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<pre class="metadata">
Group: WHATWG
H1: Web IDL
Shortname: webidl
Text Macro: TWITTER webidl
Text Macro: LATESTRD 2024-03
Abstract: This standard defines an interface definition language, Web IDL, that can be used to describe interfaces that
Abstract: are intended to be implemented in web browsers.
Translation: ja https://triple-underscore.github.io/WebIDL-ja.html
Complain About: accidental-2119 no
</pre>
<pre class="link-defaults">
spec: infra; type: dfn;
text: code unit
text: list
spec: ecmascript; type: dfn;
for: ECMAScript;
text: constructor
text: realm
spec: dom; type: dfn; text: element
</pre>
<pre class="anchors">
urlPrefix: https://tc39.es/ecma262/; spec: ecmascript
type: method; for: ECMAScript
text: JSON.stringify(); url: sec-json.stringify
text: %Array.prototype.entries%; url: sec-array.prototype.entries
text: %Array.prototype.forEach%; url: sec-array.prototype.foreach
text: %Array.prototype.keys%; url: sec-array.prototype.keys
text: %Array.prototype.values%; url: sec-array.prototype.values
text: %Promise.reject%; url: sec-promise.reject
text: %Promise.prototype.then%; url: sec-promise.prototype.then
type: argument
text: NewTarget; url: sec-built-in-function-objects
type: abstract-op
text: Completion; url: sec-completion-record-specification-type
text: IsInteger; url: sec-isinteger
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: Abstract Closure; url: sec-abstract-closure
text: NumericLiteral; url: sec-literals-numeric-literals
text: modulo; url: eqn-modulo
url: sec-returnifabrupt-shorthands
text: !
text: ?
text: ECMA-262 Immutable Prototype Exotic Objects; url: sec-immutable-prototype-exotic-objects
text: abrupt completion; url: sec-completion-record-specification-type
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: 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
url: sec-object-internal-methods-and-internal-slots
text: internal method
text: internal slot
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: PromiseCapability; url: sec-promisecapability-records
text: Iterator; url: sec-iterator-records
text: element size; url: table-the-typedarray-constructors
urlPrefix: https://tc39.es/proposal-resizablearraybuffer/; spec: RESIZABLE-BUFFERS-PROPOSAL
type: abstract-op
text: IsResizableArrayBuffer; url: sec-isresizablearraybuffer
</pre>
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<h2 id="introduction">Introduction</h2>
<i>This section is informative.</i>
This standard defines an interface definition language, Web IDL, that can be used to describe
interfaces that are intended to be implemented in web browsers. Web IDL is an IDL variant with a
number of features that allow the behavior of common script objects in the web platform to be
specified more readily. How interfaces described with Web IDL correspond to constructs within
JavaScript execution environments is also detailed here.
Concretely, Web IDL provides a syntax for specifying the surface APIs of web platform objects, as
well as JavaScript bindings that detail how those APIs manifest as JavaScript constructs. This
ensures common tasks, such as installing global properties, processing numeric inputs, or exposing
iteration behavior, remain uniform across web platform specifications: such specifications describe
their interfaces using Web IDL, and then use prose to specify API-specific details.
<p class="note">The term "JavaScript" is used to refer to ECMA-262, rather than the official term ECMAScript, since the term JavaScript is more widely known.</p>
<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=],
[=callback functions=],
[=callback interfaces=],
[=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 JavaScript language binding are discussed
in [[#js-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" id="example-25f60a7c">
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]
interface GraphicalWindow {
constructor();
readonly attribute unsigned long width;
readonly attribute unsigned long height;
attribute Paint currentPaint;
undefined drawRectangle(double x, double y, double width, double height);
undefined 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 JavaScript, 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 JavaScript object that implements <code class="idl">GraphicalWindow</code>
will have that prototype object in its prototype chain.
The [=constructor operation=] that appears on <code class="idl">GraphicalWindow</code>
causes a [=constructor=] to exist in JavaScript implementations,
so that calling <code>new GraphicalWindow()</code> would return a new object
that implemented the interface.
All [=interfaces=] have the [{{Exposed}}] [=extended attribute=], which ensures the interfaces
are only available in [=realms=] whose [=realm/global object=] is a {{Window}} object.
</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=],
[=callback interface=] 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... */);
callback interface <mark>callback_interface_identifier</mark> { /* interface_members... */ };
</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-OptionalOperationName">OptionalOperationName</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
U+005F (_) removed.
Note: A leading U+005F (_) 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 U+005F (_) 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 :
"async"
"attribute"
"callback"
"const"
"constructor"
"deleter"
"dictionary"
"enum"
"getter"
"includes"
"inherit"
"interface"
"iterable"
"maplike"
"mixin"
"namespace"
"partial"
"readonly"
"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
U+005F (_) 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 (except operation arguments) must not be "<code>constructor</code>",
"<code>toString</code>",
or begin with a U+005F (_). 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 can 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=],
[=callback interface=] and
[=typedef=]
must not
be the same as the identifier of any other
[=interface=],
[=namespace=],
[=dictionary=],
[=enumeration=],
[=callback function=],
[=callback interface=] 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" id="example-679e8e2c">
Therefore, the following [=IDL fragment=] is valid:
<pre highlight="webidl">
[Exposed=Window]
interface B : A {
undefined f(SequenceOfLongs x);
};
[Exposed=Window]
interface A {
};
typedef sequence<long> SequenceOfLongs;
</pre>
</div>
<div class="example" id="example-1192ff1f">
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>) 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
U+003A (:)
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="#js-interfaces">JavaScript 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" id="example-367b9481">
Consider the following two interfaces.
<pre highlight="webidl">
[Exposed=Window]
interface A {
undefined f();
undefined g();
};
[Exposed=Window]
interface B : A {
undefined f();
undefined g(DOMString x);
};
</pre>
In the JavaScript 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 JavaScript 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.
<div algorithm>
The [=list=] of <dfn for=interface>inclusive inherited interfaces</dfn> of an [=interface=] |I|
is defined as follows:
1. Let |result| be « ».
1. Let |interface| be |I|.
1. While |interface| is not null:
1. [=list/Append=] |interface| to |result|.
1. Set |interface| to the [=interface=] that |I| [=interface/inherits=] from, if any, and
null otherwise.
1. Return |result|.
</div>
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>
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-t>interface</emu-t>
<emu-nt><a href="#prod-PartialInterfaceRest">PartialInterfaceRest</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 is to be specified on the original [=interface=]
definition.
The relevant language binding determines how interfaces correspond to constructs
in the language.
The following extended attributes are applicable to interfaces:
[{{CrossOriginIsolated}}],
[{{Exposed}}],
[{{Global}}],
[{{LegacyFactoryFunction}}],
[{{LegacyNoInterfaceObject}}],
[{{LegacyOverrideBuiltIns}}],
[{{LegacyWindowAlias}}], and
[{{SecureContext}}].
The following extended attributes are applicable to [=partial interfaces=]:
[{{CrossOriginIsolated}}],
[{{Exposed}}],
[{{LegacyOverrideBuiltIns}}], and
[{{SecureContext}}].
[=Interfaces=] must be annotated with an [{{Exposed}}] [=extended attribute=].
<div algorithm>
The <dfn>qualified name</dfn> of an [=interface=] |interface| is defined as follows:
1. Let |identifier| be the [=identifier=] of |interface|.
1. If |interface| has a [{{LegacyNamespace}}] [=extended attribute=], then:
1. Let |namespace| be the identifier argument of the [{{LegacyNamespace}}]
[=extended attribute=].
1. Return the [=concatenation=] of « |namespace|, |identifier| » with
separator U+002E (.).
1. Return |identifier|.
</div>
<pre class="grammar" id="prod-CallbackOrInterfaceOrMixin">
CallbackOrInterfaceOrMixin :
"callback" CallbackRestOrInterface
"interface" InterfaceOrMixin
</pre>
<pre class="grammar" id="prod-InterfaceOrMixin">
InterfaceOrMixin :
InterfaceRest
MixinRest
</pre>
<pre class="grammar" id="prod-InterfaceRest">
InterfaceRest :
identifier Inheritance "{" InterfaceMembers "}" ";"
</pre>
<pre class="grammar" id="prod-Partial">
Partial :
"partial" PartialDefinition
</pre>
<pre class="grammar" id="prod-PartialDefinition">
PartialDefinition :
"interface" PartialInterfaceOrPartialMixin
PartialDictionary
Namespace
</pre>
<pre class="grammar" id="prod-PartialInterfaceOrPartialMixin">
PartialInterfaceOrPartialMixin :
PartialInterfaceRest
MixinRest
</pre>
<pre class="grammar" id="prod-PartialInterfaceRest">
PartialInterfaceRest :
identifier "{" PartialInterfaceMembers "}" ";"
</pre>
<pre class="grammar" id="prod-InterfaceMembers">
InterfaceMembers :
ExtendedAttributeList InterfaceMember InterfaceMembers
ε
</pre>
<pre class="grammar" id="prod-InterfaceMember">
InterfaceMember :
PartialInterfaceMember
Constructor
</pre>
<pre class="grammar" id="prod-PartialInterfaceMembers">
PartialInterfaceMembers :
ExtendedAttributeList PartialInterfaceMember PartialInterfaceMembers
ε
</pre>
<pre class="grammar" id="prod-PartialInterfaceMember">
PartialInterfaceMember :
Const
Operation
Stringifier
StaticMember
Iterable
AsyncIterable
ReadOnlyMember
ReadWriteAttribute
ReadWriteMaplike
ReadWriteSetlike
InheritAttribute
</pre>
<pre class="grammar" id="prod-Inheritance">
Inheritance :
":" identifier
ε
</pre>
<div class="example" id="example-0f0ea08a">
The following [=IDL fragment=]
demonstrates the definition of two mutually referential [=interfaces=].
Both <code class="idl">Human</code> and <code class="idl">Dog</code>
inherit from <code class="idl">Animal</code>. Objects that implement
either of those two interfaces will thus have a <code>name</code> attribute.
<pre highlight="webidl">
[Exposed=Window]
interface Animal {
attribute DOMString name;
};
[Exposed=Window]
interface Human : Animal {
attribute Dog? pet;
};
[Exposed=Window]
interface Dog : Animal {
attribute Human? owner;
};
</pre>
</div>
<div class="example" id="example-25ab6a82">
The following [=IDL fragment=] defines
simplified versions of a DOM [=interfaces=]
and a [=callback interface=].
<pre highlight="webidl">
[Exposed=Window]
interface Node {
readonly attribute DOMString nodeName;
readonly attribute Node? parentNode;
Node appendChild(Node newChild);
undefined addEventListener(DOMString type, EventListener listener);
};
callback interface EventListener {
undefined handleEvent(Event event);
};
</pre>
Plain objects can implement a [=callback interface=] like
<code class="idl">EventListener</code>:
<pre highlight="js">
var node = getNode(); // Obtain an instance of Node.
var listener = {
handleEvent: function(event) {
// ...
}
};
node.addEventListener("click", listener); // This works.
node.addEventListener("click", function() { ... }); // As does this.
</pre>
It is not possible for such an object to implement an [=interface=] like
<code class="idl">Node</code>, however:
<pre highlight="js">
var node = getNode(); // Obtain an instance of Node.
var newNode = {
nodeName: "span",
parentNode: null,
appendChild: function(newchild) {
// ...
},
addEventListener: function(type, listener) {
// ...
}
};
node.appendChild(newNode); // This will throw a TypeError exception.
</pre>
</div>
<h3 id="idl-interface-mixins">Interface mixins</h3>
An <dfn export>interface mixin</dfn> is a definition (matching <emu-t>interface</emu-t> <emu-nt><a href="#prod-MixinRest">MixinRest</a></emu-nt>)
that declares state and behavior that can be [=included=] by one or more [=interfaces=],
and that are exposed by objects that implement an [=interface=]
that [=includes=] the [=interface mixin=].
<pre highlight="webidl" class="syntax">
interface mixin identifier {
/* mixin_members... */
};
</pre>
Note: [=Interface mixins=], much like [=partial interfaces=],
are intended for use as a specification editorial aide,
allowing a coherent set of functionalities to be grouped together,
and included in multiple interfaces, possibly across documents.
They are not meant to be exposed through language bindings.
Guidance on when to choose [=partial interfaces=], [=interface mixins=],
or [=partial interface mixins=] can be found in [[#using-mixins-and-partials]].
An [=interface mixin=] is a specification of a set of <dfn export lt="interface mixin member">interface mixin members</dfn>
(matching <emu-nt><a href="#prod-MixinMembers">MixinMembers</a></emu-nt>),
which are the [=constants=], [=regular operations=], [=regular attributes=], and [=stringifiers=]
that appear between the braces in the [=interface mixin=] declaration.
These [=constants=], [=regular operations=], [=regular attributes=], and [=stringifiers=]
describe the behaviors that can be implemented by an object,
as if they were specified on the [=interface=] that [=includes=] them.
[=Static attributes=], [=static operations=], [=special operations=], and
[=iterable declaration|iterable=], [=asynchronously iterable declaration|asynchronously iterable=],
[=maplike declaration|maplike=], and [=setlike declarations=] cannot appear in [=interface mixin=]
declarations.
As with interfaces, the IDL for [=interface mixins=] can be split into multiple parts by using
<dfn export>partial interface mixin</dfn> definitions
(matching <emu-t>partial</emu-t> <emu-t>interface</emu-t> <emu-nt><a href="#prod-MixinRest">MixinRest</a></emu-nt>).
The [=identifier=] of a [=partial interface mixin=] [=definition=] must
be the same as the [=identifier=] of an [=interface mixin=] [=definition=].
All of the [=interface mixin member|members=] that appear on each of the [=partial interface mixin=] [=definitions=]
are considered to be [=members=] of the [=interface mixin=] itself,
and—by extension—of the [=interfaces=] that [=include=] the [=interface mixin=].
<pre highlight="webidl" class="syntax">
interface mixin <mark>SomeMixin</mark> {
/* mixin_members... */
};
partial interface mixin <mark>SomeMixin</mark> {