HTML
Living Standard — Last Updated 11 December 2024
Living Standard — Last Updated 11 December 2024
CustomElementRegistry interfaceSupport in all current engines.
This section is non-normative.
Custom elements provide a way for authors to build their own fully-featured DOM elements. Although authors could always use non-standard elements in their documents, with application-specific behavior added after the fact by scripting or similar, such elements have historically been non-conforming and not very functional. By defining a custom element, authors can inform the parser how to properly construct an element and how elements of that class should react to changes.
Custom elements are part of a larger effort to "rationalise the platform", by explaining existing platform features (like the elements of HTML) in terms of lower-level author-exposed extensibility points (like custom element definition). Although today there are many limitations on the capabilities of custom elements—both functionally and semantically—that prevent them from fully explaining the behaviors of HTML's existing elements, we hope to shrink this gap over time.
This section is non-normative.
For the purposes of illustrating how to create an autonomous custom element, let's define a custom element that encapsulates rendering a small icon for a country flag. Our goal is to be able to use it like so:
< flag-icon country = "nl" ></ flag-icon > To do this, we first declare a class for the custom element, extending
HTMLElement:
class FlagIcon extends HTMLElement {
constructor() {
super ();
this . _countryCode = null ;
}
static observedAttributes = [ "country" ];
attributeChangedCallback( name, oldValue, newValue) {
// name will always be "country" due to observedAttributes
this . _countryCode = newValue;
this . _updateRendering();
}
connectedCallback() {
this . _updateRendering();
}
get country() {
return this . _countryCode;
}
set country( v) {
this . setAttribute( "country" , v);
}
_updateRendering() {
// Left as an exercise for the reader. But, you'll probably want to
// check this.ownerDocument.defaultView to see if we've been
// inserted into a document with a browsing context, and avoid
// doing any work if not.
}
} We then need to use this class to define the element:
customElements. define( "flag-icon" , FlagIcon); At this point, our above code will work! The parser, whenever it sees the flag-icon tag, will construct a new instance of our FlagIcon class, and tell our code about its new country
attribute, which we then use to set the element's internal state and update its rendering (when
appropriate).
You can also create flag-icon elements using the DOM API:
const flagIcon = document. createElement( "flag-icon" )
flagIcon. country = "jp"
document. body. appendChild( flagIcon) Finally, we can also use the custom element constructor itself. That is, the above code is equivalent to:
const flagIcon = new FlagIcon()
flagIcon. country = "jp"
document. body. appendChild( flagIcon) This section is non-normative.
Adding a static formAssociated property, with a true value, makes an
autonomous custom element a form-associated custom element. The
ElementInternals interface helps you to implement functions and properties common
to form control elements.
class MyCheckbox extends HTMLElement {
static formAssociated = true ;
static observedAttributes = [ 'checked' ];
constructor() {
super ();
this . _internals = this . attachInternals();
this . addEventListener( 'click' , this . _onClick. bind( this ));
}
get form() { return this . _internals. form; }
get name() { return this . getAttribute( 'name' ); }
get type() { return this . localName; }
get checked() { return this . hasAttribute( 'checked' ); }
set checked( flag) { this . toggleAttribute( 'checked' , Boolean( flag)); }
attributeChangedCallback( name, oldValue, newValue) {
// name will always be "checked" due to observedAttributes
this . _internals. setFormValue( this . checked ? 'on' : null );
}
_onClick( event) {
this . checked = ! this . checked;
}
}
customElements. define( 'my-checkbox' , MyCheckbox); You can use the custom element my-checkbox like a built-in
form-associated element. For example, putting it in form or label
associates the my-checkbox element with them, and submitting the
form will send data provided by my-checkbox implementation.
< form action = "..." method = "..." >
< label >< my-checkbox name = "agreed" ></ my-checkbox > I read the agreement.</ label >
< input type = "submit" >
</ form >
This section is non-normative.
By using the appropriate properties of ElementInternals, your custom element can
have default accessibility semantics. The following code expands our form-associated checkbox from
the previous section to properly set its default role and checkedness, as viewed by accessibility
technology:
class MyCheckbox extends HTMLElement {
static formAssociated = true ;
static observedAttributes = [ 'checked' ];
constructor() {
super ();
this . _internals = this . attachInternals();
this . addEventListener( 'click' , this . _onClick. bind( this ));
this . _internals. role = 'checkbox' ;
this . _internals. ariaChecked = 'false' ;
}
get form() { return this . _internals. form; }
get name() { return this . getAttribute( 'name' ); }
get type() { return this . localName; }
get checked() { return this . hasAttribute( 'checked' ); }
set checked( flag) { this . toggleAttribute( 'checked' , Boolean( flag)); }
attributeChangedCallback( name, oldValue, newValue) {
// name will always be "checked" due to observedAttributes
this . _internals. setFormValue( this . checked ? 'on' : null );
this . _internals. ariaChecked = this . checked;
}
_onClick( event) {
this . checked = ! this . checked;
}
}
customElements. define( 'my-checkbox' , MyCheckbox); Note that, like for built-in elements, these are only defaults, and can be overridden by the
page author using the role and aria-* attributes:
<!-- This markup is non-conforming -->
< input type = "checkbox" checked role = "button" aria-checked = "false" > <!-- This markup is probably not what the custom element author intended -->
< my-checkbox role = "button" checked aria-checked = "false" > Custom element authors are encouraged to state what aspects of their accessibility semantics
are strong native semantics, i.e., should not be overridden by users of the custom element. In our
example, the author of the my-checkbox element would state that its
role and aria-checked values are strong
native semantics, thus discouraging code such as the above.
This section is non-normative.
Customized built-in elements are a distinct kind of custom element, which are defined slightly differently and used very differently compared to autonomous custom elements. They exist to allow reuse of behaviors from the existing elements of HTML, by extending those elements with new custom functionality. This is important since many of the existing behaviors of HTML elements can unfortunately not be duplicated by using purely autonomous custom elements. Instead, customized built-in elements allow the installation of custom construction behavior, lifecycle hooks, and prototype chain onto existing elements, essentially "mixing in" these capabilities on top of the already-existing element.
Customized built-in elements require a distinct syntax from autonomous custom elements because user agents and other software key off an element's local name in order to identify the element's semantics and behavior. That is, the concept of customized built-in elements building on top of existing behavior depends crucially on the extended elements retaining their original local name.
In this example, we'll be creating a customized built-in element named plastic-button, which behaves like a normal button but gets fancy animation
effects added whenever you click on it. We start by defining a class, just like before, although
this time we extend HTMLButtonElement instead of HTMLElement:
class PlasticButton extends HTMLButtonElement {
constructor() {
super ();
this . addEventListener( "click" , () => {
// Draw some fancy animation effects!
});
}
} When defining our custom element, we have to also specify the extends option:
customElements. define( "plastic-button" , PlasticButton, { extends : "button" }); In general, the name of the element being extended cannot be determined simply by looking at
what element interface it extends, as many elements share the same interface (such as
q and blockquote both sharing HTMLQuoteElement).
To construct our customized built-in element from parsed HTML source text, we use
the is attribute on a button element:
< button is = "plastic-button" > Click Me!</ button > Trying to use a customized built-in element as an autonomous custom
element will not work; that is, <plastic-button>Click
me?</plastic-button> will simply create an HTMLElement with no special
behavior.
If you need to create a customized built-in element programmatically, you can use the following
form of createElement():
const plasticButton = document. createElement( "button" , { is: "plastic-button" });
plasticButton. textContent = "Click me!" ; And as before, the constructor will also work:
const plasticButton2 = new PlasticButton();
console. log( plasticButton2. localName); // will output "button"
console. assert( plasticButton2 instanceof PlasticButton);
console. assert( plasticButton2 instanceof HTMLButtonElement); Note that when creating a customized built-in element programmatically, the is attribute will not be present in the DOM, since it was not explicitly
set. However, it will be added to the output when
serializing:
console. assert( ! plasticButton. hasAttribute( "is" ));
console. log( plasticButton. outerHTML); // will output '<button is="plastic-button"></button>' Regardless of how it is created, all of the ways in which button is special
apply to such "plastic buttons" as well: their focus behavior, ability to participate in form submission, the disabled attribute, and so on.
Customized built-in elements are designed to allow extension of existing HTML
elements that have useful user-agent supplied behavior or APIs. As such, they can only extend
existing HTML elements defined in this specification, and cannot extend legacy elements such as
bgsound, blink, isindex, keygen,
multicol, nextid, or spacer that have been defined to use
HTMLUnknownElement as their element interface.
One reason for this requirement is future-compatibility: if a customized built-in
element was defined that extended a currently-unknown element, for example combobox, this would prevent this specification from defining a combobox element in the future, as consumers of the derived customized
built-in element would have come to depend on their base element having no interesting
user-agent-supplied behavior.
This section is non-normative.
As specified below, and alluded to above, simply defining and using an element called
taco-button does not mean that such elements represent buttons. That is, tools such as web browsers, search engines,
or accessibility technology will not automatically treat the resulting element as a button just
based on its defined name.
To convey the desired button semantics to a variety of users, while still using an autonomous custom element, a number of techniques would need to be employed:
The addition of the tabindex attribute would make the
taco-button focusable. Note that if the
taco-button were to become logically disabled, the tabindex attribute would need to be removed.
The addition of an ARIA role and various ARIA states and properties helps convey semantics
to accessibility technology. For example, setting the role to "button" will convey the semantics that this is a button,
enabling users to successfully interact with the control using usual button-like interactions in
their accessibility technology. Setting the aria-label
property is necessary to give the button an accessible
name, instead of having accessibility technology traverse its child text nodes and
announce them. And setting the aria-disabled state to
"true" when the button is logically disabled conveys to accessibility
technology the button's disabled state.
The addition of event handlers to handle commonly-expected button behaviors helps convey
the semantics of the button to web browser users. In this case, the most relevant event handler
would be one that proxies appropriate keydown events to
become click events, so that you can activate the button both
with keyboard and by clicking.
In addition to any default visual styling provided for taco-button
elements, the visual styling will also need to be updated to reflect changes in logical state,
such as becoming disabled; that is, whatever style sheet has rules for taco-button will also need to have rules for taco-button[disabled].
With these points in mind, a full-featured taco-button that took on the
responsibility of conveying button semantics (including the ability to be disabled) might look
something like this:
class TacoButton extends HTMLElement {
static observedAttributes = [ "disabled" ];
constructor() {
super ();
this . _internals = this . attachInternals();
this . _internals. role = "button" ;
this . addEventListener( "keydown" , e => {
if ( e. code === "Enter" || e. code === "Space" ) {
this . dispatchEvent( new PointerEvent( "click" , {
bubbles: true ,
cancelable: true
}));
}
});
this . addEventListener( "click" , e => {
if ( this . disabled) {
e. preventDefault();
e. stopImmediatePropagation();
}
});
this . _observer = new MutationObserver(() => {
this . _internals. ariaLabel = this . textContent;
});
}
connectedCallback() {
this . setAttribute( "tabindex" , "0" );
this . _observer. observe( this , {
childList: true ,
characterData: true ,
subtree: true
});
}
disconnectedCallback() {
this . _observer. disconnect();
}
get disabled() {
return this . hasAttribute( "disabled" );
}
set disabled( flag) {
this . toggleAttribute( "disabled" , Boolean( flag));
}
attributeChangedCallback( name, oldValue, newValue) {
// name will always be "disabled" due to observedAttributes
if ( this . disabled) {
this . removeAttribute( "tabindex" );
this . _internals. ariaDisabled = "true" ;
} else {
this . setAttribute( "tabindex" , "0" );
this . _internals. ariaDisabled = "false" ;
}
}
} Even with this rather-complicated element definition, the element is not a pleasure to use for
consumers: it will be continually "sprouting" tabindex
attributes of its own volition, and its choice of tabindex="0" focusability
behavior may not match the button behavior on the current platform. This is because
as of now there is no way to specify default focus behavior for custom elements, forcing the use
of the tabindex attribute to do so (even though it is usually
reserved for allowing the consumer to override default behavior).
In contrast, a simple customized built-in element, as shown in the previous
section, would automatically inherit the semantics and behavior of the button
element, with no need to implement these behaviors manually. In general, for any elements with
nontrivial behavior and semantics that build on top of existing elements of HTML, customized built-in elements will be easier to
develop, maintain, and consume.
This section is non-normative.
Because element definition can occur at any time, a non-custom element could be created, and then later become a custom element after an appropriate definition is registered. We call this process "upgrading" the element, from a normal element into a custom element.
Upgrades enable scenarios where it may be
preferable for custom element definitions to be
registered after relevant elements have been initially created, such as by the parser. They allow
progressive enhancement of the content in the custom element. For example, in the following HTML
document the element definition for img-viewer is loaded
asynchronously:
<!DOCTYPE html>
< html lang = "en" >
< title > Image viewer example</ title >
< img-viewer filter = "Kelvin" >
< img src = "images/tree.jpg" alt = "A beautiful tree towering over an empty savannah" >
</ img-viewer >
< script src = "js/elements/img-viewer.js" async ></ script > The definition for the img-viewer element here is loaded using a
script element marked with the async
attribute, placed after the <img-viewer> tag in the markup. While the
script is loading, the img-viewer element will be treated as an undefined
element, similar to a span. Once the script loads, it will define the img-viewer element, and the existing img-viewer element on
the page will be upgraded, applying the custom element's definition (which presumably includes
applying an image filter identified by the string "Kelvin", enhancing the image's visual
appearance).
Note that upgrades only apply to elements in the document tree. (Formally, elements that are connected.) An element that is not inserted into a document will stay un-upgraded. An example illustrates this point:
<!DOCTYPE html>
< html lang = "en" >
< title > Upgrade edge-cases example</ title >
< example-element ></ example-element >
< script >
"use strict" ;
const inDocument = document. querySelector( "example-element" );
const outOfDocument = document. createElement( "example-element" );
// Before the element definition, both are HTMLElement:
console. assert( inDocument instanceof HTMLElement);
console. assert( outOfDocument instanceof HTMLElement);
class ExampleElement extends HTMLElement {}
customElements. define( "example-element" , ExampleElement);
// After element definition, the in-document element was upgraded:
console. assert( inDocument instanceof ExampleElement);
console. assert( ! ( outOfDocument instanceof ExampleElement));
document. body. appendChild( outOfDocument);
// Now that we've moved the element into the document, it too was upgraded:
console. assert( outOfDocument instanceof ExampleElement);
</ script > Built-in elements provided by user agents have certain states that can change over time
depending on user interaction and other factors, and are exposed to web authors through pseudo-classes. For example, some form controls have the "invalid"
state, which is exposed through the :invalid
pseudo-class.
Like built-in elements, custom elements can have various states to be in too, and custom element authors want to expose these states in a similar fashion as the built-in elements.
This is done via the :state() pseudo-class. A custom
element author can use the states property of
ElementInternals to add and remove such custom states, which are then exposed as
arguments to the :state() pseudo-class.
The following shows how :state() can be used to style a
custom checkbox element. Assume that LabeledCheckbox doesn't expose its
"checked" state via a content attribute.
< script >
class LabeledCheckbox extends HTMLElement {
constructor() {
super ();
this . _internals = this . attachInternals();
this . addEventListener( 'click' , this . _onClick. bind( this ));
const shadowRoot = this . attachShadow({ mode: 'closed' });
shadowRoot. innerHTML =
`<style>
:host::before {
content: '[ ]';
white-space: pre;
font-family: monospace;
}
:host(:state(checked))::before { content: '[x]' }
</style>
<slot>Label</slot>` ;
}
get checked() { return this . _internals. states. has( 'checked' ); }
set checked( flag) {
if ( flag)
this . _internals. states. add( 'checked' );
else
this . _internals. states. delete ( 'checked' );
}
_onClick( event) {
this . checked = ! this . checked;
}
}
customElements. define( 'labeled-checkbox' , LabeledCheckbox);
</ script >
< style >
labeled-checkbox { border : dashed red ; }
labeled-checkbox : state ( checked ) { border : solid ; }
</ style >
< labeled-checkbox > You need to check this</ labeled-checkbox > Custom pseudo-classes can even target shadow parts. An extension of the above example shows this:
< script >
class QuestionBox extends HTMLElement {
constructor() {
super ();
const shadowRoot = this . attachShadow({ mode: 'closed' });
shadowRoot. innerHTML =
`<div><slot>Question</slot></div>
<labeled-checkbox part='checkbox'>Yes</labeled-checkbox>` ;
}
}
customElements. define( 'question-box' , QuestionBox);
</ script >
< style >
question-box :: part ( checkbox ) { color : red ; }
question-box :: part ( checkbox ) : state ( checked ) { color : green ; }
</ style >
< question-box > Continue?</ question-box > When authoring custom element constructors, authors are bound by the following conformance requirements:
A parameter-less call to super() must be the first statement in the
constructor body, to establish the correct prototype chain and this value before any
further code is run.
A return statement must not appear anywhere inside the constructor
body, unless it is a simple early-return (return or return
this).
The constructor must not use the document.write()
or document.open() methods.
The element's attributes and children must not be inspected, as in the non-upgrade case none will be present, and relying on upgrades makes the element less usable.
The element must not gain any attributes or children, as this violates the expectations of
consumers who use the createElement or createElementNS methods.
In general, work should be deferred to connectedCallback as much as
possible—especially work involving fetching resources or rendering. However, note that connectedCallback can be called more than once, so any initialization work that
is truly one-time will need a guard to prevent it from running twice.
In general, the constructor should be used to set up initial state and default values, and to set up event listeners and possibly a shadow root.
Several of these requirements are checked during element creation, either directly or indirectly, and failing to follow them will result in a custom element that cannot be instantiated by the parser or DOM APIs. This is true even if the work is done inside a constructor-initiated microtask, as a microtask checkpoint can occur immediately after construction.
When authoring custom element reactions, authors should avoid manipulating the node tree as this can lead to unexpected results.
An element's connectedCallback can be queued before the element is
disconnected, but as the callback queue is still processed, it results in a connectedCallback for an element that is no longer connected:
class CParent extends HTMLElement {
connectedCallback() {
this . firstChild. remove();
}
}
customElements. define( "c-parent" , CParent);
class CChild extends HTMLElement {
connectedCallback() {
console. log( "CChild connectedCallback: isConnected =" , this . isConnected);
}
}
customElements. define( "c-child" , CChild);
const parent = new CParent(),
child = new CChild();
parent. append( child);
document. body. append( parent);
// Logs:
// CChild connectedCallback: isConnected = false A custom element is an element that is custom. Informally, this means that its constructor and prototype are defined by the author, instead of by the user agent. This author-supplied constructor function is called the custom element constructor.
Two distinct types of custom elements can be defined:
An autonomous custom element, which is defined with no extends option. These types of custom
elements have a local name equal to their defined name.
A customized built-in element, which is defined with an extends option. These types of custom
elements have a local name equal to the value passed in their extends option, and their defined name is used as the value of the
is attribute, which
therefore must be a valid custom element name.
After a custom element is created,
changing the value of the is attribute does not
change the element's behavior, as it is saved on the element as its is value.
Autonomous custom elements have the following element definition:
is attributeform, for form-associated custom elements — Associates the element with a form element
disabled, for form-associated custom elements — Whether the form control is disabled
readonly, for form-associated custom elements — Affects willValidate, plus any behavior added by the custom element author
name, for form-associated custom elements — Name of the element to use for form submission and in the form.elements API
HTMLElement)An autonomous custom element does not have any special meaning: it represents its children. A customized built-in element inherits the semantics of the element that it extends.
Any namespace-less attribute that is relevant to the element's functioning, as determined by
the element's author, may be specified on an autonomous custom element, so long as
the attribute name is XML-compatible and contains no ASCII upper alphas. The exception is the is attribute,
which must not be specified on an autonomous custom element (and which will have no
effect if it is).
Customized built-in elements follow the
normal requirements for attributes, based on the elements they extend. To add custom
attribute-based behavior, use data-* attributes.
An autonomous custom element is called a form-associated custom element if the element is associated with a custom element definition whose form-associated field is set to true.
The name attribute represents the form-associated
custom element's name. The disabled attribute is
used to make the form-associated custom element non-interactive and to prevent its
submission value from being submitted. The form attribute is used to explicitly associate the
form-associated custom element with its form owner.
The readonly attribute of form-associated custom elements specifies that the element is barred
from constraint validation. User agents don't provide any other behavior for the attribute,
but custom element authors should, where possible, use its presence to make their control
non-editable in some appropriate fashion, similar to the behavior for the readonly attribute on built-in form controls.
Constraint validation: If the readonly attribute is specified on a form-associated
custom element, the element is barred from constraint validation.
The reset algorithm for form-associated custom elements is to enqueue
a custom element callback reaction with the element, callback name "formResetCallback", and an empty argument list.
A valid custom element name is a sequence of characters name that meets all of the following requirements:
name must match the PotentialCustomElementName production:
PotentialCustomElementName ::=[a-z] (PCENChar)* '-'
(PCENChar)*PCENChar ::="-" | "." | [0-9] | "_" | [a-z] | #xB7 | [#xC0-#xD6] | [#xD8-#xF6] |
[#xF8-#x37D] | [#x37F-#x1FFF] | [#x200C-#x200D] | [#x203F-#x2040] | [#x2070-#x218F] |
[#x2C00-#x2FEF] | [#x3001-#xD7FF] | [#xF900-#xFDCF] | [#xFDF0-#xFFFD] |
[#x10000-#xEFFFF]This uses the EBNF notation from the XML specification. [XML]
name must not be any of the following:
annotation-xmlcolor-profilefont-facefont-face-srcfont-face-urifont-face-formatfont-face-namemissing-glyphThe list of names above is the summary of all hyphen-containing element names from the applicable specifications, namely SVG 2 and MathML. [SVG] [MATHML]
These requirements ensure a number of goals for valid custom element names:
They start with an ASCII lower alpha, ensuring that the HTML parser will treat them as tags instead of as text.
They do not contain any ASCII upper alphas, ensuring that the user agent can always treat HTML elements ASCII-case-insensitively.
They contain a hyphen, used for namespacing and to ensure forward compatibility (since no elements will be added to HTML, SVG, or MathML with hyphen-containing local names in the future).
They can always be created with createElement() and createElementNS(), which have restrictions that go
beyond the parser's.
Apart from these restrictions, a large variety of names is allowed, to give maximum
flexibility for use cases like <math-α> or <emotion-😍>.
A custom element definition describes a custom element and consists of:
CustomElementConstructor callback function type value wrapping
the custom element constructorsequence<DOMString>connectedCallback",
"disconnectedCallback", "adoptedCallback",
"attributeChangedCallback",
"formAssociatedCallback",
"formDisabledCallback",
"formResetCallback", and "formStateRestoreCallback".
The corresponding values are either a Web IDL Function
callback function type value, or null. By default the value of each entry is null.attachInternals().
attachShadow().
To look up a custom element definition, given a document, namespace, localName, and is, perform the following steps. They will return either a custom element definition or null:
If namespace is not the HTML namespace, then return null.
If document's browsing context is null, then return null.
Let registry be document's relevant global object's
CustomElementRegistry object.
If registry's custom element definition set contains an item with name and local name both equal to localName, then return that item.
If registry's custom element definition set contains an item with name equal to is and local name equal to localName, then return that item.
Return null.
CustomElementRegistry interfaceSupport in all current engines.
Each Window object is associated with a unique instance of a
CustomElementRegistry object, allocated when the Window object is
created.
Custom element registries are associated with Window objects, instead
of Document objects, since each custom element constructor inherits from
the HTMLElement interface, and there is exactly one HTMLElement
interface per Window object.
Support in all current engines.
The customElements attribute of the
Window interface must return the CustomElementRegistry object for that
Window object.
[Exposed =Window ]
interface CustomElementRegistry {
[CEReactions ] undefined define (DOMString name , CustomElementConstructor constructor , optional ElementDefinitionOptions options = {});
(CustomElementConstructor or undefined ) get (DOMString name );
DOMString ? getName (CustomElementConstructor constructor );
Promise <CustomElementConstructor > whenDefined (DOMString name );
[CEReactions ] undefined upgrade (Node root );
};
callback CustomElementConstructor = HTMLElement ();
dictionary ElementDefinitionOptions {
DOMString extends ;
};Every CustomElementRegistry has a custom element definition set, a
set of custom element definitions,
initially « ». Lookup of items in this set uses their name, local name, or constructor.
Every CustomElementRegistry also has an element definition is running
boolean which is used to prevent reentrant invocations of element definition. It is
initially false.
Every CustomElementRegistry also has a when-defined promise map, a
map of valid custom
element names to promises. It is used to implement the whenDefined() method.
window.customElements.define(name,
constructor)Support in all current engines.
window.customElements.define(name, constructor,
{ extends: baseLocalName })NotSupportedError"
DOMException will be thrown upon trying to extend a custom element or
an unknown element.window.customElements.get(name)Support in all current engines.
window.customElements.getName(constructor)window.customElements.whenDefined(name)CustomElementRegistry/whenDefined
Support in all current engines.
SyntaxError" DOMException if not
given a valid custom element name.window.customElements.upgrade(root)Support in all current engines.
Element definition is a process of adding a custom element definition
to the CustomElementRegistry. This is accomplished by the define() method. The define(name, constructor,
options) method steps are:
If IsConstructor(constructor) is false, then throw a
TypeError.
If name is not a valid custom element name, then throw a
"SyntaxError" DOMException.
If this's custom element definition set contains an item with name name, then throw a
"NotSupportedError" DOMException.
If this's custom element definition set contains an item with constructor constructor,
then throw a "NotSupportedError" DOMException.
Let localName be name.
Let extends be options["extends"] if it exists; otherwise null.
If extends is not null:
If extends is a valid custom element name, then throw a
"NotSupportedError" DOMException.
If the element interface for extends and the HTML
namespace is HTMLUnknownElement (e.g., if extends does not
indicate an element definition in this specification), then throw a
"NotSupportedError" DOMException.
Set localName to extends.
If this's element definition is running is true, then throw
a "NotSupportedError" DOMException.
Set this's element definition is running to true.
Let formAssociated be false.
Let disableInternals be false.
Let disableShadow be false.
Let observedAttributes be an empty sequence<DOMString>.
Run the following steps while catching any exceptions:
Let prototype be ? Get(constructor, "prototype").
If prototype is not an Object, then throw a
TypeError exception.
Let lifecycleCallbacks be the ordered map «[ "connectedCallback" → null, "disconnectedCallback" →
null, "adoptedCallback" → null, "attributeChangedCallback" → null ]».
For each callbackName of the keys of lifecycleCallbacks:
Let callbackValue be ? Get(prototype, callbackName).
If callbackValue is not undefined, then set
lifecycleCallbacks[callbackName] to the result of converting callbackValue to the Web IDL Function callback type.
If lifecycleCallbacks["attributeChangedCallback"] is not
null:
Let observedAttributesIterable be ? Get(constructor, "observedAttributes").
If observedAttributesIterable is not undefined, then set
observedAttributes to the result of converting observedAttributesIterable to a
sequence<DOMString>. Rethrow any exceptions from the
conversion.
Let disabledFeatures be an empty sequence<DOMString>.
Let disabledFeaturesIterable be ? Get(constructor, "disabledFeatures").
If disabledFeaturesIterable is not undefined, then set
disabledFeatures to the result of converting disabledFeaturesIterable to a sequence<DOMString>. Rethrow any exceptions from the conversion.
If disabledFeatures contains "internals", then set disableInternals to true.
If disabledFeatures contains "shadow", then set disableShadow to true.
Let formAssociatedValue be ? Get( constructor, "formAssociated").
Set formAssociated to the result of converting formAssociatedValue to a boolean.
If formAssociated is true, then for each callbackName of « "formAssociatedCallback", "formResetCallback", "formDisabledCallback", "formStateRestoreCallback"
»:
Let callbackValue be ? Get(prototype, callbackName).
If callbackValue is not undefined, then set
lifecycleCallbacks[callbackName] to the result of converting callbackValue to the Web IDL Function callback type.
Then, regardless of whether the above steps threw an exception or not: set this's element definition is running to false.
Finally, if the steps threw an exception, rethrow that exception.
Let definition be a new custom element definition with name name, local name localName, constructor constructor, observed attributes observedAttributes, lifecycle callbacks lifecycleCallbacks, form-associated formAssociated, disable internals disableInternals, and disable shadow disableShadow.
Append definition to this's custom element definition set.
Let document be this's relevant global object's associated Document.
Let upgradeCandidates be all elements that are shadow-including descendants of document, whose namespace
is the HTML namespace and whose local name is localName, in
shadow-including tree order. Additionally, if extends is non-null, only
include elements whose is
value is equal to name.
For each element element of upgradeCandidates, enqueue a custom element upgrade reaction given element and definition.
If this's when-defined promise map[name] exists:
Resolve this's when-defined promise map[name] with constructor.
Remove this's when-defined promise map[name].
The get(name) method steps are:
If this's custom element definition set contains an item with name name, then return that item's constructor.
Return undefined.
The getName(constructor) method
steps are:
If this's custom element definition set contains an item with constructor constructor, then return that item's name.
Return null.
The whenDefined(name) method
steps are:
If name is not a valid custom element name, then return a
promise rejected with a "SyntaxError"
DOMException.
If this's custom element definition set contains an item with name name, then return a promise resolved with that item's constructor.
If this's when-defined promise map[name] does not exist, then set this's when-defined promise map[name] to a new promise.
Return this's when-defined promise map[name].
The whenDefined() method can be
used to avoid performing an action until all appropriate custom
elements are defined. In this example, we
combine it with the :defined pseudo-class to hide a
dynamically-loaded article's contents until we're sure that all of the autonomous custom elements it uses are defined.
articleContainer. hidden = true ;
fetch( articleURL)
. then( response => response. text())
. then( text => {
articleContainer. innerHTML = text;
return Promise. all(
[... articleContainer. querySelectorAll( ":not(:defined)" )]
. map( el => customElements. whenDefined( el. localName))
);
})
. then(() => {
articleContainer. hidden = false ;
}); When invoked, the upgrade(root) method must run
these steps:
Let candidates be a list of all of root's shadow-including inclusive descendant elements, in shadow-including tree order.
For each candidate of candidates, try to upgrade candidate.
The upgrade() method allows upgrading
of elements at will. Normally elements are automatically upgraded when they become
connected, but this method can be used if you need to upgrade before you're ready to
connect the element.
const el = document. createElement( "spider-man" );
class SpiderMan extends HTMLElement {}
customElements. define( "spider-man" , SpiderMan);
console. assert( ! ( el instanceof SpiderMan)); // not yet upgraded
customElements. upgrade( el);
console. assert( el instanceof SpiderMan); // upgraded! To upgrade an element, given as input a custom element definition definition and an element element, run the following steps:
If element's custom element state is not "undefined" or "uncustomized", then return.
One scenario where this can occur due to reentrant invocation of this algorithm, as in the following example:
<!DOCTYPE html>
< x-foo id = "a" ></ x-foo >
< x-foo id = "b" ></ x-foo >
< script >
// Defining enqueues upgrade reactions for both "a" and "b"
customElements. define( "x-foo" , class extends HTMLElement {
constructor() {
super ();
const b = document. querySelector( "#b" );
b. remove();
// While this constructor is running for "a", "b" is still
// undefined, and so inserting it into the document will enqueue a
// second upgrade reaction for "b" in addition to the one enqueued
// by defining x-foo.
document. body. appendChild( b);
}
})
</ script > This step will thus bail out the algorithm early when upgrade an element is invoked
with "b" a second time.
Set element's custom element definition to definition.
Set element's custom element state to "failed".
It will be set to "custom" after the upgrade succeeds. For now,
we set it to "failed" so that any reentrant invocations will hit the above early-exit step.
For each attribute in element's attribute list, in
order, enqueue a custom element callback reaction with element, callback
name "attributeChangedCallback", and an argument list containing
attribute's local name, null, attribute's value, and attribute's
namespace.
If element is connected, then enqueue a custom element
callback reaction with element, callback name "connectedCallback", and an empty argument list.
Add element to the end of definition's construction stack.
Let C be definition's constructor.
Run the following substeps while catching any exceptions:
If definition's disable shadow is true and
element's shadow root is
non-null, then throw a "NotSupportedError"
DOMException.
This is needed as attachShadow() does not use look up a custom
element definition while attachInternals()
does.
Set element's custom element state to "precustomized".
Let constructResult be the result of constructing C, with no arguments.
If C non-conformantly
uses an API decorated with the [CEReactions] extended
attribute, then the reactions enqueued at the beginning of this algorithm will execute during
this step, before C finishes and control returns to this algorithm. Otherwise, they
will execute after C and the rest of the upgrade process finishes.
If SameValue(constructResult, element) is false, then
throw a TypeError.
This can occur if C constructs another instance of the same custom
element before calling super(), or if C uses JavaScript's
return-override feature to return an arbitrary HTMLElement
object from the constructor.
Then, perform the following substep, regardless of whether the above steps threw an exception or not:
Remove the last entry from the end of definition's construction stack.
Assuming C calls super() (as it will if it is conformant), and that the call succeeds, this will be
the already
constructed marker that replaced the element we pushed at the beginning
of this algorithm. (The HTML element constructor
carries out this replacement.)
If C does not call super() (i.e. it is not conformant), or if any step in the HTML element constructor throws, then this entry will
still be element.
Finally, if the above steps threw an exception, then:
Set element's custom element definition to null.
Empty element's custom element reaction queue.
Rethrow the exception (thus terminating this algorithm).
If the above steps threw an exception, then element's custom
element state will remain "failed" or "precustomized".
If element is a form-associated custom element, then:
Reset the form owner of element. If element is
associated with a form element, then
enqueue a custom element callback reaction with element, callback
name "formAssociatedCallback", and « the associated form
».
If element is disabled, then
enqueue a custom element callback reaction with element, callback name
"formDisabledCallback" and « true ».
Set element's custom
element state to "custom".
To try to upgrade an element, given as input an element element, run the following steps:
Let definition be the result of looking up a custom element definition given element's node
document, element's namespace, element's local name, and
element's is
value.
If definition is not null, then enqueue a custom element upgrade reaction given element and definition.
A custom element possesses the ability to respond to certain occurrences by running author code:
When upgraded, its constructor is run, with no arguments.
When it becomes connected, its connectedCallback is
called, with no arguments.
When it becomes disconnected, its disconnectedCallback
is called, with no arguments.
When it is adopted into a new document, its adoptedCallback is called, given the old document and new document as
arguments.
When any of its attributes are changed, appended, removed, or replaced, its attributeChangedCallback is called, given the attribute's local name, old value,
new value, and namespace as arguments. (An attribute's old or new value is considered to be null
when the attribute is added or removed, respectively.)
When the user agent resets the form owner of a
form-associated custom element and doing so changes the form owner, its formAssociatedCallback is called, given the new form owner (or null if no owner)
as an argument.
When the form owner of a form-associated custom element is reset, its formResetCallback is
called.
When the disabled state of a
form-associated custom element is changed, its formDisabledCallback is called, given the new state as an
argument.
When user agent updates a form-associated custom element's value on behalf of
a user or as part of navigation, its formStateRestoreCallback is called, given the new state and a string indicating
a reason, "autocomplete" or "restore", as
arguments.
We call these reactions collectively custom element reactions.
The way in which custom element reactions are invoked is done with special care, to avoid running author code during the middle of delicate operations. Effectively, they are delayed until "just before returning to user script". This means that for most purposes they appear to execute synchronously, but in the case of complicated composite operations (like cloning, or range manipulation), they will instead be delayed until after all the relevant user agent processing steps have completed, and then run together as a batch.
Additionally, the precise ordering of these reactions is managed via a somewhat-complicated stack-of-queues system, described below. The intention behind this system is to guarantee that custom element reactions always are invoked in the same order as their triggering actions, at least within the local context of a single custom element. (Because custom element reaction code can perform its own mutations, it is not possible to give a global ordering guarantee across multiple elements.)
Each similar-origin window agent has a custom element reactions stack, which is initially empty. A similar-origin window agent's current element queue is the element queue at the top of its custom element reactions stack. Each item in the stack is an element queue, which is initially empty as well. Each item in an element queue is an element. (The elements are not necessarily custom yet, since this queue is used for upgrades as well.)
Each custom element reactions stack has an associated backup element
queue, which is an initially-empty element queue. Elements are pushed onto the
backup element queue during operations that affect the DOM without going through an
API decorated with [CEReactions], or through the parser's
create an element for the token algorithm. An example of this is a user-initiated
editing operation which modifies the descendants or attributes of an editable
element. To prevent reentrancy when processing the backup element queue, each
custom element reactions stack also has a processing the backup element
queue flag, initially unset.
All elements have an associated custom element reaction queue, initially empty. Each item in the custom element reaction queue is of one of two types:
An upgrade reaction, which will upgrade the custom element and contains a custom element definition; or
A callback reaction, which will call a lifecycle callback, and contains a callback function as well as a list of arguments.
This is all summarized in the following schematic diagram:
To enqueue an element on the appropriate element queue, given an element element, run the following steps:
Let reactionsStack be element's relevant agent's custom element reactions stack.
If reactionsStack is empty, then:
Add element to reactionsStack's backup element queue.
If reactionsStack's processing the backup element queue flag is set, then return.
Set reactionsStack's processing the backup element queue flag.
Queue a microtask to perform the following steps:
Invoke custom element reactions in reactionsStack's backup element queue.
Unset reactionsStack's processing the backup element queue flag.
Otherwise, add element to element's relevant agent's current element queue.
To enqueue a custom element callback reaction, given a custom element element, a callback name callbackName, and a list of arguments args, run the following steps:
Let definition be element's custom element definition.
Let callback be the value of the entry in definition's lifecycle callbacks with key callbackName.
If callback is null, then return.
If callbackName is "attributeChangedCallback", then:
Let attributeName be the first element of args.
If definition's observed attributes does not contain attributeName, then return.
Add a new callback reaction to element's custom element reaction queue, with callback function callback and arguments args.
Enqueue an element on the appropriate element queue given element.
To enqueue a custom element upgrade reaction, given an element element and custom element definition definition, run the following steps:
Add a new upgrade reaction to element's custom element reaction queue, with custom element definition definition.
Enqueue an element on the appropriate element queue given element.
To invoke custom element reactions in an element queue queue, run the following steps:
While queue is not empty:
Let element be the result of dequeuing from queue.
Let reactions be element's custom element reaction queue.
Repeat until reactions is empty:
Remove the first element of reactions, and let reaction be that element. Switch on reaction's type:
Upgrade element using reaction's custom element definition.
If this throws an exception, catch it, and report it for reaction's custom element definition's constructor's corresponding JavaScript object's associated realm's global object.
Invoke reaction's
callback function with reaction's arguments and "report", and callback this value
set to element.
To ensure custom element reactions are
triggered appropriately, we introduce the [CEReactions] IDL extended attribute. It
indicates that the relevant algorithm is to be supplemented with additional steps in order to
appropriately track and invoke custom element
reactions.
The [CEReactions] extended attribute must take no
arguments, and must not appear on anything other than an operation, attribute, setter, or deleter.
Additionally, it must not appear on readonly attributes.
Operations, attributes, setters, or deleters annotated with the [CEReactions] extended attribute must run the following steps in place
of the ones specified in their description:
Push a new element queue onto this object's relevant agent's custom element reactions stack.
Run the originally-specified steps for this construct, catching any exceptions. If the steps return a value, let value be the returned value. If they throw an exception, let exception be the thrown exception.
Let queue be the result of popping from this object's relevant agent's custom element reactions stack.
Invoke custom element reactions in queue.
If an exception exception was thrown by the original steps, rethrow exception.
If a value value was returned from the original steps, return value.
The intent behind this extended attribute is somewhat subtle. One way of accomplishing its goals would be to say that every operation, attribute, setter, and deleter on the platform must have these steps inserted, and to allow implementers to optimize away unnecessary cases (where no DOM mutation is possible that could cause custom element reactions to occur).
However, in practice this imprecision could lead to non-interoperable implementations of custom element reactions, as some implementations might forget to invoke these steps in some cases. Instead, we settled on the approach of explicitly annotating all relevant IDL constructs, as a way of ensuring interoperable behavior and helping implementations easily pinpoint all cases where these steps are necessary.
Any nonstandard APIs introduced by the user agent that could modify the DOM in such a way as to
cause enqueuing a custom element
callback reaction or enqueuing a
custom element upgrade reaction, for example by modifying any attributes or child elements,
must also be decorated with the [CEReactions] attribute.
As of the time of this writing, the following nonstandard or not-yet-standardized APIs are known to fall into this category:
HTMLInputElement's webkitdirectory and incremental IDL attributes
HTMLLinkElement's scope IDL attribute
Certain capabilities are meant to be available to a custom element author, but not to a custom
element consumer. These are provided by the element.attachInternals() method, which returns an instance of
ElementInternals. The properties and methods of ElementInternals allow
control over internal features which the user agent provides to all elements.
element.attachInternals()Returns an ElementInternals object targeting the custom element
element. Throws an exception if element is not a custom
element, if the "internals" feature was disabled as part of the
element definition, or if it is called twice on the same element.
Each HTMLElement has an attached internals (null or an
ElementInternals object), initially null.
Support in all current engines.
The attachInternals() method steps are:
If this's is
value is not null, then throw a "NotSupportedError"
DOMException.
Let definition be the result of looking up a custom element definition given this's node
document, its namespace, its local name, and null as the is value.
If definition is null, then throw an
"NotSupportedError" DOMException.
If definition's disable internals is true,
then throw a "NotSupportedError" DOMException.
If this's attached internals is non-null, then throw an
"NotSupportedError" DOMException.
If this's custom element state is not "precustomized" or "custom", then throw a
"NotSupportedError" DOMException.
Set this's attached internals to a new
ElementInternals instance whose target
element is this.
Return this's attached internals.
ElementInternals interfaceSupport in all current engines.
The IDL for the ElementInternals interface is as follows, with the various operations and attributes
defined in the following sections:
[Exposed =Window ]
interface ElementInternals {
// Shadow root access
readonly attribute ShadowRoot shadowRoot ;
// Form-associated custom elements
undefined setFormValue ((File or USVString or FormData )? value ,
optional (File or USVString or FormData )? state );
readonly attribute HTMLFormElement ? form ;
undefined setValidity (optional ValidityStateFlags flags = {},
optional DOMString message ,
optional HTMLElement anchor );
readonly attribute boolean willValidate ;
readonly attribute ValidityState validity ;
readonly attribute DOMString validationMessage ;
boolean checkValidity ();
boolean reportValidity ();
readonly attribute NodeList labels ;
// Custom state pseudo-class
[SameObject ] readonly attribute CustomStateSet states ;
};
// Accessibility semantics
ElementInternals includes ARIAMixin ;
dictionary ValidityStateFlags {
boolean valueMissing = false ;
boolean typeMismatch = false ;
boolean patternMismatch = false ;
boolean tooLong = false ;
boolean tooShort = false ;
boolean rangeUnderflow = false ;
boolean rangeOverflow = false ;
boolean stepMismatch = false ;
boolean badInput = false ;
boolean customError = false ;
};Each ElementInternals has a target element,
which is a custom element.
internals.shadowRootReturns the ShadowRoot for internals's target element, if the target
element is a shadow host, or null otherwise.
Support in all current engines.
The shadowRoot getter steps are:
Let target be this's target element.
If target is not a shadow host, then return null.
Let shadow be target's shadow root.
If shadow's available to element internals is false, then return null.
Return shadow.
internals.setFormValue(value)Sets both the state and submission value of internals's target element to value.
If value is null, the element won't participate in form submission.
internals.setFormValue(value,
state)Sets the submission value of internals's target element to value, and its state to state.
If value is null, the element won't participate in form submission.
internals.formReturns the form owner of internals's target element.
internals.setValidity(flags,
message [, anchor ])Marks internals's target element as
suffering from the constraints indicated by the flags argument, and sets the element's
validation message to message. If anchor is specified, the user agent might
use it to indicate problems with the constraints of internals's target element when the form owner is validated
interactively or reportValidity() is
called.
internals.setValidity({})Marks internals's target element as satisfying its constraints.
internals.willValidateReturns true if internals's target element will be validated when the form is submitted; false otherwise.
internals.validityReturns the ValidityState object for internals's target element.
internals.validationMessageReturns the error message that would be shown to the user if internals's target element was to be checked for validity.
valid = internals.checkValidity()Returns true if internals's target
element has no validity problems; false otherwise. Fires an invalid event at the element in the latter case.
valid = internals.reportValidity()Returns true if internals's target
element has no validity problems; otherwise, returns false, fires an invalid event at the element, and (if the event isn't canceled)
reports the problem to the user.
internals.labelsReturns a NodeList of all the label elements that
internals's target element is associated
with.
Each form-associated custom element has submission value. It is used to provide one or more
entries on form submission.
The initial value of submission value is null, and
submission value can be null, a string, a
File, or a list of entries.
Each form-associated custom element has state.
It is information with which the user agent can restore a user's input for the element.
The initial value of state is null, and state can be null, a string, a File, or a
list of entries.
The setFormValue() method is used by
the custom element author to set the element's submission
value and state, thus communicating these to the user
agent.
When the user agent believes it is a good idea to restore a form-associated custom
element's state, for example after navigation or restarting the user agent, they may enqueue a
custom element callback reaction with that element, callback name "formStateRestoreCallback", an argument list containing the state to be
restored, and "restore".
If the user agent has a form-filling assist feature, then when the feature is invoked,
it may enqueue a custom element callback reaction with
a form-associated custom element, callback name
"formStateRestoreCallback", an argument list containing the state value
determined by history of state value and some heuristics, and
"autocomplete".
In general, the state is information specified by a user, and the submission value is a value after canonicalization or sanitization, suitable for submission to the server. The following examples makes this concrete:
Suppose that we have a form-associated custom element which asks a
user to specify a date. The user specifies "3/15/2019", but the control wishes to
submit "2019-03-15" to the server. "3/15/2019" would be a state of the element, and "2019-03-15" would be
a submission value.
Suppose you develop a custom element emulating a the behavior of the existing
checkbox input type. Its submission value would be the value of its value content attribute, or the string "on". Its state would be one of "checked", "unchecked", "checked/indeterminate", or "unchecked/indeterminate".
Support in all current engines.
The setFormValue(value,
state) method steps are:
Let element be this's target element.
If element is not a form-associated custom element, then throw a
"NotSupportedError" DOMException.
Set target element's submission value to value if value is
not a FormData object, or to a clone of
value's entry list otherwise.
If the state argument of the function is omitted, set element's state to its submission value.
Otherwise, if state is a FormData object, set element's
state to a clone of
state's entry list.
Otherwise, set element's state to state.
Each form-associated custom element has validity flags named
valueMissing, typeMismatch,
patternMismatch, tooLong,
tooShort, rangeUnderflow,
rangeOverflow, stepMismatch, and
customError. They are false initially.
Each form-associated custom element has a validation message string. It is the empty string initially.
Each form-associated custom element has a validation anchor element. It is null initially.
Support in all current engines.
The setValidity(flags, message,
anchor) method steps are:
Let element be this's target element.
If element is not a form-associated custom element, then throw a
"NotSupportedError" DOMException.
If flags contains one or more true values and message is not given
or is the empty string, then throw a TypeError.
For each entry flag → value of flags, set element's validity flag with the name flag to value.
Set element's validation message to the empty string if message is not given or all of element's validity flags are false, or to message otherwise.
If element's customError validity flag is true, then set
element's custom validity error message to element's
validation message. Otherwise, set
element's custom validity error message to the empty string.
Set element's validation anchor to
null if anchor is not given. Otherwise, if anchor is not a
shadow-including descendant of element, then throw a
"NotFoundError" DOMException. Otherwise, set
element's validation anchor to
anchor.
ElementInternals/validationMessage
Support in all current engines.
The validationMessage getter steps
are:
Let element be this's target element.
If element is not a form-associated custom element, then throw a
"NotSupportedError" DOMException.
Return element's validation message.
The entry construction algorithm for a form-associated custom element, given an element element and an entry list entry list, consists of the following steps:
If element's submission value is a list of entries, then append each item of element's submission value to entry list, and return.
In this case, user agent does not refer to the
name content attribute value. An implementation of
form-associated custom element is responsible to decide names of
entries. They can be the
name content attribute value, they can be strings based on
the name content attribute value, or they can be unrelated
to the name content attribute.
If the element does not have a name attribute
specified, or its name attribute's value is the empty string,
then return.
If the element's submission value is not
null, create an entry with the name attribute
value and the submission value, and append it to entry list.
internals.role [ = value ]Sets or retrieves the default ARIA role for internals's target element, which will be used unless the page author
overrides it using the role attribute.
internals.aria* [ = value ]Sets or retrieves various default ARIA states or property values for
internals's target element, which will be used
unless the page author overrides them using the aria-*
attributes.
Each custom element has an internal content attribute map, which is a map, initially empty. See the Requirements related to ARIA and to platform accessibility APIs section for information on how this impacts platform accessibility APIs.
internals.states.add(value)Adds the string value to the element's states set to be exposed as a pseudo-class.
internals.states.has(value)Returns true if value is in the element's states set, otherwise false.
internals.states.delete(value)If the element's states set has value, then it will be removed and true will be returned. Otherwise, false will be returned.
internals.states.clear()Removes all values from the element's states set.
for (const stateName of internals.states)for (const stateName of internals.states.entries())for (const stateName of internals.states.keys())for (const stateName of internals.states.values())Iterates over all values in the element's states set.
internals.states.forEach(callback)Iterates over all values in the element's states set by calling callback once for each value.
internals.states.sizeReturns the number of values in the element's states set.
Each custom element has a states set, which is a
CustomStateSet, initially empty.
[Exposed =Window ]
interface CustomStateSet {
setlike <DOMString >;
};The states getter steps are to return
this's target element's states
set.
The states set can expose boolean states represented by existence/non-existence
of string values. If an author wants to expose a state which can have three values, it can be
converted to three exclusive boolean states. For example, a state called readyState with "loading", "interactive", and "complete" values can be mapped to
three exclusive boolean states, "loading", "interactive", and "complete":
// Change the readyState from anything to "complete".
this . _readyState = "complete" ;
this . _internals. states. delete ( "loading" );
this . _internals. states. delete ( "interactive" );
this . _internals. states. add( "complete" );