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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
Default Biblio Status: current
</pre>
<pre class="link-defaults">
spec: infra; type: dfn; text: list
spec: url; type: interface; text: URL
spec: dom; type: interface; text: Document
spec: ecma-262; type: dfn; for: /; text: internal method
spec: ecma-262; type: dfn; for: /; text: internal slot
</pre>
<pre class="anchors">
urlPrefix: https://www.khronos.org/registry/webgl/extensions/WEBGL_compressed_texture_astc/; spec: WEBGL_compressed_texture_astc
type: interface; text: WEBGL_compressed_texture_astc
urlPrefix: https://www.khronos.org/registry/webgl/extensions/WEBGL_compressed_texture_s3tc_srgb/; spec: WEBGL_compressed_texture_s3tc_srgb
type: interface; text: WEBGL_compressed_texture_s3tc_srgb
urlPrefix: https://www.khronos.org/registry/webgl/extensions/WEBGL_draw_buffers/; spec: WEBGL_draw_buffers
type: interface; text: WEBGL_draw_buffers
urlPrefix: https://www.khronos.org/registry/webgl/extensions/WEBGL_lose_context/; spec: WEBGL_lose_context
type: interface; text: WEBGL_lose_context
urlPrefix: https://www.khronos.org/registry/webgl/extensions/ANGLE_instanced_arrays/; spec: ANGLE_instanced_arrays
type: interface; text: ANGLE_instanced_arrays
urlPrefix: https://www.khronos.org/registry/webgl/extensions/EXT_blend_minmax/; spec: EXT_blend_minmax
type: interface; text: EXT_blend_minmax
urlPrefix: https://www.khronos.org/registry/webgl/extensions/EXT_color_buffer_float/; spec: EXT_color_buffer_float
type: interface; text: EXT_color_buffer_float
urlPrefix: https://www.khronos.org/registry/webgl/extensions/EXT_disjoint_timer_query/; spec: EXT_disjoint_timer_query
type: interface; text: EXT_disjoint_timer_query
urlPrefix: https://www.khronos.org/registry/webgl/extensions/OES_standard_derivatives/; spec: OES_standard_derivatives
type: interface; text: OES_standard_derivatives
urlPrefix: https://www.khronos.org/registry/webgl/extensions/OES_vertex_array_object/; spec: OES_vertex_array_object
type: interface; text: OES_vertex_array_object
urlPrefix: https://www.w3.org/TR/geolocation-API/; spec: GEOLOCATION-API
type: interface
text: Geolocation; url: geolocation;
text: Coordinates; url: coordinates;
text: Position; url: position;
text: PositionError; url: position-error;
urlPrefix: https://w3c.github.io/deviceorientation/spec-source-orientation.html; spec: ORIENTATION-EVENT
type: interface
text: DeviceRotationRate; url: device_rotation_rate;
text: DeviceAcceleration; url: device_acceleration;
urlPrefix: https://w3c.github.io/mediacapture-main/; spec: MEDIACAPTURE-STREAMS
type: interface
text: ConstrainablePattern; url: dom-constrainablepattern;
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: Array; url: sec-array-objects
text: ArrayBuffer; url: sec-arraybuffer-objects
text: DataView; url: sec-dataview-objects
text: Map; url: sec-map-objects
text: Promise; url: sec-promise-objects
text: Set; url: sec-set-objects
text: SharedArrayBuffer; url: sec-sharedarraybuffer-objects
text: %ErrorPrototype%; url: sec-properties-of-the-error-prototype-object
text: %FunctionPrototype%; url: sec-properties-of-the-function-prototype-object
text: %IteratorPrototype%; url: sec-%iteratorprototype%-object
text: %MapPrototype%; url: sec-properties-of-the-map-prototype-object
text: %ObjectPrototype%; url: sec-properties-of-the-object-prototype-object
text: %Promise%; url: sec-promise-constructor
text: %SetPrototype%; url: sec-properties-of-the-set-prototype-object
type: exception; for: ECMAScript
text: Error; url: sec-error-objects
text: SyntaxError; url: sec-native-error-types-used-in-this-standard-syntaxerror
text: TypeError; url: sec-native-error-types-used-in-this-standard-typeerror
type: method; for: ECMAScript
text: JSON.stringify(); url: sec-json.stringify
text: %ArrayProto_entries%; url: sec-array.prototype.entries
text: %ArrayProto_forEach%; url: sec-array.prototype.foreach
text: %ArrayProto_keys%; url: sec-array.prototype.keys
text: %ArrayProto_values%; url: sec-array.prototype.values
text: %ObjProto_toString%; url: sec-object.prototype.tostring
text: %Promise_reject%; url: sec-promise.reject
text: %Promise_resolve%; url: sec-promise.resolve
type: const; for: ECMAScript
url: sec-well-known-symbols
text: @@iterator
text: @@toStringTag
text: @@unscopables
type: argument
text: NewTarget; url: sec-built-in-function-objects
type: abstract-op
text: ArrayCreate; url: sec-arraycreate
text: Call; url: sec-call
text: CanonicalNumericIndexString; url: sec-canonicalnumericindexstring
text: Completion; url: sec-completion
text: Construct; url: sec-construct
text: CreateArrayIterator; url: sec-createarrayiterator
text: CreateBuiltinFunction; url: sec-createbuiltinfunction
text: CreateDataProperty; url: sec-createdataproperty
text: CreateIterResultObject; url: sec-createiterresultobject
text: CreateMapIterator; url: sec-createmapiterator
text: CreateSetIterator; url: sec-createsetiterator
text: DefinePropertyOrThrow; url: sec-definepropertyorthrow
text: DetachArrayBuffer; url: sec-detacharraybuffer
text: Get; url: sec-get-o-p
text: GetIterator; url: sec-getiterator
text: GetMethod; url: sec-getmethod
text: IsAccessorDescriptor; url: sec-isaccessordescriptor
text: IsCallable; url: sec-iscallable
text: IsConstructor; url: sec-isconstructor
text: IsDataDescriptor; url: sec-isdatadescriptor
text: IsDetachedBuffer; url: sec-isdetachedbuffer
text: IsInteger; url: sec-isinteger
text: IsSharedArrayBuffer; url: sec-issharedarraybuffer
text: IteratorStep; url: sec-iteratorstep
text: IteratorValue; url: sec-iteratorvalue
text: NormalCompletion; url: sec-normalcompletion
text: ObjectCreate; url: sec-objectcreate
text: OrdinaryDefineOwnProperty; url: sec-ordinarydefineownproperty
text: OrdinaryGetOwnProperty; url: sec-ordinarygetownproperty
text: OrdinaryPreventExtensions; url: sec-ordinarypreventextensions
text: OrdinarySetWithOwnDescriptor; url: sec-ordinarysetwithowndescriptor
text: Set; url: sec-set-o-p-v-throw
text: SetFunctionLength; url: sec-setfunctionlength
text: SetFunctionName; url: sec-setfunctionname
text: SetImmutablePrototype; url: sec-set-immutable-prototype
text: SetIntegrityLevel; url: sec-setintegritylevel
text: ToBoolean; url: sec-toboolean
text: ToInt32; url: sec-toint32
text: ToNumber; url: sec-tonumber
text: ToObject; url: sec-toobject
text: ToString; url: sec-tostring
text: ToUint16; url: sec-touint16
text: ToUint32; url: sec-touint32
text: Type; url: sec-ecmascript-data-types-and-values
text: abs; url: eqn-abs
text: floor; url: eqn-floor
text: max; url: eqn-max
text: min; url: eqn-min
type: dfn
text: NumericLiteral; url: sec-literals-numeric-literals
text: modulo; url: eqn-modulo
url: sec-returnifabrupt-shorthands
text: !
text: ?
text: ECMA-262 §5.2; url: sec-algorithm-conventions
text: ECMA-262 §9.1; url: sec-ordinary-object-internal-methods-and-internal-slots
text: ECMA-262 §9.3; url: sec-built-in-function-objects
text: ECMA-262 §9.4.7; url: sec-immutable-prototype-exotic-objects
text: abrupt completion; url: sec-completion-record-specification-type
text: array index; url: array-index
text: array iterator object; url: sec-array-iterator-objects
text: built-in function object; url: sec-built-in-function-objects
text: callable; for: ECMAScript; url: sec-iscallable
text: Completion Record; url: sec-completion-record-specification-type
text: constructor; url: constructor
text: conventions; for: ECMAScript; url: sec-algorithm-conventions
text: current Realm; url: current-realm
text: element; for: ECMAScript String; url: sec-ecmascript-language-types-string-type
text: enumerable; url: sec-property-attributes
text: equally close values; url: sec-ecmascript-language-types-number-type
text: error objects; for: ECMAScript; url: sec-error-objects
text: function object; url: function-object
text: immutable prototype exotic object; url: sec-immutable-prototype-exotic-objects
url: sec-object-internal-methods-and-internal-slots
text: internal method
text: internal slot
text: Number type; url: sec-ecmascript-language-types-number-type
for: ordinary object; url: sec-ordinary-object-internal-methods-and-internal-slots
text: internal method
text: internal slot
text: own property; url: sec-own-property
text: Property Descriptor; url: sec-property-descriptor-specification-type
text: realm; url: realm
</pre>
<pre class=biblio>
{
"GEOMETRY": {
"aliasOf": "GEOMETRY-1"
}
}
</pre>
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<div boilerplate=status>
<p>
This is a public copy of the editors’ draft.
It is provided for discussion only and may change at any moment.
Its publication here does not imply endorsement of its contents
by <abbr title="World Wide Web Consortium">W3C</abbr>.
Don’t cite this document other than as work in progress.
<p>
<strong>Changes to this document may be tracked at
<a href="[REPOSITORYURL]">[REPOSITORYURL]</a>.</strong>
<p>
<a href="[REPOSITORYURL]/issues">GitHub issues</a> are preferred for discussion of this specification.
There is also a <a href="[MAILINGLISTARCHIVES]">historical mailing-list archive</a>.
<p>
This document was produced by the
<a href="https://www.w3.org/WebPlatform/WG/">Web Platform Working Group</a>.
<p>
This document was produced by a group operating under the
<a href="https://www.w3.org/Consortium/Patent-Policy-20040205/">5 February 2004
<abbr title="World Wide Web Consortium">W3C</abbr> Patent Policy</a>.
<abbr title="World Wide Web Consortium">W3C</abbr> maintains a
<a href="https://www.w3.org/2004/01/pp-impl/83482/status" rel="disclosure">public list of any patent disclosures</a>
made in connection with the deliverables of the group;
that page also includes instructions for disclosing a patent.
An individual who has actual knowledge of a patent which the individual believes contains
<a href="https://www.w3.org/Consortium/Patent-Policy-20040205/#def-essential">Essential Claim(s)</a>
must disclose the information in accordance with
<a href="https://www.w3.org/Consortium/Patent-Policy-20040205/#sec-Disclosure">section 6 of the
<abbr title="World Wide Web Consortium">W3C</abbr> Patent Policy</a>.
<p>
This document is governed by the
<a id="w3c_process_revision" href="https://www.w3.org/2017/Process-20170301/">1 March 2017
<abbr title="World Wide Web Consortium">W3C</abbr> Process Document</a>.
<p>
[STATUSTEXT]
</div>
<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 Number
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 the ECMAScript language 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=],
[=interface mixins=],
[=partial interface mixin|partial mixin definitions=],
[=namespaces=],
[=partial namespace|partial namespace definitions=],
[=dictionary|dictionaries=],
[=partial dictionary|partial dictionary definitions=],
[=typedefs=] and
[=includes 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 :
CallbackOrInterfaceOrMixin
Namespace
Partial
Dictionary
Enum
Typedef
IncludesStatement
</pre>
<div class="example">
The following is an example of an [=IDL fragment=].
<pre highlight="webidl">
[Exposed=Window]
interface Paint { };
[Exposed=Window]
interface SolidColor : Paint {
attribute double red;
attribute double green;
attribute double blue;
};
[Exposed=Window]
interface Pattern : Paint {
attribute DOMString imageURL;
};
[Exposed=Window, 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 data properties whose value is a [=built-in function object=] 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">
[extended_attributes]
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 "<code>interface</code>" 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"
"includes"
"inherit"
"interface"
"iterable"
"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 "<code>constructor</code>",
"<code>toString</code>",
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 "<code>toJSON</code>" [=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">
[Exposed=Window]
interface B : A {
void f(SequenceOfLongs x);
};
[Exposed=Window]
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"
[Exposed=Window]
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"
[Exposed=Window]
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 "<code>value</code>" 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-t>interface</emu-t> <emu-nt><a href="#prod-InterfaceRest">InterfaceRest</a></emu-nt> or
<emu-t>callback</emu-t> <emu-t>interface</emu-t> <emu-nt><a href="#prod-InterfaceRest">InterfaceRest</a></emu-nt>) that declares some
state and behavior that an object implementing that interface will expose.
<pre highlight="webidl" class="syntax">
[extended_attributes]
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>).
These are the [=members=] that appear between the braces in the interface declaration.
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">
[Exposed=Window]
interface A {
void f();
void g();
};
[Exposed=Window]
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 [=includes statement=] can be used
to define that objects implementing an [=interface=] |A|
will always also include the [=interface mixin member|members=]
of the [=interface mixins=] |A| [=includes=].
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">
[extended_attributes]
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 [=include=] any [=interface mixins=].
[=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 <code class="idl">handleEvent</code>) 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 [=function 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;
};
[Exposed=Window]
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;
};
[Exposed=Window]