DOM

1. Infrastructure

This specification depends on the Infra Standard. [INFRA]

Some of the terms used in this specification are defined in Encoding, Selectors, Web IDL, XML, and Namespaces in XML. [ENCODING] [SELECTORS4] [WEBIDL] [XML] [XML-NAMES]

The term context object is an alias for this.

Usage of context object is deprecated in favor of this.

When extensions are needed, the DOM Standard can be updated accordingly, or a new standard can be written that hooks into the provided extensibility hooks for applicable specifications.

1.1. Trees

A tree is a finite hierarchical tree structure. In tree order is preorder, depth-first traversal of a tree.

An object that participates in a tree has a parent, which is either null or an object, and has children, which is an ordered set of objects. An object A whose parent is object B is a child of B.

The root of an object is itself, if its parent is null, or else it is the root of its parent. The root of a tree is any object participating in that tree whose parent is null.

An object A is called a descendant of an object B, if either A is a child of B or A is a child of an object C that is a descendant of B.

An inclusive descendant is an object or one of its descendants.

An object A is called an ancestor of an object B if and only if B is a descendant of A.

An inclusive ancestor is an object or one of its ancestors.

An object A is called a sibling of an object B, if and only if B and A share the same non-null parent.

An inclusive sibling is an object or one of its siblings.

An object A is preceding an object B if A and B are in the same tree and A comes before B in tree order.

An object A is following an object B if A and B are in the same tree and A comes after B in tree order.

The first child of an object is its first child or null if it has no children.

The last child of an object is its last child or null if it has no children.

The previous sibling of an object is its first preceding sibling or null if it has no preceding sibling.

The next sibling of an object is its first following sibling or null if it has no following sibling.

The index of an object is its number of preceding siblings, or 0 if it has none.

1.2. Ordered sets

The ordered set parser takes a string input and then runs these steps:

1. Let inputTokens be the result of splitting input on ASCII whitespace.

2. Let tokens be a new ordered set.

3. For each token in inputTokens, append token to tokens.

4. Return tokens.

The ordered set serializer takes a set and returns the concatenation of set using U+0020 SPACE.

1.3. Selectors

To scope-match a selectors string selectors against a node, run these steps:

1. Let s be the result of parse a selector selectors. [SELECTORS4]

2. If s is failure, then throw a "SyntaxError" DOMException.

3. Return the result of match a selector against a tree with s and node’s root using scoping root node. [SELECTORS4].

Support for namespaces within selectors is not planned and will not be added.

1.4. Namespaces

To validate a qualifiedName, throw an "InvalidCharacterError" DOMException if qualifiedName does not match the Name or QName production.

To validate and extract a namespace and qualifiedName, run these steps:

1. If namespace is the empty string, set it to null.
2. Validate qualifiedName.
3. Let prefix be null.
4. Let localName be qualifiedName.
5. If qualifiedName contains a ":" (U+003E), then split the string on it and set prefix to the part before and localName to the part after.
6. If prefix is non-null and namespace is null, then throw a "NamespaceError" DOMException.
7. If prefix is "xml" and namespace is not the XML namespace, then throw a "NamespaceError" DOMException.
8. If either qualifiedName or prefix is "xmlns" and namespace is not the XMLNS namespace, then throw a "NamespaceError" DOMException.
9. If namespace is the XMLNS namespace and neither qualifiedName nor prefix is "xmlns", then throw a "NamespaceError" DOMException.
10. Return namespace, prefix, and localName.

2. Events

2.1. Introduction to "DOM Events"

Throughout the web platform events are dispatched to objects to signal an occurrence, such as network activity or user interaction. These objects implement the EventTarget interface and can therefore add event listeners to observe events by calling addEventListener():

obj.addEventListener("load", imgFetched)

function imgFetched(ev) {
  // great success
  …
}

Event listeners can be removed by utilizing the removeEventListener() method, passing the same arguments.

Events are objects too and implement the Event interface (or a derived interface). In the example above ev is the event. ev is passed as an argument to the event listener’s callback (typically a JavaScript Function as shown above). Event listeners key off the event’s type attribute value ("load" in the above example). The event’s target attribute value returns the object to which the event was dispatched (obj above).

Although events are typically dispatched by the user agent as the result of user interaction or the completion of some task, applications can dispatch events themselves by using what are commonly known as synthetic events:

// add an appropriate event listener
obj.addEventListener("cat", function(e) { process(e.detail) })

// create and dispatch the event
var event = new CustomEvent("cat", {"detail":{"hazcheeseburger":true}})
obj.dispatchEvent(event)

Apart from signaling, events are sometimes also used to let an application control what happens next in an operation. For instance as part of form submission an event whose type attribute value is "submit" is dispatched. If this event’s preventDefault() method is invoked, form submission will be terminated. Applications who wish to make use of this functionality through events dispatched by the application (synthetic events) can make use of the return value of the dispatchEvent() method:

if(obj.dispatchEvent(event)) {
  // event was not canceled, time for some magic
  …
}

When an event is dispatched to an object that participates in a tree (e.g., an element), it can reach event listeners on that object’s ancestors too. Effectively, all the object’s inclusive ancestor event listeners whose capture is true are invoked, in tree order. And then, if event’s bubbles is true, all the object’s inclusive ancestor event listeners whose capture is false are invoked, now in reverse tree order.

Let’s look at an example of how events work in a tree:

<!doctype html>
<html>
 <head>
  <title>Boring example</title>
 </head>
 <body>
  <p>Hello <span id=x>world</span>!</p>
  <script>
   function test(e) {
     debug(e.target, e.currentTarget, e.eventPhase)
   }
   document.addEventListener("hey", test, {capture: true})
   document.body.addEventListener("hey", test)
   var ev = new Event("hey", {bubbles:true})
   document.getElementById("x").dispatchEvent(ev)
  </script>
 </body>
</html>

The debug function will be invoked twice. Each time the event’s target attribute value will be the span element. The first time currentTarget attribute’s value will be the document, the second time the body element. eventPhase attribute’s value switches from CAPTURING_PHASE to BUBBLING_PHASE. If an event listener was registered for the span element, eventPhase attribute’s value would have been AT_TARGET.

2.2. Interface Event

[Exposed=(Window,Worker,AudioWorklet)]
interface Event {
  constructor(DOMString type, optional EventInit eventInitDict = {});

  readonly attribute DOMString type;
  readonly attribute EventTarget? target;
  readonly attribute EventTarget? srcElement; // legacy
  readonly attribute EventTarget? currentTarget;
  sequence<EventTarget> composedPath();

  const unsigned short NONE = 0;
  const unsigned short CAPTURING_PHASE = 1;
  const unsigned short AT_TARGET = 2;
  const unsigned short BUBBLING_PHASE = 3;
  readonly attribute unsigned short eventPhase;

  undefined stopPropagation();
           attribute boolean cancelBubble; // legacy alias of .stopPropagation()
  undefined stopImmediatePropagation();

  readonly attribute boolean bubbles;
  readonly attribute boolean cancelable;
           attribute boolean returnValue;  // legacy
  undefined preventDefault();
  readonly attribute boolean defaultPrevented;
  readonly attribute boolean composed;

  [LegacyUnforgeable] readonly attribute boolean isTrusted;
  readonly attribute DOMHighResTimeStamp timeStamp;

  undefined initEvent(DOMString type, optional boolean bubbles = false, optional boolean cancelable = false); // legacy
};

dictionary EventInit {
  boolean bubbles = false;
  boolean cancelable = false;
  boolean composed = false;
};

An Event object is simply named an event. It allows for signaling that something has occurred, e.g., that an image has completed downloading.

A potential event target is null or an EventTarget object.

An event has an associated target (a potential event target). Unless stated otherwise it is null.

An event has an associated relatedTarget (a potential event target). Unless stated otherwise it is null.

Other specifications use relatedTarget to define a relatedTarget attribute. [UIEVENTS]

An event has an associated touch target list (a list of zero or more potential event targets). Unless stated otherwise it is the empty list.

The touch target list is for the exclusive use of defining the TouchEvent interface and related interfaces. [TOUCH-EVENTS]

An event has an associated path. A path is a list of structs. Each struct consists of an invocation target (an EventTarget object), an invocation-target-in-shadow-tree (a boolean), a shadow-adjusted target (a potential event target), a relatedTarget (a potential event target), a touch target list (a list of potential event targets), a root-of-closed-tree (a boolean), and a slot-in-closed-tree (a boolean). A path is initially the empty list.

event = new Event(type [, eventInitDict])

Returns a new event whose type attribute value is set to type. The eventInitDict argument allows for setting the bubbles and cancelable attributes via object members of the same name.

event . type

Returns the type of event, e.g. "click", "hashchange", or "submit".

event . target

Returns the object to which event is dispatched (its target).

event . currentTarget

Returns the object whose event listener’s callback is currently being invoked.

event . composedPath()

Returns the invocation target objects of event’s path (objects on which listeners will be invoked), except for any nodes in shadow trees of which the shadow root’s mode is "closed" that are not reachable from event’s currentTarget.

event . eventPhase

Returns the event’s phase, which is one of NONE, CAPTURING_PHASE, AT_TARGET, and BUBBLING_PHASE.

event . stopPropagation()

When dispatched in a tree, invoking this method prevents event from reaching any objects other than the current object.

event . stopImmediatePropagation()

Invoking this method prevents event from reaching any registered event listeners after the current one finishes running and, when dispatched in a tree, also prevents event from reaching any other objects.

event . bubbles

Returns true or false depending on how event was initialized. True if event goes through its target’s ancestors in reverse tree order, and false otherwise.

event . cancelable

Returns true or false depending on how event was initialized. Its return value does not always carry meaning, but true can indicate that part of the operation during which event was dispatched, can be canceled by invoking the preventDefault() method.

event . preventDefault()

If invoked when the cancelable attribute value is true, and while executing a listener for the event with passive set to false, signals to the operation that caused event to be dispatched that it needs to be canceled.

event . defaultPrevented

Returns true if preventDefault() was invoked successfully to indicate cancelation, and false otherwise.

event . composed

Returns true or false depending on how event was initialized. True if event invokes listeners past a ShadowRoot node that is the root of its target, and false otherwise.

event . isTrusted

Returns true if event was dispatched by the user agent, and false otherwise.

event . timeStamp

Returns the event’s timestamp as the number of milliseconds measured relative to the time origin.

The type attribute must return the value it was initialized to. When an event is created the attribute must be initialized to the empty string.

The target attribute’s getter, when invoked, must return this’s target.

The srcElement attribute’s getter, when invoked, must return this’s target.

The currentTarget attribute must return the value it was initialized to. When an event is created the attribute must be initialized to null.

The composedPath() method, when invoked, must run these steps:

1. Let composedPath be an empty list.

2. Let path be this’s path.

3. If path is empty, then return composedPath.

4. Let currentTarget be this’s currentTarget attribute value.

5. Append currentTarget to composedPath.

6. Let currentTargetIndex be 0.

7. Let currentTargetHiddenSubtreeLevel be 0.

8. Let index be path’s size − 1.

9. While index is greater than or equal to 0:

   1. If path[index]'s root-of-closed-tree is true, then increase currentTargetHiddenSubtreeLevel by 1.

   2. If path[index]'s invocation target is currentTarget, then set currentTargetIndex to index and break.

   3. If path[index]'s slot-in-closed-tree is true, then decrease currentTargetHiddenSubtreeLevel by 1.

   4. Decrease index by 1.

10. Let currentHiddenLevel and maxHiddenLevel be currentTargetHiddenSubtreeLevel.

11. Set index to currentTargetIndex − 1.

12. While index is greater than or equal to 0:

    1. If path[index]'s root-of-closed-tree is true, then increase currentHiddenLevel by 1.

    2. If currentHiddenLevel is less than or equal to maxHiddenLevel, then prepend path[index]'s invocation target to composedPath.

    3. If path[index]'s slot-in-closed-tree is true, then:

       1. Decrease currentHiddenLevel by 1.

       2. If currentHiddenLevel is less than maxHiddenLevel, then set maxHiddenLevel to currentHiddenLevel.

    4. Decrease index by 1.

13. Set currentHiddenLevel and maxHiddenLevel to currentTargetHiddenSubtreeLevel.

14. Set index to currentTargetIndex + 1.

15. While index is less than path’s size:

    1. If path[index]'s slot-in-closed-tree is true, then increase currentHiddenLevel by 1.

    2. If currentHiddenLevel is less than or equal to maxHiddenLevel, then append path[index]'s invocation target to composedPath.

    3. If path[index]'s root-of-closed-tree is true, then:

       1. Decrease currentHiddenLevel by 1.

       2. If currentHiddenLevel is less than maxHiddenLevel, then set maxHiddenLevel to currentHiddenLevel.

    4. Increase index by 1.

16. Return composedPath.

The eventPhase attribute must return the value it was initialized to, which must be one of the following:

NONE (numeric value 0)

Events not currently dispatched are in this phase.

CAPTURING_PHASE (numeric value 1)

When an event is dispatched to an object that participates in a tree it will be in this phase before it reaches its target.

AT_TARGET (numeric value 2)

When an event is dispatched it will be in this phase on its target.

BUBBLING_PHASE (numeric value 3)

When an event is dispatched to an object that participates in a tree it will be in this phase after it reaches its target.

Initially the attribute must be initialized to NONE.

Each event has the following associated flags that are all initially unset:

• stop propagation flag
• stop immediate propagation flag
• canceled flag
• in passive listener flag
• composed flag
• initialized flag
• dispatch flag

The stopPropagation() method, when invoked, must set this’s stop propagation flag.

The cancelBubble attribute’s getter, when invoked, must return true if this’s stop propagation flag is set, and false otherwise.

The cancelBubble attribute’s setter, when invoked, must set this’s stop propagation flag if the given value is true, and do nothing otherwise.

The stopImmediatePropagation() method, when invoked, must set this’s stop propagation flag and this’s stop immediate propagation flag.

The bubbles and cancelable attributes must return the values they were initialized to.

To set the canceled flag, given an event event, if event’s cancelable attribute value is true and event’s in passive listener flag is unset, then set event’s canceled flag, and do nothing otherwise.

The returnValue attribute’s getter, when invoked, must return false if this’s canceled flag is set, and true otherwise.

The returnValue attribute’s setter, when invoked, must set the canceled flag with this if the given value is false, and do nothing otherwise.

The preventDefault() method, when invoked, must set the canceled flag with this.

There are scenarios where invoking preventDefault() has no effect. User agents are encouraged to log the precise cause in a developer console, to aid debugging.

The defaultPrevented attribute’s getter, when invoked, must return true if this’s canceled flag is set, and false otherwise.

The composed attribute’s getter, when invoked, must return true if this’s composed flag is set, and false otherwise.

The isTrusted attribute must return the value it was initialized to. When an event is created the attribute must be initialized to false.

isTrusted is a convenience that indicates whether an event is dispatched by the user agent (as opposed to using dispatchEvent()). The sole legacy exception is click(), which causes the user agent to dispatch an event whose isTrusted attribute is initialized to false.

The timeStamp attribute must return the value it was initialized to.

To initialize an event, with type, bubbles, and cancelable, run these steps:

1. Set event’s initialized flag.

2. Unset event’s stop propagation flag, stop immediate propagation flag, and canceled flag.

3. Set event’s isTrusted attribute to false.

4. Set event’s target to null.

5. Set event’s type attribute to type.

6. Set event’s bubbles attribute to bubbles.

7. Set event’s cancelable attribute to cancelable.

The initEvent(type, bubbles, cancelable) method, when invoked, must run these steps:

1. If this’s dispatch flag is set, then return.

2. Initialize this with type, bubbles, and cancelable.

initEvent() is redundant with event constructors and incapable of setting composed. It has to be supported for legacy content.

2.3. Legacy extensions to the Window interface

partial interface Window {
  [Replaceable] readonly attribute any event; // legacy
};

Each Window object has an associated current event (undefined or an Event object). Unless stated otherwise it is undefined.

The event attribute’s getter, when invoked, must return this’s current event.

Web developers are strongly encouraged to instead rely on the Event object passed to event listeners, as that will result in more portable code. This attribute is not available in workers or worklets, and is inaccurate for events dispatched in shadow trees.

2.4. Interface CustomEvent

[Exposed=(Window,Worker)]
interface CustomEvent : Event {
  constructor(DOMString type, optional CustomEventInit eventInitDict = {});

  readonly attribute any detail;

  undefined initCustomEvent(DOMString type, optional boolean bubbles = false, optional boolean cancelable = false, optional any detail = null); // legacy
};

dictionary CustomEventInit : EventInit {
  any detail = null;
};

Events using the CustomEvent interface can be used to carry custom data.

event = new CustomEvent(type [, eventInitDict])

Works analogously to the constructor for Event except that the eventInitDict argument now allows for setting the detail attribute too.

event . detail

Returns any custom data event was created with. Typically used for synthetic events.

The detail attribute must return the value it was initialized to.

The initCustomEvent(type, bubbles, cancelable, detail) method must, when invoked, run these steps:

1. If this’s dispatch flag is set, then return.

2. Initialize this with type, bubbles, and cancelable.

3. Set this’s detail attribute to detail.

2.5. Constructing events

Specifications may define event constructing steps for all or some events. The algorithm is passed an event as indicated in the inner event creation steps.

This construct can be used by Event subclasses that have a more complex structure than a simple 1:1 mapping between their initializing dictionary members and IDL attributes.

When a constructor of the Event interface, or of an interface that inherits from the Event interface, is invoked, these steps must be run, given the arguments type and eventInitDict:

1. Let event be the result of running the inner event creation steps with this interface, null, now, and eventInitDict.

2. Initialize event’s type attribute to type.

3. Return event.

To create an event using eventInterface, which must be either Event or an interface that inherits from it, and optionally given a Realm realm, run these steps:

1. If realm is not given, then set it to null.

2. Let dictionary be the result of converting the JavaScript value undefined to the dictionary type accepted by eventInterface’s constructor. (This dictionary type will either be EventInit or a dictionary that inherits from it.)

   This does not work if members are required; see whatwg/dom#600.

3. Let event be the result of running the inner event creation steps with eventInterface, realm, the time of the occurrence that the event is signaling, and dictionary.

   In macOS the time of the occurrence for input actions is available via the timestamp property of NSEvent objects.

4. Initialize event’s isTrusted attribute to true.

5. Return event.

Create an event is meant to be used by other specifications which need to separately create and dispatch events, instead of simply firing them. It ensures the event’s attributes are initialized to the correct defaults.

The inner event creation steps, given an interface, realm, time, and dictionary, are as follows:

1. Let event be the result of creating a new object using eventInterface. If realm is non-null, then use that Realm; otherwise, use the default behavior defined in Web IDL.

   As of the time of this writing Web IDL does not yet define any default behavior; see heycam/webidl#135.

2. Set event’s initialized flag.

3. Initialize event’s timeStamp attribute to a DOMHighResTimeStamp representing the high resolution time from the time origin to time.

   User agents should set a minimum resolution of event’s timeStamp attribute to 5 microseconds following the existing clock resolution recommendation. [HR-TIME]

4. For each member → value in dictionary, if event has an attribute whose identifier is member, then initialize that attribute to value.

5. Run the event constructing steps with event.

6. Return event.

2.6. Defining event interfaces

In general, when defining a new interface that inherits from Event please always ask feedback from the WHATWG or the W3C WebApps WG community.

The CustomEvent interface can be used as starting point. However, do not introduce any init*Event() methods as they are redundant with constructors. Interfaces that inherit from the Event interface that have such a method only have it for historical reasons.

2.7. Interface EventTarget

[Exposed=(Window,Worker,AudioWorklet)]
interface EventTarget {
  constructor();

  undefined addEventListener(DOMString type, EventListener? callback, optional (AddEventListenerOptions or boolean) options = {});
  undefined removeEventListener(DOMString type, EventListener? callback, optional (EventListenerOptions or boolean) options = {});
  boolean dispatchEvent(Event event);
};

callback interface EventListener {
  undefined handleEvent(Event event);
};

dictionary EventListenerOptions {
  boolean capture = false;
};

dictionary AddEventListenerOptions : EventListenerOptions {
  boolean passive = false;
  boolean once = false;
};

An EventTarget object represents a target to which an event can be dispatched when something has occurred.

Each EventTarget object has an associated event listener list (a list of zero or more event listeners). It is initially the empty list.

An event listener can be used to observe a specific event and consists of:

• type (a string)
• callback (null or an EventListener object)
• capture (a boolean, initially false)
• passive (a boolean, initially false)
• once (a boolean, initially false)
• removed (a boolean for bookkeeping purposes, initially false)

Although callback is an EventListener object, an event listener is a broader concept as can be seen above.

Each EventTarget object also has an associated get the parent algorithm, which takes an event event, and returns an EventTarget object. Unless specified otherwise it returns null.

Nodes, shadow roots, and documents override the get the parent algorithm.

Each EventTarget object can have an associated activation behavior algorithm. The activation behavior algorithm is passed an event, as indicated in the dispatch algorithm.

This exists because user agents perform certain actions for certain EventTarget objects, e.g., the area element, in response to synthetic MouseEvent events whose type attribute is click. Web compatibility prevented it from being removed and it is now the enshrined way of defining an activation of something. [HTML]

Each EventTarget object that has activation behavior, can additionally have both (not either) a legacy-pre-activation behavior algorithm and a legacy-canceled-activation behavior algorithm.

These algorithms only exist for checkbox and radio input elements and are not to be used for anything else. [HTML]

target = new EventTarget();

Creates a new EventTarget object, which can be used by developers to dispatch and listen for events.

target . addEventListener(type, callback [, options])

Appends an event listener for events whose type attribute value is type. The callback argument sets the callback that will be invoked when the event is dispatched.

The options argument sets listener-specific options. For compatibility this can be a boolean, in which case the method behaves exactly as if the value was specified as options’s capture.

When set to true, options’s capture prevents callback from being invoked when the event’s eventPhase attribute value is BUBBLING_PHASE. When false (or not present), callback will not be invoked when event’s eventPhase attribute value is CAPTURING_PHASE. Either way, callback will be invoked if event’s eventPhase attribute value is AT_TARGET.

When set to true, options’s passive indicates that the callback will not cancel the event by invoking preventDefault(). This is used to enable performance optimizations described in § 2.8 Observing event listeners.

When set to true, options’s once indicates that the callback will only be invoked once after which the event listener will be removed.

The event listener is appended to target’s event listener list and is not appended if it has the same type, callback, and capture.

target . removeEventListener(type, callback [, options])

Removes the event listener in target’s event listener list with the same type, callback, and options.

target . dispatchEvent(event)

Dispatches a synthetic event event to target and returns true if either event’s cancelable attribute value is false or its preventDefault() method was not invoked, and false otherwise.

To flatten options, run these steps:

1. If options is a boolean, then return options.

2. Return options’s capture.

To flatten more options, run these steps:

1. Let capture be the result of flattening options.

2. Let once and passive be false.

3. If options is a dictionary, then set passive to options’s passive and once to options’s once.

4. Return capture, passive, and once.

The EventTarget() constructor, when invoked, must return a new EventTarget.

Because of the defaults stated elsewhere, the returned EventTarget's get the parent algorithm will return null, and it will have no activation behavior, legacy-pre-activation behavior, or legacy-canceled-activation behavior.

In the future we could allow custom get the parent algorithms. Let us know if this would be useful for your programs. For now, all author-created EventTargets do not participate in a tree structure.

To add an event listener, given an EventTarget object eventTarget and an event listener listener, run these steps:

1. If eventTarget is a ServiceWorkerGlobalScope object, its service worker’s script resource’s has ever been evaluated flag is set, and listener’s type matches the type attribute value of any of the service worker events, then report a warning to the console that this might not give the expected results. [SERVICE-WORKERS]

2. If listener’s callback is null, then return.

3. If eventTarget’s event listener list does not contain an event listener whose type is listener’s type, callback is listener’s callback, and capture is listener’s capture, then append listener to eventTarget’s event listener list.

The add an event listener concept exists to ensure event handlers use the same code path. [HTML]

The addEventListener(type, callback, options) method, when invoked, must run these steps:

1. Let capture, passive, and once be the result of flattening more options.

2. Add an event listener with this and an event listener whose type is type, callback is callback, capture is capture, passive is passive, and once is once.

To remove an event listener, given an EventTarget object eventTarget and an event listener listener, run these steps:

1. If eventTarget is a ServiceWorkerGlobalScope object and its service worker’s set of event types to handle contains type, then report a warning to the console that this might not give the expected results. [SERVICE-WORKERS]

2. Set listener’s removed to true and remove listener from eventTarget’s event listener list.

HTML needs this to define event handlers. [HTML]

To remove all event listeners, given an EventTarget object eventTarget, for each listener of eventTarget’s event listener list, remove an event listener with eventTarget and listener.

HTML needs this to define document.open(). [HTML]

The removeEventListener(type, callback, options) method, when invoked, must run these steps:

1. Let capture be the result of flattening options.

2. If this’s event listener list contains an event listener whose type is type, callback is callback, and capture is capture, then remove an event listener with this and that event listener.

The event listener list will not contain multiple event listeners with equal type, callback, and capture, as add an event listener prevents that.

The dispatchEvent(event) method, when invoked, must run these steps:

1. If event’s dispatch flag is set, or if its initialized flag is not set, then throw an "InvalidStateError" DOMException.

2. Initialize event’s isTrusted attribute to false.

3. Return the result of dispatching event to this.

2.8. Observing event listeners

In general, developers do not expect the presence of an event listener to be observable. The impact of an event listener is determined by its callback. That is, a developer adding a no-op event listener would not expect it to have any side effects.

Unfortunately, some event APIs have been designed such that implementing them efficiently requires observing event listeners. This can make the presence of listeners observable in that even empty listeners can have a dramatic performance impact on the behavior of the application. For example, touch and wheel events which can be used to block asynchronous scrolling. In some cases this problem can be mitigated by specifying the event to be cancelable only when there is at least one non-passive listener. For example, non-passive TouchEvent listeners must block scrolling, but if all listeners are passive then scrolling can be allowed to start in parallel by making the TouchEvent uncancelable (so that calls to preventDefault() are ignored). So code dispatching an event is able to observe the absence of non-passive listeners, and use that to clear the cancelable property of the event being dispatched.

Ideally, any new event APIs are defined such that they do not need this property (use public-script-coord@w3.org for discussion).

2.9. Dispatching events

To dispatch an event to a target, with an optional legacy target override flag and an optional legacyOutputDidListenersThrowFlag, run these steps:

1. Set event’s dispatch flag.

2. Let targetOverride be target, if legacy target override flag is not given, and target’s associated Document otherwise. [HTML]

   legacy target override flag is only used by HTML and only when target is a Window object.

3. Let activationTarget be null.

4. Let relatedTarget be the result of retargeting event’s relatedTarget against target.

5. If target is not relatedTarget or target is event’s relatedTarget, then:

   1. Let touchTargets be a new list.

   2. For each touchTarget of event’s touch target list, append the result of retargeting touchTarget against target to touchTargets.

   3. Append to an event path with event, target, targetOverride, relatedTarget, touchTargets, and false.

   4. Let isActivationEvent be true, if event is a MouseEvent object and event’s type attribute is "click", and false otherwise.

   5. If isActivationEvent is true and target has activation behavior, then set activationTarget to target.

   6. Let slottable be target, if target is a slottable and is assigned, and null otherwise.

   7. Let slot-in-closed-tree be false.

   8. Let parent be the result of invoking target’s get the parent with event.

   9. While parent is non-null:

      1. If slottable is non-null:

         1. Assert: parent is a slot.

         2. Set slottable to null.

         3. If parent’s root is a shadow root whose mode is "closed", then set slot-in-closed-tree to true.

      2. If parent is a slottable and is assigned, then set slottable to parent.

      3. Let relatedTarget be the result of retargeting event’s relatedTarget against parent.

      4. Let touchTargets be a new list.

      5. For each touchTarget of event’s touch target list, append the result of retargeting touchTarget against parent to touchTargets.

      6. If parent is a Window object, or parent is a node and target’s root is a shadow-including inclusive ancestor of parent, then:

         1. If isActivationEvent is true, event’s bubbles attribute is true, activationTarget is null, and parent has activation behavior, then set activationTarget to parent.

         2. Append to an event path with event, parent, null, relatedTarget, touchTargets, and slot-in-closed-tree.

      7. Otherwise, if parent is relatedTarget, then set parent to null.

      8. Otherwise, set target to parent and then:

         1. If isActivationEvent is true, activationTarget is null, and target has activation behavior, then set activationTarget to target.

         2. Append to an event path with event, parent, target, relatedTarget, touchTargets, and slot-in-closed-tree.

      9. If parent is non-null, then set parent to the result of invoking parent’s get the parent with event.

      10. Set slot-in-closed-tree to false.

   10. Let clearTargetsStruct be the last struct in event’s path whose shadow-adjusted target is non-null.

   11. Let clearTargets be true if clearTargetsStruct’s shadow-adjusted target, clearTargetsStruct’s relatedTarget, or an EventTarget object in clearTargetsStruct’s touch target list is a node and its root is a shadow root, and false otherwise.

   12. If activationTarget is non-null and activationTarget has legacy-pre-activation behavior, then run activationTarget’s legacy-pre-activation behavior.

   13. For each struct in event’s path, in reverse order:

       1. If struct’s shadow-adjusted target is non-null, then set event’s eventPhase attribute to AT_TARGET.

       2. Otherwise, set event’s eventPhase attribute to CAPTURING_PHASE.

       3. Invoke with struct, event, "capturing", and legacyOutputDidListenersThrowFlag if given.

   14. For each struct in event’s path:

       1. If struct’s shadow-adjusted target is non-null, then set event’s eventPhase attribute to AT_TARGET.

       2. Otherwise:

          1. If event’s bubbles attribute is false, then continue.

          2. Set event’s eventPhase attribute to BUBBLING_PHASE.

       3. Invoke with struct, event, "bubbling", and legacyOutputDidListenersThrowFlag if given.

6. Set event’s eventPhase attribute to NONE.

7. Set event’s currentTarget attribute to null.

8. Set event’s path to the empty list.

9. Unset event’s dispatch flag, stop propagation flag, and stop immediate propagation flag.

10. If clearTargets, then:

    1. Set event’s target to null.

    2. Set event’s relatedTarget to null.

    3. Set event’s touch target list to the empty list.

11. If activationTarget is non-null, then:

    1. If event’s canceled flag is unset, then run activationTarget’s activation behavior with event.

    2. Otherwise, if activationTarget has legacy-canceled-activation behavior, then run activationTarget’s legacy-canceled-activation behavior.

12. Return false if event’s canceled flag is set, and true otherwise.

To append to an event path, given an event, invocationTarget, shadowAdjustedTarget, relatedTarget, touchTargets, and a slot-in-closed-tree, run these steps:

1. Let invocationTargetInShadowTree be false.

2. If invocationTarget is a node and its root is a shadow root, then set invocationTargetInShadowTree to true.

3. Let root-of-closed-tree be false.

4. If invocationTarget is a shadow root whose mode is "closed", then set root-of-closed-tree to true.

5. Append a new struct to event’s path whose invocation target is invocationTarget, invocation-target-in-shadow-tree is invocationTargetInShadowTree, shadow-adjusted target is shadowAdjustedTarget, relatedTarget is relatedTarget, touch target list is touchTargets, root-of-closed-tree is root-of-closed-tree, and slot-in-closed-tree is slot-in-closed-tree.

To invoke, given a struct, event, phase, and an optional legacyOutputDidListenersThrowFlag, run these steps:

1. Set event’s target to the shadow-adjusted target of the last struct in event’s path, that is either struct or preceding struct, whose shadow-adjusted target is non-null.

2. Set event’s relatedTarget to struct’s relatedTarget.

3. Set event’s touch target list to struct’s touch target list.

4. If event’s stop propagation flag is set, then return.

5. Initialize event’s currentTarget attribute to struct’s invocation target.

6. Let listeners be a clone of event’s currentTarget attribute value’s event listener list.

   This avoids event listeners added after this point from being run. Note that removal still has an effect due to the removed field.

7. Let invocationTargetInShadowTree be struct’s invocation-target-in-shadow-tree.

8. Let found be the result of running inner invoke with event, listeners, phase, invocationTargetInShadowTree, and legacyOutputDidListenersThrowFlag if given.

9. If found is false and event’s isTrusted attribute is true, then:

   1. Let originalEventType be event’s type attribute value.

   2. If event’s type attribute value is a match for any of the strings in the first column in the following table, set event’s type attribute value to the string in the second column on the same row as the matching string, and return otherwise.

      Event type	Legacy event type
      "animationend"	"webkitAnimationEnd"
      "animationiteration"	"webkitAnimationIteration"
      "animationstart"	"webkitAnimationStart"
      "transitionend"	"webkitTransitionEnd"

   3. Inner invoke with event, listeners, phase, invocationTargetInShadowTree, and legacyOutputDidListenersThrowFlag if given.

   4. Set event’s type attribute value to originalEventType.

To inner invoke, given an event, listeners, phase, invocationTargetInShadowTree, and an optional legacyOutputDidListenersThrowFlag, run these steps:

1. Let found be false.

2. For each listener in listeners, whose removed is false:

   1. If event’s type attribute value is not listener’s type, then continue.

   2. Set found to true.

   3. If phase is "capturing" and listener’s capture is false, then continue.

   4. If phase is "bubbling" and listener’s capture is true, then continue.

   5. If listener’s once is true, then remove listener from event’s currentTarget attribute value’s event listener list.

   6. Let global be listener callback’s associated Realm’s global object.

   7. Let currentEvent be undefined.

   8. If global is a Window object, then:

      1. Set currentEvent to global’s current event.

      2. If invocationTargetInShadowTree is false, then set global’s current event to event.

   9. If listener’s passive is true, then set event’s in passive listener flag.

   10. Call a user object’s operation with listener’s callback, "handleEvent", « event », and event’s currentTarget attribute value. If this throws an exception, then:

       1. Report the exception.

       2. Set legacyOutputDidListenersThrowFlag if given.

          The legacyOutputDidListenersThrowFlag is only used by Indexed Database API. [INDEXEDDB]

   11. Unset event’s in passive listener flag.

   12. If global is a Window object, then set global’s current event to currentEvent.

   13. If event’s stop immediate propagation flag is set, then return found.

3. Return found.

2.10. Firing events

To fire an event named e at target, optionally using an eventConstructor, with a description of how IDL attributes are to be initialized, and a legacy target override flag, run these steps:

1. If eventConstructor is not given, then let eventConstructor be Event.

2. Let event be the result of creating an event given eventConstructor, in the relevant Realm of target.

3. Initialize event’s type attribute to e.

4. Initialize any other IDL attributes of event as described in the invocation of this algorithm.

   This also allows for the isTrusted attribute to be set to false.

5. Return the result of dispatching event at target, with legacy target override flag set if set.

Fire in the context of DOM is short for creating, initializing, and dispatching an event. Fire an event makes that process easier to write down.

2.11. Action versus occurrence

An event signifies an occurrence, not an action. Phrased differently, it represents a notification from an algorithm and can be used to influence the future course of that algorithm (e.g., through invoking preventDefault()). Events must not be used as actions or initiators that cause some algorithm to start running. That is not what they are for.

This is called out here specifically because previous iterations of the DOM had a concept of "default actions" associated with events that gave folks all the wrong ideas. Events do not represent or cause actions, they can only be used to influence an ongoing one.

3. Aborting ongoing activities

Though promises do not have a built-in aborting mechanism, many APIs using them require abort semantics. AbortController is meant to support these requirements by providing an abort() method that toggles the state of a corresponding AbortSignal object. The API which wishes to support aborting can accept an AbortSignal object, and use its state to determine how to proceed.

APIs that rely upon AbortController are encouraged to respond to abort() by rejecting any unsettled promise with a new "AbortError" DOMException.

const controller = new AbortController();
const signal = controller.signal;

startSpinner();

doAmazingness({ ..., signal })
  .then(result => ...)
  .catch(err => {
    if (err.name == 'AbortError') return;
    showUserErrorMessage();
  })
  .then(() => stopSpinner());

// …

controller.abort();

function doAmazingness({signal}) {
  if (signal.aborted) {
    return Promise.reject(new DOMException('Aborted', 'AbortError'));
  }

  return new Promise((resolve, reject) => {
    // Begin doing amazingness, and call resolve(result) when done.
    // But also, watch for signals:
    signal.addEventListener('abort', () => {
      // Stop doing amazingness, and:
      reject(new DOMException('Aborted', 'AbortError'));
    });
  });
}

3.1. Interface AbortController

[Exposed=(Window,Worker)]
interface AbortController {
  constructor();

  [SameObject] readonly attribute AbortSignal signal;

  undefined abort();
};

controller = new AbortController()

Returns a new controller whose signal is set to a newly created AbortSignal object.

controller . signal

Returns the AbortSignal object associated with this object.

controller . abort()

Invoking this method will set this object’s AbortSignal's aborted flag and signal to any observers that the associated activity is to be aborted.

An AbortController object has an associated signal (an AbortSignal object).

The new AbortController() constructor steps are:

1. Let signal be a new AbortSignal object.

2. Set this’s signal to signal.

The signal getter steps are to return this’s signal.

The abort() method steps are to signal abort on this’s signal.

3.2. Interface AbortSignal

[Exposed=(Window,Worker)]
interface AbortSignal : EventTarget {
  readonly attribute boolean aborted;

  attribute EventHandler onabort;
};

signal . aborted

Returns true if this AbortSignal's AbortController has signaled to abort, and false otherwise.

An AbortSignal object has an associated aborted flag. It is unset unless specified otherwise.

An AbortSignal object has associated abort algorithms, which is a set of algorithms which are to be executed when its aborted flag is set. Unless specified otherwise, its value is the empty set.

To add an algorithm algorithm to an AbortSignal object signal, run these steps:

1. If signal’s aborted flag is set, then return.

2. Append algorithm to signal’s abort algorithms.

To remove an algorithm algorithm from an AbortSignal signal, remove algorithm from signal’s abort algorithms.

The abort algorithms enable APIs with complex requirements to react in a reasonable way to abort(). For example, a given API’s aborted flag might need to be propagated to a cross-thread environment, such as a service worker.

The aborted getter steps are to return true if this’s aborted flag is set; otherwise false.

The onabort attribute is an event handler IDL attribute for the onabort event handler, whose event handler event type is abort.

Changes to an AbortSignal object represent the wishes of the corresponding AbortController object, but an API observing the AbortSignal object can chose to ignore them. For instance, if the operation has already completed.

To signal abort, given a AbortSignal object signal, run these steps:

1. If signal’s aborted flag is set, then return.

2. Set signal’s aborted flag.

3. For each algorithm in signal’s abort algorithms: run algorithm.

4. Empty signal’s abort algorithms.

5. Fire an event named abort at signal.

A followingSignal (an AbortSignal) is made to follow a parentSignal (an AbortSignal) by running these steps:

1. If followingSignal’s aborted flag is set, then return.

2. If parentSignal’s aborted flag is set, then signal abort on followingSignal.

3. Otherwise, add the following abort steps to parentSignal:

   1. Signal abort on followingSignal.

3.3. Using AbortController and AbortSignal objects in APIs

Any web platform API using promises to represent operations that can be aborted must adhere to the following:

• Accept AbortSignal objects through a signal dictionary member.
• Convey that the operation got aborted by rejecting the promise with an "AbortError" DOMException.
• Reject immediately if the AbortSignal's aborted flag is already set, otherwise:
• Use the abort algorithms mechanism to observe changes to the AbortSignal object and do so in a manner that does not lead to clashes with other observers.

APIs not using promises should still adhere to the above as much as possible.

4. Nodes

4.1. Introduction to "The DOM"

In its original sense, "The DOM" is an API for accessing and manipulating documents (in particular, HTML and XML documents). In this specification, the term "document" is used for any markup-based resource, ranging from short static documents to long essays or reports with rich multimedia, as well as to fully-fledged interactive applications.

Each such document is represented as a node tree. Some of the nodes in a tree can have children, while others are always leaves.

To illustrate, consider this HTML document:

<!DOCTYPE html>
<html class=e>
 <head><title>Aliens?</title></head>
 <body>Why yes.</body>
</html>

It is represented as follows:

• Document
  • Doctype: html
  • Element: html class="e"
    • Element: head
      • Element: title
        • Text: Aliens?
    • Text: ⏎␣
    • Element: body
      • Text: Why yes.⏎

Note that, due to the magic that is HTML parsing, not all ASCII whitespace were turned into Text nodes, but the general concept is clear. Markup goes in, a tree of nodes comes out.

The most excellent Live DOM Viewer can be used to explore this matter in more detail.

4.2. Node tree

Document, DocumentType, DocumentFragment, Element, Text, ProcessingInstruction, and Comment objects (simply called nodes) participate in a tree, simply named the node tree.

A node tree is constrained as follows, expressed as a relationship between the type of node and its allowed children:

Document

In tree order:

1. Zero or more nodes each of which is ProcessingInstruction or Comment.

2. Optionally one DocumentType node.

3. Zero or more nodes each of which is ProcessingInstruction or Comment.

4. Optionally one Element node.

5. Zero or more nodes each of which is ProcessingInstruction or Comment.

DocumentFragment

Element

Zero or more nodes each of which is Element, Text, ProcessingInstruction, or Comment.

DocumentType

Text

ProcessingInstruction

Comment

None.

To determine the length of a node node, switch on node:

DocumentType

Zero.

Text

ProcessingInstruction

Comment

Its data’s length.

Any other node

Its number of children.

A node is considered empty if its length is zero.

4.2.1. Document tree

A document tree is a node tree whose root is a document.

The document element of a document is the element whose parent is that document, if it exists, and null otherwise.

Per the node tree constraints, there can be only one such element.

An element is in a document tree if its root is a document.

An element is in a document if it is in a document tree. The term in a document is no longer supposed to be used. It indicates that the standard using it has not been updated to account for shadow trees.

4.2.2. Shadow tree

A shadow tree is a node tree whose root is a shadow root.

A shadow root is always attached to another node tree through its host. A shadow tree is therefore never alone. The node tree of a shadow root’s host is sometimes referred to as the light tree.

A shadow tree’s corresponding light tree can be a shadow tree itself.

An element is connected if its shadow-including root is a document.

4.2.2.1. Slots

A shadow tree contains zero or more elements that are slots.

A slot can only be created through HTML’s slot element.

A slot has an associated name (a string). Unless stated otherwise it is the empty string.

Use these attribute change steps to update a slot’s name:

1. If element is a slot, localName is name, and namespace is null, then:

   1. If value is oldValue, then return.

   2. If value is null and oldValue is the empty string, then return.

   3. If value is the empty string and oldValue is null, then return.

   4. If value is null or the empty string, then set element’s name to the empty string.

   5. Otherwise, set element’s name to value.

   6. Run assign slottables for a tree with element’s root.

The first slot in a shadow tree, in tree order, whose name is the empty string, is sometimes known as the "default slot".

A slot has an associated assigned nodes (a list of slottables). Unless stated otherwise it is empty.

4.2.2.2. Slottables

Element and Text nodes are slottables.

A slot can be a slottable.

A slottable has an associated name (a string). Unless stated otherwise it is the empty string.

Use these attribute change steps to update a slottable’s name:

1. If localName is slot and namespace is null, then:

   1. If value is oldValue, then return.

   2. If value is null and oldValue is the empty string, then return.

   3. If value is the empty string and oldValue is null, then return.

   4. If value is null or the empty string, then set element’s name to the empty string.

   5. Otherwise, set element’s name to value.

   6. If element is assigned, then run assign slottables for element’s assigned slot.

   7. Run assign a slot for element.

A slottable has an associated assigned slot (null or a slot). Unless stated otherwise it is null. A slottable is assigned if its assigned slot is non-null.

4.2.2.3. Finding slots and slottables

To find a slot for a given slottable slottable and an optional open flag (unset unless stated otherwise), run these steps:

1. If slottable’s parent is null, then return null.

2. Let shadow be slottable’s parent’s shadow root.

3. If shadow is null, then return null.

4. If the open flag is set and shadow’s mode is not "open", then return null.

5. Return the first slot in tree order in shadow’s descendants whose name is slottable’s name, if any, and null otherwise.

To find slottables for a given slot slot, run these steps:

1. Let result be an empty list.

2. If slot’s root is not a shadow root, then return result.

3. Let host be slot’s root’s host.

4. For each slottable child of host, slottable, in tree order:

   1. Let foundSlot be the result of finding a slot given slottable.

   2. If foundSlot is slot, then append slottable to result.

5. Return result.

To find flattened slottables for a given slot slot, run these steps:

1. Let result be an empty list.

2. If slot’s root is not a shadow root, then return result.

3. Let slottables be the result of finding slottables given slot.

4. If slottables is the empty list, then append each slottable child of slot, in tree order, to slottables.

5. For each node in slottables:

   1. If node is a slot whose root is a shadow root, then:

      1. Let temporaryResult be the result of finding flattened slottables given node.

      2. Append each slottable in temporaryResult, in order, to result.

   2. Otherwise, append node to result.

6. Return result.

4.2.2.4. Assigning slottables and slots

To assign slottables for a slot slot, run these steps:

1. Let slottables be the result of finding slottables for slot.

2. If slottables and slot’s assigned nodes are not identical, then run signal a slot change for slot.

3. Set slot’s assigned nodes to slottables.

4. For each slottable in slottables, set slottable’s assigned slot to slot.

To assign slottables for a tree, given a node root, run assign slottables for each slot slot in root’s inclusive descendants, in tree order.

To assign a slot, given a slottable slottable, run these steps:

1. Let slot be the result of finding a slot with slottable.

2. If slot is non-null, then run assign slottables for slot.

4.2.2.5. Signaling slot change

Each similar-origin window agent has signal slots (a set of slots), which is initially empty. [HTML]

To signal a slot change, for a slot slot, run these steps:

1. Append slot to slot’s relevant agent’s signal slots.

2. Queue a mutation observer microtask.

4.2.3. Mutation algorithms

To ensure pre-insertion validity of a node into a parent before a child, run these steps:

1. If parent is not a Document, DocumentFragment, or Element node, then throw a "HierarchyRequestError" DOMException.

2. If node is a host-including inclusive ancestor of parent, then throw a "HierarchyRequestError" DOMException.

3. If child is non-null and its parent is not parent, then throw a "NotFoundError" DOMException.

4. If node is not a DocumentFragment, DocumentType, Element, Text, ProcessingInstruction, or Comment node, then throw a "HierarchyRequestError" DOMException.

5. If either node is a Text node and parent is a document, or node is a doctype and parent is not a document, then throw a "HierarchyRequestError" DOMException.

6. If parent is a document, and any of the statements below, switched on node, are true, then throw a "HierarchyRequestError" DOMException.

   DocumentFragment node

   If node has more than one element child or has a Text node child.

   Otherwise, if node has one element child and either parent has an element child, child is a doctype, or child is non-null and a doctype is following child.

   element

   parent has an element child, child is a doctype, or child is non-null and a doctype is following child.

   doctype

   parent has a doctype child, child is non-null and an element is preceding child, or child is null and parent has an element child.

To pre-insert a node into a parent before a child, run these steps:

1. Ensure pre-insertion validity of node into parent before child.

2. Let referenceChild be child.

3. If referenceChild is node, then set referenceChild to node’s next sibling.

4. Insert node into parent before referenceChild.

5. Return node.

Specifications may define insertion steps for all or some nodes. The algorithm is passed insertedNode, as indicated in the insert algorithm below.

Specifications may define children changed steps for all or some nodes. The algorithm is passed no argument and is called from insert, remove, and replace data.

To insert a node into a parent before a child, with an optional suppress observers flag, run these steps:

1. Let nodes be node’s children, if node is a DocumentFragment node; otherwise « node ».

2. Let count be nodes’s size.

3. If count is 0, then return.

4. If node is a DocumentFragment node, then:

   1. Remove its children with the suppress observers flag set.

   2. Queue a tree mutation record for node with « », nodes, null, and null.

      This step intentionally does not pay attention to the suppress observers flag.

5. If child is non-null, then:

   1. For each live range whose start node is parent and start offset is greater than child’s index, increase its start offset by count.

   2. For each live range whose end node is parent and end offset is greater than child’s index, increase its end offset by count.

6. Let previousSibling be child’s previous sibling or parent’s last child if child is null.

7. For each node in nodes, in tree order:

   1. Adopt node into parent’s node document.

   2. If child is null, then append node to parent’s children.

   3. Otherwise, insert node into parent’s children before child’s index.

   4. If parent is a shadow host and node is a slottable, then assign a slot for node.

   5. If parent’s root is a shadow root, and parent is a slot whose assigned nodes is the empty list, then run signal a slot change for parent.

   6. Run assign slottables for a tree with node’s root.

   7. For each shadow-including inclusive descendant inclusiveDescendant of node, in shadow-including tree order:

      1. Run the insertion steps with inclusiveDescendant.

      2. If inclusiveDescendant is connected, then:

         1. If inclusiveDescendant is custom, then enqueue a custom element callback reaction with inclusiveDescendant, callback name "connectedCallback", and an empty argument list.

         2. Otherwise, try to upgrade inclusiveDescendant.

            If this successfully upgrades inclusiveDescendant, its connectedCallback will be enqueued automatically during the upgrade an element algorithm.

8. If suppress observers flag is unset, then queue a tree mutation record for parent with nodes, « », previousSibling, and child.

9. Run the children changed steps for parent.

To append a node to a parent, pre-insert node into parent before null.

To replace a child with node within a parent, run these steps:

1. If parent is not a Document, DocumentFragment, or Element node, then throw a "HierarchyRequestError" DOMException.

2. If node is a host-including inclusive ancestor of parent, then throw a "HierarchyRequestError" DOMException.

3. If child’s parent is not parent, then throw a "NotFoundError" DOMException.

4. If node is not a DocumentFragment, DocumentType, Element, Text, ProcessingInstruction, or Comment node, then throw a "HierarchyRequestError" DOMException.

5. If either node is a Text node and parent is a document, or node is a doctype and parent is not a document, then throw a "HierarchyRequestError" DOMException.

6. If parent is a document, and any of the statements below, switched on node, are true, then throw a "HierarchyRequestError" DOMException.

   DocumentFragment node

   If node has more than one element child or has a Text node child.

   Otherwise, if node has one element child and either parent has an element child that is not child or a doctype is following child.

   element

   parent has an element child that is not child or a doctype is following child.

   doctype

   parent has a doctype child that is not child, or an element is preceding child.

   The above statements differ from the pre-insert algorithm.

7. Let referenceChild be child’s next sibling.

8. If referenceChild is node, then set referenceChild to node’s next sibling.

9. Let previousSibling be child’s previous sibling.

10. Let removedNodes be the empty set.

11. If child’s parent is non-null, then:

    1. Set removedNodes to « child ».

    2. Remove child with the suppress observers flag set.

    The above can only be false if child is node.

12. Let nodes be node’s children if node is a DocumentFragment node; otherwise « node ».

13. Insert node into parent before referenceChild with the suppress observers flag set.

14. Queue a tree mutation record for parent with nodes, removedNodes, previousSibling, and referenceChild.

15. Return child.

To replace all with a node within a parent, run these steps:

1. Let removedNodes be parent’s children.

2. Let addedNodes be the empty set.

3. If node is a DocumentFragment node, then set addedNodes to node’s children.

4. Otherwise, if node is non-null, set addedNodes to « node ».

5. Remove all parent’s children, in tree order, with the suppress observers flag set.

6. If node is non-null, then insert node into parent before null with the suppress observers flag set.

7. If either addedNodes or removedNodes is not empty, then queue a tree mutation record for parent with addedNodes, removedNodes, null, and null.

This algorithm does not make any checks with regards to the node tree constraints. Specification authors need to use it wisely.

To pre-remove a child from a parent, run these steps:

1. If child’s parent is not parent, then throw a "NotFoundError" DOMException.

2. Remove child.

3. Return child.

Specifications may define removing steps for all or some nodes. The algorithm is passed removedNode, and optionally oldParent, as indicated in the remove algorithm below.

To remove a node, with an optional suppress observers flag, run these steps:

1. Let parent be node’s parent

2. Assert: parent is non-null.

3. Let index be node’s index.

4. For each live range whose start node is an inclusive descendant of node, set its start to (parent, index).

5. For each live range whose end node is an inclusive descendant of node, set its end to (parent, index).

6. For each live range whose start node is parent and start offset is greater than index, decrease its start offset by 1.

7. For each live range whose end node is parent and end offset is greater than index, decrease its end offset by 1.

8. For each NodeIterator object iterator whose root’s node document is node’s node document, run the NodeIterator pre-removing steps given node and iterator.

9. Let oldPreviousSibling be node’s previous sibling.

10. Let oldNextSibling be node’s next sibling.

11. Remove node from its parent’s children.

12. If node is assigned, then run assign slottables for node’s assigned slot.

13. If parent’s root is a shadow root, and parent is a slot whose assigned nodes is the empty list, then run signal a slot change for parent.

14. If node has an inclusive descendant that is a slot, then:

    1. Run assign slottables for a tree with parent’s root.

    2. Run assign slottables for a tree with node.

15. Run the removing steps with node and parent.

16. Let isParentConnected be parent’s connected.

17. If node is custom and isParentConnected is true, then enqueue a custom element callback reaction with node, callback name "disconnectedCallback", and an empty argument list.

    It is intentional for now that custom elements do not get parent passed. This might change in the future if there is a need.

18. For each shadow-including descendant descendant of node, in shadow-including tree order, then:

    1. Run the removing steps with descendant.

    2. If descendant is custom and isParentConnected is true, then enqueue a custom element callback reaction with descendant, callback name "disconnectedCallback", and an empty argument list.

19. For each inclusive ancestor inclusiveAncestor of parent, and then for each registered of inclusiveAncestor’s registered observer list, if registered’s options’s subtree is true, then append a new transient registered observer whose observer is registered’s observer, options is registered’s options, and source is registered to node’s registered observer list.

20. If suppress observers flag is unset, then queue a tree mutation record for parent with « », « node », oldPreviousSibling, and oldNextSibling.

21. Run the children changed steps for parent.

4.2.4. Mixin NonElementParentNode

Web compatibility prevents the getElementById() method from being exposed on elements (and therefore on ParentNode).

interface mixin NonElementParentNode {
  Element? getElementById(DOMString elementId);
};
Document includes NonElementParentNode;
DocumentFragment includes NonElementParentNode;

node . getElementById(elementId)

Returns the first element within node’s descendants whose ID is elementId.

The getElementById(elementId) method, when invoked, must return the first element, in tree order, within this’s descendants, whose ID is elementId, and null if there is no such element otherwise.

4.2.5. Mixin DocumentOrShadowRoot

interface mixin DocumentOrShadowRoot {
};
Document includes DocumentOrShadowRoot;
ShadowRoot includes DocumentOrShadowRoot;

The DocumentOrShadowRoot mixin is expected to be used by other standards that want to define APIs shared between documents and shadow roots.

4.2.6. Mixin ParentNode

To convert nodes into a node, given nodes and document, run these steps:

1. Let node be null.

2. Replace each string in nodes with a new Text node whose data is the string and node document is document.

3. If nodes contains one node, set node to that node.

4. Otherwise, set node to a new DocumentFragment whose node document is document, and then append each node in nodes, if any, to it.

5. Return node.

interface mixin ParentNode {
  [SameObject] readonly attribute HTMLCollection children;
  readonly attribute Element? firstElementChild;
  readonly attribute Element? lastElementChild;
  readonly attribute unsigned long childElementCount;

  [CEReactions, Unscopable] undefined prepend((Node or DOMString)... nodes);
  [CEReactions, Unscopable] undefined append((Node or DOMString)... nodes);
  [CEReactions, Unscopable] undefined replaceChildren((Node or DOMString)... nodes);

  Element? querySelector(DOMString selectors);
  [NewObject] NodeList querySelectorAll(DOMString selectors);
};
Document includes ParentNode;
DocumentFragment includes ParentNode;
Element includes ParentNode;

collection = node . children

Returns the child elements.

element = node . firstElementChild

Returns the first child that is an element, and null otherwise.

element = node . lastElementChild

Returns the last child that is an element, and null otherwise.

node . prepend(nodes)

Inserts nodes before the first child of node, while replacing strings in nodes with equivalent Text nodes.

Throws a "HierarchyRequestError" DOMException if the constraints of the node tree are violated.

node . append(nodes)

Inserts nodes after the last child of node, while replacing strings in nodes with equivalent Text nodes.

Throws a "HierarchyRequestError" DOMException if the constraints of the node tree are violated.

node . replaceChildren(nodes)

Replace all children of node with nodes, while replacing strings in nodes with equivalent Text nodes.

Throws a "HierarchyRequestError" DOMException if the constraints of the node tree are violated.

node . querySelector(selectors)

Returns the first element that is a descendant of node that matches selectors.

node . querySelectorAll(selectors)

Returns all element descendants of node that match selectors.

The children attribute’s getter must return an HTMLCollection collection rooted at this matching only element children.

The firstElementChild attribute’s getter must return the first child that is an element, and null otherwise.

The lastElementChild attribute’s getter must return the last child that is an element, and null otherwise.

The childElementCount attribute’s getter must return the number of children of this that are elements.

The prepend(nodes) method, when invoked, must run these steps:

1. Let node be the result of converting nodes into a node given nodes and this’s node document.

2. Pre-insert node into this before this’s first child.

The append(nodes) method, when invoked, must run these steps:

1. Let node be the result of converting nodes into a node given nodes and this’s node document.

2. Append node to this.

The replaceChildren(nodes) method, when invoked, must run these steps:

1. Let node be the result of converting nodes into a node given nodes and this’s node document.

2. Ensure pre-insertion validity of node into this before null.

3. Replace all with node within this.

The querySelector(selectors) method, when invoked, must return the first result of running scope-match a selectors string selectors against this, if the result is not an empty list, and null otherwise.