfaces define event handler attributes, like:

attribute EventHandler onthingchange;

If you need to implement an event handler attribute for an interface, in the definition (header file), you use the handy “IMPL_EVENT_HANDLER” macro:

IMPL_EVENT_HANDLER(onthingchange);

The “onthingchange” needs to be added to the StaticAtoms.py file:

Atom("onthingchange", "onthingchange")

The actual implementation (.cpp) for firing the event would then look something like:

nsresult
MyInterface::DispatchThingChangeEvent()
{
  NS_NAMED_LITERAL_STRING(type, "thingchange");
  EventInit init;
  init.mBubbles = false;
  init.mCancelable = false;
  RefPtr<Event> event = Event::Constructor(this, type, init);
  event->SetTrusted(true);
  ErrorResult rv;
  DispatchEvent(*event, rv);
  return rv.StealNSResult();  // Assuming the caller cares about the return code.
}

Bindings.conf details

Write me. In particular, need to describe at least use of concrete, prefable, and addExternalInterface.

How to get a JSContext passed to a given method

In some rare cases you may need a JSContext* argument to be passed to a C++ method that wouldn’t otherwise get such an argument. To see how to achieve this, search for implicitJSContext in dom/bindings/Bindings.conf.

Implementing Web IDL using Javascript

Warning

Implementing Web IDL using Javascript is deprecated. New interfaces should always be implemented in C++!

It is possible to implement Web IDL interfaces in JavaScript within Gecko – however, this is limited to interfaces that are not exposed in Web Workers. When the binding occurs, two objects are created:

• Content-exposed object: what gets exposed to the web page.

• Implementation object: running as a chrome-privileged script. This allows the implementation object to have various APIs that the content-exposed object does not.

Because there are two types of objects, you have to be careful about which object you are creating.

Creating JS-implemented Web IDL objects

To create a JS-implemented Web IDL object, one must create both the chrome-side implementation object and the content-side page-exposed object. There are three ways to do this.

Using the Web IDL constructor

If the interface has a constructor, a content-side object can be created by getting that constructor from the relevant content window and invoking it. For example:

var contentObject = new contentWin.RTCPeerConnection();

The returned object will be an Xray wrapper for the content-side object. Creating the object this way will automatically create the chrome-side object using its contractID.

This method is limited to the constructor signatures exposed to webpages. Any additional configuration of the object needs to be done methods on the interface.

Creating many objects this way can be slow due to the createInstance overhead involved.

Using a _create method

A content-side object can be created for a given chrome-side object by invoking the static _create method on the interface. This method takes two arguments: the content window in which to create the object and the chrome-side object to use. For example:

var contentObject = RTCPeerConnection._create(contentWin, new
MyPeerConnectionImpl());

However, if you are in a JS component, you may only be able to get to the correct interface object via some window object. In this case, the code would look more like:

var contentObject = contentWin.RTCPeerConnection._create(contentWin,
new MyPeerConnectionImpl());

Creating the object this way will not invoke its __init method or init method.

By returning a chrome-side object from a JS-implemented Web IDL method

If a JS-implemented Web IDL method is declared as returning a JS-implemented interface, then a non-Web IDL object returned from that method will be treated as the chrome-side part of a JS-implemented WebIdL object and the content-side part will be automatically created.

Creating the object this way will not invoke its __init method or init method.

Implementing a Web IDL object in JavaScript

To implement a Web IDL interface in JavaScript, first add a Web IDL file, in the same way as you would for a C++-implemented interface. To support implementation in JS, you must add an extended attribute JSImplementation="CONTRACT_ID_STRING" on your interface, where CONTRACT_ID_STRING is the XPCOM component contract ID of the JS implementation – note “;1” is just a Mozilla convention for versioning APIs. Here’s an example:

[JSImplementation="@mozilla.org/my-number;1"]
interface MyNumber {
  constructor(optional long firstNumber);
  attribute long value;
  readonly attribute long otherValue;
  undefined doNothing();
};

Next, create an XPCOM component that implements this interface. Use the same contract ID as you specified in the Web IDL file. The class ID doesn’t matter, except that it should be a newly generated one. For QueryInterface, you only need to implement nsISupports, not anything corresponding to the Web IDL interface. The name you use for the XPCOM component should be distinct from the name of the interface, to avoid confusing error messages.

Web IDL attributes are implemented as properties on the JS object or its prototype chain, whereas Web IDL methods are implemented as methods on the object or prototype. Note that any other instances of the interface that you are passed in as arguments are the full web-facing version of the object, and not the JS implementation, so you currently cannot access any private data.

The Web IDL constructor invocation will first create your object. If the XPCOM component implements nsIDOMGlobalPropertyInitializer, then the object’s init method will be invoked with a single argument: the content window the constructor came from. This allows the JS implementation to know which content window it’s associated with. The init method should not return anything. After this, the content-side object will be created. Then,if there are any constructor arguments, the object’s __init method will be invoked, with the constructor arguments as its arguments.

Static Members

Static attributes and methods are not supported on JS-implemented Web IDL (see bug 863952). However, with the changes in bug 1172785 you can route static methods to a C++ implementation on another object using a StaticClassOverride annotation. This annotation includes the full, namespace-qualified name of the class that contains an implementation of the named method. The include for that class must be found in a directory based on its name.

[JSImplementation="@mozilla.org/dom/foo;1"]
interface Foo {
  [StaticClassOverride="mozilla::dom::OtherClass"]
  static Promise<undefined> doSomething();
};

Rather than calling into a method on the JS implementation; calling Foo.doSomething() will result in calling mozilla::dom::OtherClass::DoSomething().

Checking for Permissions or Preferences

With JS-implemented Web IDL, the init method should only return undefined. If any other value, such as null, is returned, the bindings code will assert or crash. In other words, it acts like it has an “undefined” return type. Preference or permission checking should be implemented by adding an extended attribute to the Web IDL interface. This has the advantage that if the check fails, the constructor or object will not show up at all.

For preference checking, add an extended attribute Pref="myPref.enabled" where myPref.enabled is the preference that should be checked. SettingsLock is an example of this.

For permissions or other kinds of checking, add an extended attribute Func="MyPermissionChecker" where MyPermissionChecker is a function implemented in C++ that returns true if the interface should be enabled. This function can do whatever checking is needed. One example of this is PushManager.

Example

Here’s an example JS implementation of the above interface. The invisibleValue field will not be accessible to web content, but is usable by the doNothing() method.

function MyNumberInner() {
  this.value = 111;
  this.invisibleValue = 12345;
}

MyNumberInner.prototype = {
  classDescription: "Get my number XPCOM Component",
  contractID: "@mozilla.org/my-number;1",
  QueryInterface: ChromeUtils.generateQI([]),
  doNothing: function() {},
  get otherValue() { return this.invisibleValue - 4; },
  __init: function(firstNumber) {
    if (arguments.length > 0) {
      this.value = firstNumber;
    }
  }
}

Finally, add a component and a contract and whatever other manifest stuff you need to implement an XPCOM component.

Guarantees provided by bindings

When implementing a Web IDL interface in JavaScript, certain guarantees will be provided by the binding implementation. For example, string or numeric arguments will actually be primitive strings or numbers. Dictionaries will contain only the properties that they are declared to have, and they will have the right types. Interface arguments will actually be objects implementing that interface.

What the bindings will NOT guarantee is much of anything about object and any arguments. They will get cross-compartment wrappers that make touching them from chrome code not be an immediate security bug, but otherwise they can have quite surprising behavior if the page is trying to be malicious. Try to avoid using these types if possible.

Accessing the content object from the implementation

If the JS implementation of the Web IDL interface needs to access the content object, it is available as a property called __DOM_IMPL__ on the chrome implementation object. This property only appears after the content-side object has been created. So it is available in __init but not in init.

Determining the principal of the caller that invoked the Web IDL API

This can be done by calling Component.utils.getWebIDLCallerPrincipal().

Throwing exceptions from JS-implemented APIs

There are two reasons a JS implemented API might throw. The first reason is that some unforeseen condition occurred and the second is that a specification requires an exception to be thrown.

When throwing for an unforeseen condition, the exception will be reported to the console, and a sanitized NS_ERROR_UNEXPECTED exception will be thrown to the calling content script, with the file/line of the content code that invoked your API. This will avoid exposing chrome URIs and other implementation details to the content code.

When throwing because a specification requires an exception, you need to create the exception from the window your Web IDL object is associated with (the one that was passed to your init method). The binding code will then rethrow that exception to the web page. An example of how this could work:

if (!isValid(passedInObject)) {
  throw new this.contentWindow.TypeError("Object is invalid");
}

or

if (!isValid(passedInObject)) {
  throw new this.contentWindow.DOMException("Object is invalid", "InvalidStateError");
}

depending on which exact exception the specification calls for throwing in this situation.

In some cases you may need to perform operations whose exception message you just want to propagate to the content caller. This can be done like so:

try {
  someOperationThatCanThrow();
} catch (e) {
  throw new this.contentWindow.Error(e.message);
}

Inheriting from interfaces implemented in C++

It’s possible to have an interface implemented in JavaScript inherit from an interface implemented in C++. To do so, simply have one interface inherit from the other and the bindings code will auto-generate a C++ object inheriting from the implementation of the parent interface. The class implementing the parent interface will need a constructor that takes an nsPIDOMWindow* (though it doesn’t have to do anything with that argument).

If the class implementing the parent interface is abstract and you want to use a specific concrete class as the implementation to inherit from, you will need to add a defaultImpl annotation to the descriptor for the parent interface in Bindings.conf. The value of the annotation is the C++ class to use as the parent for JS-implemented descendants; if defaultImpl is not specified, the nativeType will be used.

For example, consider this interface that we wish to implement in JavaScript:

[JSImplementation="some-contract"]
interface MyEventTarget : EventTarget {
  attribute EventHandler onmyevent;
  undefined dispatchTheEvent(); // Sends a "myevent" event to this EventTarget
}

The implementation would look something like this, ignoring most of the XPCOM boilerplate:

function MyEventTargetImpl() {
}
MyEventTargetImpl.prototype = {
  // QI to nsIDOMGlobalPropertyInitializer so we get init() called on us.
  QueryInterface: ChromeUtils.generateQI(["nsIDOMGlobalPropertyInitializer"]),

  init: function(contentWindow) {
    this.contentWindow = contentWindow;
  },

  get onmyevent() {
    return this.__DOM_IMPL__.getEventHandler("onmyevent");
  },

  set onmyevent(handler) {
    this.__DOM_IMPL__.setEventHandler("onmyevent", handler);
  },

  dispatchTheEvent: function() {
    var event = new this.contentWindow.Event("myevent");
    this.__DOM_IMPL__.dispatchEvent(event);
  },
};

The implementation would automatically support the API exposed on EventTarget (so, for example, addEventListener). Calling the dispatchTheEvent method would cause dispatch of an event that content script can see via listeners it has added.

Note that in this case the chrome implementation is relying on some [ChromeOnly] methods on EventTarget that were added specifically to make it possible to easily implement event handlers. Other cases can do similar things as needed.