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<pre class="metadata">
Title: Web IDL
Shortname: WebIDL
Level: 2
Status: ED
Group: webplatform
Mailing list: public-script-coord@w3.org
Mailing List Archives: https://lists.w3.org/Archives/Public/public-script-coord/
Repository: heycam/webidl
!Feedback: <a href="https://github.com/heycam/webidl">GitHub</a> (<a href="https://github.com/heycam/webidl/issues/new">new issue</a>, <a href="https://github.com/heycam/webidl/issues">open issues</a>, <a href="https://www.w3.org/Bugs/Public/buglist.cgi?product=WebAppsWG&component=WebIDL&resolution=---">legacy bug tracker</a>)
ED: https://heycam.github.io/webidl/
Editor: Cameron McCormack, Mozilla Corporation, http://mcc.id.au/, cam@mcc.id.au
Editor: Boris Zbarsky, Mozilla Corporation, bzbarsky@mit.edu
Editor: Tobie Langel, http://tobie.me, tobie@codespeaks.com
Abstract: This document defines an interface definition language, Web IDL,
Abstract: that can be used to describe interfaces that are intended to be
Abstract: implemented in web browsers. Web IDL is an IDL variant with a
Abstract: number of features that allow the behavior of common script objects in
Abstract: the web platform to be specified more readily. How interfaces
Abstract: described with Web IDL correspond to constructs within ECMAScript
Abstract: execution environments is also detailed in this document.
Abstract: It is expected that this document acts
Abstract: as a guide to implementors of already-published specifications,
Abstract: and that newly published specifications reference this
Abstract: document to ensure conforming implementations of interfaces
Abstract: are interoperable.
Boilerplate: omit issues-index, omit conformance, omit feedback-header
</pre>
<pre class="link-defaults">
spec: infra; type: dfn; text: list
</pre>
<pre class="anchors">
urlPrefix: https://html.spec.whatwg.org/; spec: HTML
type: interface
text: Audio; url: dom-audio
text: Option; url: dom-option
text: Image; url: dom-image
urlPrefix: http://www.unicode.org/glossary/; spec: UNICODE
type: dfn
text: Unicode scalar value; url: unicode_scalar_value
urlPrefix: https://tc39.github.io/ecma262/; spec: ECMA-262
type: interface; for: ECMAScript
text: ArrayBuffer; url: sec-arraybuffer-objects
text: DataView; url: sec-dataview-objects
text: SharedArrayBuffer; url: sec-sharedarraybuffer-objects
type: dfn
text: NumericLiteral; url: sec-literals-numeric-literals
text: ECMAScript error objects; url: sec-error-objects
text: ToBoolean; url: sec-toboolean
text: ToNumber; url: sec-tonumber
text: ToUint16; url: sec-touint16
text: ToInt32; url: sec-toint32
text: ToUint32; url: sec-touint32
text: ToString; url: sec-tostring
text: ToObject; url: sec-toobject
text: isFinite; url: sec-isfinite-number
text: IsAccessorDescriptor; url: sec-isaccessordescriptor
text: IsDataDescriptor; url: sec-isdatadescriptor
url: sec-ecmascript-data-types-and-values
text: Type
text: Type(x)
text: sign; url: eqn-sign
text: floor; url: eqn-floor
text: min; url: eqn-min
text: max; url: eqn-max
text: abs; url: eqn-abs
text: modulo; url: eqn-modulo
url: sec-algorithm-conventions
text: !
text: ?
text: ECMA-262 section 5.2
text: conventions of the ECMAScript specification
text: element; url: sec-ecmascript-language-types-string-type
url: sec-iscallable
text: IsCallable
text: callable
url: sec-well-known-intrinsic-objects
text: %ArrayPrototype%
text: %ArrayProto_values%
text: %MapPrototype%
text: %SetPrototype%
text: %Error%
text: %ErrorPrototype%
text: %ObjProto_toString%
text: %IteratorPrototype%
text: %ObjectPrototype%; url: sec-properties-of-the-object-prototype-object
text: %FunctionPrototype%; url: sec-properties-of-the-function-prototype-object
text: %Promise%; url: sec-promise-constructor
text: Property Descriptor; url: sec-property-descriptor-specification-type
text: array index; url: sec-array-exotic-objects
text: OrdinaryGetOwnProperty; url: sec-ordinarygetownproperty
text: OrdinaryDefineOwnProperty; url: sec-ordinarydefineownproperty
text: OrdinaryPreventExtensions; url: sec-ordinarypreventextensions
text: OrdinarySet; url: sec-ordinaryset
text: equally close values; url: sec-ecmascript-language-types-number-type
text: internal slot; url: sec-object-internal-methods-and-internal-slots
text: JavaScript execution context stack; url: execution-context-stack
text: running JavaScript execution context; url: running-execution-context
text: ReturnIfAbrupt; url: sec-returnifabrupt
text: GetIterator; url: sec-getiterator
text: IteratorStep; url: sec-iteratorstep
text: NormalCompletion; url: sec-normalcompletion
text: IteratorValue; url: sec-iteratorvalue
url: sec-well-known-symbols
text: @@iterator
text: @@toStringTag
text: CreateArrayIterator; url: sec-createarrayiterator
text: CreateIterResultObject; url: sec-createiterresultobject
text: CreateMapIterator; url: sec-createmapiterator
text: CreateSetIterator; url: sec-createsetiterator
text: ArrayCreate; url: sec-arraycreate
text: CreateDataProperty; url: sec-createdataproperty
text: DetachArrayBuffer; url: sec-detacharraybuffer
text: IsDetachedBuffer; url: sec-isdetachedbuffer
text: IsSharedArrayBuffer; url: sec-issharedarraybuffer
text: SetIntegrityLevel; url: sec-setintegritylevel
url: sec-array-iterator-objects
text: array iterator object
text: array iterator objects
text: Call; url: sec-call
text: Get; url: sec-get-o-p
text: Set; url: sec-set-o-p-v-throw
text: IsConstructor; url: sec-isconstructor
text: Construct; url: sec-construct
text: own property; url: sec-own-property
text: enumerable; url: sec-property-attributes
text: DefinePropertyOrThrow; url: sec-definepropertyorthrow
url: sec-code-realms
text: Realm
text: ECMAScript global environment
text: current Realm; url: current-realm
text: Completion; url: sec-completion-record-specification-type
text: ObjectCreate; url: sec-objectcreate
text: CreateBuiltinFunction; url: sec-createbuiltinfunction
text: SetFunctionName; url: sec-setfunctionname
text: immutable prototype exotic object; url: sec-immutable-prototype-exotic-objects
text: sections 9.1; url: sec-ordinary-object-internal-methods-and-internal-slots
text: 9.3.1; url: sec-built-in-function-objects-call-thisargument-argumentslist
text: ECMA-262 section 9.3; url: sec-built-in-function-objects
text: function object; url: sec-built-in-function-objects
text: Array methods; url: sec-properties-of-the-array-prototype-object
text: typed arrays; url: sec-typedarray-objects
text: GetMethod; url: sec-getmethod
text: @@unscopables; url: sec-well-known-symbols
text: NewTarget; url: sec-built-in-function-objects
text: Number Type; url: sec-ecmascript-language-types-number-type
text: JSON.stringify; url: sec-json.stringify
</pre>
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</style>
<h2 id="introduction">Introduction</h2>
<i>This section is informative.</i>
Technical reports published by the W3C that include programming
language interfaces have typically been described using the
Object Management Group’s Interface Definition Language (IDL)
[[OMGIDL]]. The IDL provides a means to
describe these interfaces in a language independent manner. Usually,
additional language binding appendices are included in such
documents which detail how the interfaces described with the IDL
correspond to constructs in the given language.
However, the bindings in these specifications for the language most
commonly used on the web, ECMAScript, are consistently specified with
low enough precision as to result in interoperability issues. In
addition, each specification must describe the same basic information,
such as DOM interfaces described in IDL corresponding to properties
on the ECMAScript global object, or the {{unsigned long}} IDL type mapping to the <emu-val>Number</emu-val>
type in ECMAScript.
This specification defines an IDL language similar to OMG IDL
for use by specifications that define interfaces for Web APIs. A number of extensions are
given to the IDL to support common functionality that previously must
have been written in prose. In addition, precise language bindings
for ECMAScript Edition 6 are given.
<h2 id="idl">Interface definition language</h2>
This section describes a language, <em>Web IDL</em>, which can be used to define
interfaces for APIs in the Web platform. A specification that defines Web APIs
can include one or more <dfn id="dfn-idl-fragment" export lt="IDL fragment">IDL fragments</dfn> that
describe the interfaces (the state and behavior that objects can exhibit)
for the APIs defined by that specification.
An [=IDL fragment=] is
a sequence of definitions that matches the <emu-nt><a href="#prod-Definitions">Definitions</a></emu-nt> grammar symbol.
The set of [=IDL fragments=] that
an implementation supports is not ordered.
See [[#idl-grammar]] for the complete grammar and an explanation of the notation used.
The different kinds of <dfn id="dfn-definition">definitions</dfn> that can appear in an
[=IDL fragment=] are:
[=interfaces=],
[=partial interface|partial interface definitions=],
[=namespaces=],
[=partial namespace|partial namespace definitions=],
[=dictionary|dictionaries=],
[=partial dictionary|partial dictionary definitions=],
[=typedefs=] and
[=implements statements=].
These are all defined in the following sections.
Each [=definition=]
(matching <emu-nt><a href="#prod-Definition">Definition</a></emu-nt>)
can be preceded by a list of [=extended attributes=] (matching
<emu-nt><a href="#prod-ExtendedAttributeList">ExtendedAttributeList</a></emu-nt>),
which can control how the definition will be handled in language bindings.
The extended attributes defined by this specification that are language binding
agnostic are discussed in [[#idl-extended-attributes]],
while those specific to the ECMAScript language binding are discussed
in [[#es-extended-attributes]].
<pre highlight="webidl" class="syntax">
[<mark>extended_attributes</mark>]
interface identifier {
/* interface_members... */
};
</pre>
<pre class="grammar" id="prod-Definitions">
Definitions :
ExtendedAttributeList Definition Definitions
ε
</pre>
<pre class="grammar" id="prod-Definition">
Definition :
CallbackOrInterface
Namespace
Partial
Dictionary
Enum
Typedef
ImplementsStatement
</pre>
<pre class="grammar" id="prod-CallbackOrInterface">
CallbackOrInterface :
"callback" CallbackRestOrInterface
Interface
</pre>
<div class="example">
The following is an example of an [=IDL fragment=].
<pre highlight="webidl">
interface Paint { };
interface SolidColor : Paint {
attribute double red;
attribute double green;
attribute double blue;
};
interface Pattern : Paint {
attribute DOMString imageURL;
};
[Constructor]
interface GraphicalWindow {
readonly attribute unsigned long width;
readonly attribute unsigned long height;
attribute Paint currentPaint;
void drawRectangle(double x, double y, double width, double height);
void drawText(double x, double y, DOMString text);
};
</pre>
Here, four [=interfaces=]
are being defined.
The <code class="idl">GraphicalWindow</code> interface has two
[=read only=] [=attributes=],
one writable attribute, and two [=operations=]
defined on it. Objects that implement the <code class="idl">GraphicalWindow</code> interface
will expose these attributes and operations in a manner appropriate to the
particular language being used.
In ECMAScript, the attributes on the IDL interfaces will be exposed as accessor
properties and the operations as <emu-val>Function</emu-val>-valued
data properties on a prototype object for all <code class="idl">GraphicalWindow</code>
objects; each ECMAScript object that implements <code class="idl">GraphicalWindow</code>
will have that prototype object in its prototype chain.
The [{{Constructor}}] that appears on <code class="idl">GraphicalWindow</code>
is an [=extended attribute=].
This extended attribute causes a constructor to exist in ECMAScript implementations,
so that calling <code>new GraphicalWindow()</code> would return a new object
that implemented the interface.
</div>
<h3 id="idl-names">Names</h3>
Every [=interface=],
[=partial interface|partial interface definition=],
[=namespace=],
[=partial namespace|partial namespace definition=],
[=dictionary=],
[=partial dictionary|partial dictionary definition=],
[=enumeration=],
[=callback function=] and
[=typedef=] (together called <dfn id="dfn-named-definition" export lt="named definition">named definitions</dfn>)
and every [=constant=],
[=attribute=],
and [=dictionary member=] has an
<dfn id="dfn-identifier" export>identifier</dfn>, as do some
[=operations=].
The identifier is determined by an
<emu-t class="regex"><a href="#prod-identifier">identifier</a></emu-t> token somewhere
in the declaration:
* For [=named definitions=],
the <emu-t class="regex"><a href="#prod-identifier">identifier</a></emu-t> token that appears
directly after the <emu-t>interface</emu-t>, <emu-t>namespace</emu-t>,
<emu-t>dictionary</emu-t>, <emu-t>enum</emu-t>
or <emu-t>callback</emu-t> keyword
determines the identifier of that definition.
<pre highlight="webidl" class="syntax">
interface <mark>interface_identifier</mark> { /* interface_members... */ };
partial interface <mark>interface_identifier</mark> { /* interface_members... */ };
namespace <mark>namespace_identifier</mark> { /* namespace_members... */ };
partial namespace <mark>namespace_identifier</mark> { /* namespace_members... */ };
dictionary <mark>dictionary_identifier</mark> { /* dictionary_members... */ };
partial dictionary <mark>dictionary_identifier</mark> { /* dictionary_members... */ };
enum <mark>enumeration_identifier</mark> { "enum", "values" /* , ... */ };
callback <mark>callback_identifier</mark> = return_type (/* arguments... */);
</pre>
* For [=attributes=],
[=typedefs=]
and [=dictionary members=],
the final <emu-t class="regex"><a href="#prod-identifier">identifier</a></emu-t> token before the
semicolon at the end of the declaration determines the identifier.
<pre highlight="webidl" class="syntax">
interface identifier {
attribute type <mark>attribute_identifier</mark>;
};
typedef type <mark>typedef_identifier</mark>;
dictionary identifier {
type <mark>dictionary_member_identifier</mark>;
};
</pre>
* For [=constants=],
the <emu-t class="regex"><a href="#prod-identifier">identifier</a></emu-t> token before the
equals sign determines the identifier.
<pre highlight="webidl" class="syntax">const type <mark>constant_identifier</mark> = 42;</pre>
* For [=operations=], the
<emu-t class="regex"><a href="#prod-identifier">identifier</a></emu-t> token that appears
after the return type but before the opening parenthesis (that is,
one that is matched as part of the <emu-nt><a href="#prod-OptionalIdentifier">OptionalIdentifier</a></emu-nt>
grammar symbol in an <emu-nt><a href="#prod-OperationRest">OperationRest</a></emu-nt>) determines the identifier of the operation. If
there is no such <emu-t class="regex"><a href="#prod-identifier">identifier</a></emu-t> token,
then the operation does not have an identifier.
<pre highlight="webidl" class="syntax">
interface interface_identifier {
return_type <mark>operation_identifier</mark>(/* arguments... */);
};
</pre>
Note: Operations can have no identifier when they are being used to declare a
[=special operation|special kind of operation=], such as a getter or setter.
For all of these constructs, the [=identifier=]
is the value of the <emu-t class="regex"><a href="#prod-identifier">identifier</a></emu-t> token with any leading
<span class="char">U+005F LOW LINE ("_")</span> character (underscore) removed.
Note: A leading <span class="char">"_"</span> is used to escape an identifier from looking
like a reserved word so that, for example, an interface named “interface” can be
defined. The leading <span class="char">"_"</span> is dropped to unescape the
identifier.
Operation arguments can take a slightly wider set of identifiers. In an operation
declaration, the identifier of an argument is specified immediately after its
type and is given by either an <emu-t class="regex"><a href="#prod-identifier">identifier</a></emu-t>
token or by one of the keywords that match the <emu-nt><a href="#prod-ArgumentNameKeyword">ArgumentNameKeyword</a></emu-nt>
symbol. If one of these keywords is used, it need not be escaped with a leading
underscore.
<pre highlight="webidl" class="syntax">
interface interface_identifier {
return_type operation_identifier(argument_type <mark>argument_identifier</mark> /* , ... */);
};
</pre>
<pre class="grammar" id="prod-ArgumentNameKeyword">
ArgumentNameKeyword :
"attribute"
"callback"
"const"
"deleter"
"dictionary"
"enum"
"getter"
"implements"
"inherit"
"interface"
"iterable"
"legacycaller"
"maplike"
"namespace"
"partial"
"required"
"setlike"
"setter"
"static"
"stringifier"
"typedef"
"unrestricted"
</pre>
If an <emu-t class="regex"><a href="#prod-identifier">identifier</a></emu-t> token is used, then the
[=identifier=] of the operation argument
is the value of that token with any leading
<span class="char">U+005F LOW LINE ("_")</span> character (underscore) removed.
If instead one of the <emu-nt><a href="#prod-ArgumentNameKeyword">ArgumentNameKeyword</a></emu-nt>
keyword token is used, then the [=identifier=] of the operation argument
is simply that token.
The [=identifier=] of any of the abovementioned
IDL constructs must not be “constructor”,
“toString”,
or begin with a <span class="char">U+005F LOW LINE ("_")</span> character. These
are known as <dfn id="dfn-reserved-identifier" export>reserved identifiers</dfn>.
Although the “toJSON” [=identifier=] is not a [=reserved identifier=],
it must only be used for [=regular operations=]
that convert objects to [=JSON types=],
as described in [[#idl-tojson-operation]].
Note: Further restrictions on identifier names for particular constructs may be made
in later sections.
Within the set of [=IDL fragments=]
that a given implementation supports,
the [=identifier=] of every
[=interface=],
[=namespace=],
[=dictionary=],
[=enumeration=],
[=callback function=] and
[=typedef=]
must not
be the same as the identifier of any other
[=interface=],
[=namespace=],
[=dictionary=],
[=enumeration=],
[=callback function=] or
[=typedef=].
Within an [=IDL fragment=], a reference
to a [=definition=] need not appear after
the declaration of the referenced definition. References can also be made
across [=IDL fragments=].
<div class="example">
Therefore, the following [=IDL fragment=] is valid:
<pre highlight="webidl">
interface B : A {
void f(SequenceOfLongs x);
};
interface A {
};
typedef sequence<long> SequenceOfLongs;
</pre>
</div>
<div class="example">
The following [=IDL fragment=]
demonstrates how [=identifiers=]
are given to definitions and [=interface members=].
<pre highlight="webidl">
// Typedef identifier: "number"
typedef double number;
// Interface identifier: "System"
interface System {
// Operation identifier: "createObject"
// Operation argument identifier: "interface"
object createObject(DOMString _interface);
// Operation argument identifier: "interface"
sequence<object> getObjects(DOMString interface);
// Operation has no identifier; it declares a getter.
getter DOMString (DOMString keyName);
};
// Interface identifier: "TextField"
interface TextField {
// Attribute identifier: "const"
attribute boolean _const;
// Attribute identifier: "value"
attribute DOMString? _value;
};
</pre>
Note that while the second [=attribute=]
on the <code class="idl">TextField</code> [=interface=]
need not have been escaped with an underscore (because “value” is
not a keyword in the IDL grammar), it is still unescaped
to obtain the attribute’s [=identifier=].
</div>
<h3 id="idl-interfaces">Interfaces</h3>
[=IDL fragments=] are used to
describe object oriented systems. In such systems, objects are entities
that have identity and which are encapsulations of state and behavior.
An <dfn id="dfn-interface" export>interface</dfn> is a definition (matching
<emu-nt><a href="#prod-Interface">Interface</a></emu-nt> or
<emu-t>callback</emu-t> <emu-nt><a href="#prod-Interface">Interface</a></emu-nt>) that declares some
state and behavior that an object implementing that interface will expose.
<pre highlight="webidl" class="syntax">
interface identifier {
/* interface_members... */
};
</pre>
An interface is a specification of a set of
<dfn id="dfn-interface-member" export lt="interface member">interface members</dfn>
(matching <emu-nt><a href="#prod-InterfaceMembers">InterfaceMembers</a></emu-nt>),
which are the [=constants=],
[=attributes=],
[=operations=] and
other declarations that appear between the braces in the interface declaration.
Attributes describe the state that an object
implementing the interface will expose, and operations describe the
behaviors that can be invoked on the object. Constants declare
named constant values that are exposed as a convenience to users
of objects in the system.
Interfaces in Web IDL describe how objects that implement the
interface behave. In bindings for object oriented languages, it is
expected that an object that implements a particular IDL interface
provides ways to inspect and modify the object's state and to
invoke the behavior described by the interface.
An interface can be defined to <dfn id="dfn-inherit" for="interface" export>inherit</dfn> from another interface.
If the identifier of the interface is followed by a
<span class="char">U+003A COLON (":")</span> character
and an [=identifier=],
then that identifier identifies the inherited interface.
An object that implements an interface that inherits from another
also implements that inherited interface. The object therefore will also
have members that correspond to the interface members from the inherited interface.
<pre highlight="webidl" class="syntax">
interface identifier : <mark>identifier_of_inherited_interface</mark> {
/* interface_members... */
};
</pre>
The order that members appear in has significance for property enumeration in the <a href="#es-interfaces">ECMAScript binding</a>.
Interfaces may specify an interface member that has the same name as
one from an inherited interface. Objects that implement the derived
interface will expose the member on the derived interface. It is
language binding specific whether the overridden member can be
accessed on the object.
<div class="example">
Consider the following two interfaces.
<pre highlight="webidl">
interface A {
void f();
void g();
};
interface B : A {
void f();
void g(DOMString x);
};
</pre>
In the ECMAScript language binding, an instance of <code class="idl">B</code>
will have a prototype chain that looks like the following:
<pre>
[Object.prototype: the Object prototype object]
↑
[A.prototype: interface prototype object for A]
↑
[B.prototype: interface prototype object for B]
↑
[instanceOfB]
</pre>
Calling <code>instanceOfB.f()</code> in ECMAScript will invoke the f defined
on <code class="idl">B</code>. However, the f from <code class="idl">A</code>
can still be invoked on an object that implements <code class="idl">B</code> by
calling <code>A.prototype.f.call(instanceOfB)</code>.
</div>
The <dfn id="dfn-inherited-interfaces" export>inherited interfaces</dfn> of
a given interface |A| is the set of all interfaces that |A|
inherits from, directly or indirectly. If |A| does not [=interface/inherit=]
from another interface, then the set is empty. Otherwise, the set
includes the interface |B| that |A| [=interface/inherits=]
from and all of |B|’s [=inherited interfaces=].
An interface must not be declared such that
its inheritance hierarchy has a cycle. That is, an interface
|A| cannot inherit from itself, nor can it inherit from another
interface |B| that inherits from |A|, and so on.
Note that general multiple inheritance of interfaces is not supported, and
objects also cannot implement arbitrary sets of interfaces.
Objects can be defined to implement a single given interface |A|,
which means that it also implements all of |A|’s
[=inherited interfaces=]. In addition,
an [=implements statement=] can be
used to define that objects implementing an interface will always
also implement another interface.
Each interface member can be preceded by a list of [=extended attributes=] (matching
<emu-nt><a href="#prod-ExtendedAttributeList">ExtendedAttributeList</a></emu-nt>),
which can control how the interface member will be handled in language bindings.
<pre highlight="webidl" class="syntax">
interface identifier {
[<mark>extended_attributes</mark>]
const type constant_identifier = 42;
[<mark>extended_attributes</mark>]
attribute type identifier;
[<mark>extended_attributes</mark>]
return_type identifier(/* arguments... */);
};
</pre>
A <dfn id="dfn-callback-interface" export>callback interface</dfn> is
an [=interface=]
that uses the <emu-t>callback</emu-t> keyword at the start of
its definition. Callback interfaces are ones that can be
implemented by [=user objects=]
and not by [=platform objects=],
as described in [[#idl-objects]].
<pre highlight="webidl" class="syntax">
callback interface identifier {
/* interface_members... */
};
</pre>
Note: See also the similarly named [=callback function=] definition.
[=Callback interfaces=]
must not [=interface/inherit=]
from any non-callback interfaces, and non-callback interfaces must not
inherit from any callback interfaces.
Callback interfaces must not have any
[=consequential interfaces=].
[=Static attributes=] and
[=static operations=] must not
be defined on a [=callback interface=].
<div class="advisement">
Specification authors should not define
[=callback interfaces=]
that have only a single [=operation=],
unless required to describe the requirements of existing APIs.
Instead, a [=callback function=] should be used.
The definition of <code class="idl">EventListener</code> as a
[=callback interface=]
is an example of an existing API that needs to allow
[=user objects=] with a
given property (in this case “handleEvent”) to be considered to implement the interface.
For new APIs, and those for which there are no compatibility concerns,
using a [=callback function=] will allow
only a <emu-val>Function</emu-val> object (in the ECMAScript
language binding).
</div>
<p class="issue">
Perhaps this warning shouldn't apply if you are planning to extend the callback
interface in the future. That's probably a good reason to start off with a single
operation callback interface.
</p>
<p class="issue">
I think we need to support operations not being implemented on a given
user object implementing a callback interface. If specs extending an existing
callback interface, we probably want to be able to avoid calling the
operations that aren't implemented (and having some default behavior instead).
So we should perhaps define a term that means whether the operation is
implemented, which in the ECMAScript binding would correspond to checking
for the property's existence.
</p>
<div class="note">
Specification authors wanting to define APIs that take ECMAScript objects
as “property bag” like function arguments are suggested to use
[=dictionaries=] rather than
[=callback interfaces=].
For example, instead of this:
<pre highlight="webidl">
callback interface Options {
attribute DOMString? option1;
attribute DOMString? option2;
attribute long? option3;
};
interface A {
void doTask(DOMString type, Options options);
};
</pre>
to be used like this:
<pre highlight="js">
var a = getA(); // Get an instance of A.
a.doTask("something", { option1: "banana", option3: 100 });
</pre>
instead write the following:
<pre highlight="webidl">
dictionary Options {
DOMString? option1;
DOMString? option2;
long? option3;
};
interface A {
void doTask(DOMString type, optional Options options);
};
</pre>
</div>
The IDL for interfaces can be split into multiple parts by using
<dfn id="dfn-partial-interface" export>partial interface</dfn> definitions
(matching <emu-t>partial</emu-t> <emu-nt><a href="#prod-PartialInterface">PartialInterface</a></emu-nt>).
The [=identifier=] of a partial
interface definition must be the same
as the identifier of an interface definition. All of
the members that appear on each of the partial interfaces are considered to be
members of the interface itself.
<pre highlight="webidl" class="syntax">
interface <mark>SomeInterface</mark> {
/* interface_members... */
};
partial interface <mark>SomeInterface</mark> {
/* interface_members... */
};
</pre>
Note: Partial interface definitions are intended for use as a specification
editorial aide, allowing the definition of an interface to be separated
over more than one section of the document, and sometimes multiple documents.
The order of appearance of an [=interface=]
definition and any of its [=partial interface=]
definitions does not matter.
Note: A partial interface definition cannot specify that the interface
[=interface/inherits=] from another interface.
Inheritance must be specified on the original [=interface=]
definition.
[=Extended attributes=] can be specified on
[=partial interface=] definitions, with some
limitations. The following extended attributes must not
be specified on partial interface definitions:
[{{Constructor}}],
[{{LegacyArrayClass}}],
[{{LegacyWindowAlias}}],
[{{NamedConstructor}}],
[{{NoInterfaceObject}}].
Note: The above list of [=extended attributes=]
is all of those defined in this document that are applicable to
[=interfaces=] except for
[{{Exposed}}],
[{{Global}}],
[{{OverrideBuiltins}}],
[{{PrimaryGlobal}}], and
[{{SecureContext}}].
Any [=extended attribute=] specified
on a [=partial interface=] definition
is considered to appear on the [=interface=]
itself.
The relevant language binding determines how interfaces correspond to constructs
in the language.
The following extended attributes are applicable to interfaces:
[{{Constructor}}],
[{{Exposed}}],
[{{Global}}],
[{{LegacyArrayClass}}],
[{{LegacyWindowAlias}}],
[{{NamedConstructor}}],
[{{NoInterfaceObject}}],
[{{OverrideBuiltins}}],
[{{PrimaryGlobal}}], and
[{{SecureContext}}].
<div data-fill-with="grammar-CallbackOrInterface"></div>
<pre class="grammar" id="prod-CallbackRestOrInterface">
CallbackRestOrInterface :
CallbackRest
Interface
</pre>
<pre class="grammar" id="prod-Interface">
Interface :
"interface" identifier Inheritance "{" InterfaceMembers "}" ";"
</pre>
<pre class="grammar" id="prod-Partial">
Partial :
"partial" PartialDefinition
</pre>
<pre class="grammar" id="prod-PartialDefinition">
PartialDefinition :