DOM

Goals

This specification standardizes the DOM. It does so as follows:

1. By consolidating DOM Level 3 Core [DOM-Level-3-Core], Element Traversal [ELEMENTTRAVERSAL], Selectors API Level 2 [SELECTORS-API2], the "DOM Event Architecture" and "Basic Event Interfaces" chapters of DOM Level 3 Events [uievents-20031107] (specific types of events do not belong in the DOM Standard), and DOM Level 2 Traversal and Range [DOM-Level-2-Traversal-Range], and:

   • Aligning them with the JavaScript ecosystem where possible.
   • Aligning them with existing implementations.
   • Simplifying them as much as possible.

2. By moving features from the HTML Standard [HTML] that make more sense to be specified as part of the DOM Standard.

3. By defining a replacement for the "Mutation Events" and "Mutation Name Event Types" chapters of DOM Level 3 Events [uievents-20031107] as the old model was problematic.

   The old model is expected to be removed from implementations in due course.

4. By defining new features that simplify common DOM operations.

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 means the object on which the algorithm, attribute getter, attribute setter, or method being discussed was called. When the context object is unambiguous, the term can be omitted.

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. It is passed as argument to 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).

Now while typically events are dispatched by the user agent as the result of user interaction or the completion of some task, applications can dispatch events themselves, 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. First all object’s ancestor event listeners whose capture variable is set to true are invoked, in tree order. Second, object’s own event listeners are invoked. And finally, and only if event’s bubbles attribute value is true, object’s ancestor event listeners are invoked again, but now in reverse tree order.

Lets 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

[Constructor(DOMString type, optional EventInit eventInitDict),
 Exposed=(Window,Worker,AudioWorklet)]
interface Event {
  readonly attribute DOMString type;
  readonly attribute EventTarget? target;
  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;

  void stopPropagation();
           attribute boolean cancelBubble; // historical alias of .stopPropagation
  void stopImmediatePropagation();

  readonly attribute boolean bubbles;
  readonly attribute boolean cancelable;
  void preventDefault();
  readonly attribute boolean defaultPrevented;
  readonly attribute boolean composed;

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

  void initEvent(DOMString type, optional boolean bubbles = false, optional boolean cancelable = false); // historical
};

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.

An event has an associated relatedTarget (null or an EventTarget object). Unless stated otherwise it is null.

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

An event has an associated path. A path is a list of structs. Each struct consists of an item (an EventTarget object), target (null or an EventTarget object), a relatedTarget (null or an EventTarget object), root-of-closed-tree (a boolean), and a slot-in-closed-tree (a boolean). A path is initially the empty list.

Specifications may define retargeting steps for all or some events. The algorithm is passed event, as indicated in the dispatch algorithm below.

event = new Event(type [, eventInitDict])

Returns a new event whose type attribute value is set to type. The optional 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.

event . currentTarget

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

event . composedPath()

Returns the item 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 attribute value’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 attribute value, 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 and currentTarget attributes must return the values they were initialized to. When an event is created the attributes must be initialized to null.

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

1. Let reversedComposedPath be an empty list.

2. Let hiddenSubtreeLevel be 0.

3. Let hasSeenCurrentTarget be false.

4. Let currentTarget be context object’s currentTarget attribute value.

5. Let reversedPath be context object’s path, in reverse order.

6. For each struct in reversedPath:

   1. If struct’s item is currentTarget, then set hasSeenCurrentTarget to true.

   2. Otherwise, if hasSeenCurrentTarget is true and struct’s root-of-closed-tree is true, then increase hiddenSubtreeLevel by

   3. If hiddenSubtreeLevel is 0, then append struct’s item to reversedComposedPath.

   4. If struct’s slot-in-closed-tree is true and hiddenSubtreeLevel is greater than 0, then decrease hiddenSubtreeLevel by

7. Return reversedComposedPath, in reverse order.

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 attribute value.

AT_TARGET (numeric value 2)

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

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 attribute value.

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 the context object’s stop propagation flag.

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

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

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

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

The preventDefault() method, when invoked, must set the canceled flag if the cancelable attribute value is true and the in passive listener flag is unset.

This means 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 must return true if context object’s canceled flag is set, and false otherwise.

The composed attribute’s getter must return true if context object’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 the initialized flag.
2. Unset the stop propagation flag, stop immediate propagation flag, and canceled flag.
3. Set the isTrusted attribute to false.
4. Set the target attribute to null.
5. Set the type attribute to type.
6. Set the bubbles attribute to bubbles.
7. Set the cancelable attribute to cancelable.

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

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

2. Initialize context object with type, bubbles, and cancelable.

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

2.3. Interface CustomEvent

[Constructor(DOMString type, optional CustomEventInit eventInitDict),
 Exposed=(Window,Worker)]
interface CustomEvent : Event {
  readonly attribute any detail;

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

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 optional 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 context object’s dispatch flag is set, then return.
2. Initialize the context object with type, bubbles, and cancelable.
3. Set context object’s detail attribute to detail.

2.4. Constructing events

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. Return event.

2.5. 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.

2.6. Interface EventTarget

[Constructor,
 Exposed=(Window,Worker,AudioWorklet)]
interface EventTarget {
  void addEventListener(DOMString type, EventListener? callback, optional (AddEventListenerOptions or boolean) options);
  void removeEventListener(DOMString type, EventListener? callback, optional (EventListenerOptions or boolean) options);
  boolean dispatchEvent(Event event);
};

callback interface EventListener {
  void 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 §2.7 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’s relevant global object is a ServiceWorkerGlobalScope object and its associated service worker’s script resource’s has ever been evaluated flag is set, then throw a TypeError. [SERVICE-WORKERS]

   To optimize storing the event types allowed for the service worker and to avoid non-deterministic changes to the event listeners, invocation of the method is allowed only during the very first evaluation of the service worker script.

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 the context object and an event listener whose type is type, callback is callback, capture is capture, passive is passive, and once is once.

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

1. If the context object’s relevant global object is a ServiceWorkerGlobalScope object and its associated service worker’s script resource’s has ever been evaluated flag is set, then throw a TypeError. [SERVICE-WORKERS]

2. Let capture be the result of flattening options.

3. If the context object’s event listener list contains an event listener whose type is type, callback is callback, and capture is capture, then set that event listener’s removed to true and remove it from the context object’s event listener list.

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 context object.

2.7. 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).

2.8. 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 relatedTarget be the result of retargeting event’s relatedTarget against target if event’s relatedTarget is non-null, and null otherwise.

4. If target is relatedTarget and target is not event’s relatedTarget, then return true.

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

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

7. Let activationTarget be target, if isActivationEvent is true and target has activation behavior, and null otherwise.

8. Let slotable be target, if target is assigned, and null otherwise.

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

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

11. While parent is non-null:

    1. If slotable is non-null:

       1. Assert: parent is a slot.

       2. Set slotable 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 assigned, then set slotable to parent.

    3. Let relatedTarget be the result of retargeting event’s relatedTarget against parent if event’s relatedTarget is non-null, and null otherwise.

    4. If 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, and slot-in-closed-tree.

    5. Otherwise, if parent and relatedTarget are identical, then set parent to null.

    6. 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, and slot-in-closed-tree.

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

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

12. Set event’s eventPhase attribute to CAPTURING_PHASE.

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

14. For each tuple in event’s path, in reverse order:

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

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

    3. Run the retargeting steps with event.

    4. If tuple’s target is null, then invoke tuple’s item with event and legacyOutputDidListenersThrowFlag if given.

15. For each tuple in event’s path, in order:

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

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

    3. Run the retargeting steps with event.

    4. If tuple’s target is non-null, then set event’s eventPhase attribute to AT_TARGET.

    5. Otherwise, set event’s eventPhase attribute to BUBBLING_PHASE.

    6. If either event’s eventPhase attribute is BUBBLING_PHASE and event’s bubbles attribute is true or event’s eventPhase attribute is AT_TARGET, then invoke tuple’s item with event and legacyOutputDidListenersThrowFlag if given.

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

17. Set event’s eventPhase attribute to NONE.

18. If target's root is a shadow root, then set event’s target attribute and event’s relatedTarget to null.

19. Set event’s currentTarget attribute to null.

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

21. 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.

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

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

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

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

3. Append a new struct to event’s path whose item is target, target is targetOverride, relatedTarget is relatedTarget, root-of-closed-tree is root-of-closed-tree, and slot-in-closed-tree is slot-in-closed-tree.

To invoke an object with event and an optional legacyOutputDidListenersThrowFlag, run these steps:

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

2. Let listeners be a new list.

3. For each listener of object’s event listener list, append listener to listeners.

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

4. Initialize event’s currentTarget attribute to object.

5. Let found be the result of running inner invoke object with event, listeners, and legacyOutputDidListenersThrowFlag if given.

6. 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 object with event, listeners, and legacyOutputDidListenersThrowFlag if given.

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

To inner invoke an object with event, listeners, 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 event’s eventPhase attribute value is CAPTURING_PHASE and listener’s capture is false, then continue.

   4. If event’s eventPhase attribute value is BUBBLING_PHASE and listener’s capture is true, then continue.

   5. If listener’s once is true, then remove listener from object’s event listener list.

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

   7. 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]

   8. Unset event’s in passive listener flag.

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

3. Return found.

2.9. 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.

2.10. 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

[Constructor,
 Exposed=(Window,Worker)]
interface AbortController {
  [SameObject] readonly attribute AbortSignal signal;

  void 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 AbortController() constructor, when invoked, must run these steps:

1. Let signal be a new AbortSignal object.

2. Let controller be a new AbortController object whose signal is signal.

3. Return controller.

The signal attribute’s getter must return context object’s signal.

The abort() method, when invoked, must signal abort on context object’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 attribute’s getter must return true if context object’s aborted flag is set, and false otherwise.

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:

Acknowledgments

There have been a lot of people that have helped make DOM more interoperable over the years and thereby furthered the goals of this standard. Likewise many people have helped making this standard what it is today.

With that, many thanks to Adam Klein, Adrian Bateman, Aleksey Shvayka, Alex Komoroske, Alex Russell, Anthony Ramine, Arkadiusz Michalski, Arnaud Le Hors, Arun Ranganathan, Björn Höhrmann, Boris Zbarsky, Brandon Payton, Brandon Slade, Brandon Wallace, Brian Kardell, Cameron McCormack, Chris Dumez, Chris Paris, Chris Rebert, Cyrille Tuzi, Daniel Glazman, Darin Fisher, David Bruant, David Flanagan, David Håsäther, David Hyatt, Deepak Sherveghar, Dethe Elza, Dimitri Glazkov, Domenic Denicola, Dominic Cooney, Dominique Hazaël-Massieux, Don Jordan, Doug Schepers, Edgar Chen, Elisée Maurer, Elliott Sprehn, Eric Bidelman, Erik Arvidsson, Gary Kacmarcik, Gavin Nicol, Geoffrey Sneddon, Giorgio Liscio, Glen Huang, Glenn Adams, Glenn Maynard, Hajime Morrita, Harald Alvestrand, Hayato Ito, Henri Sivonen, Hongchan Choi, Hunan Rostomyan, Ian Hickson, Igor Bukanov, Jacob Rossi, Jake Archibald, Jake Verbaten, James Graham, James Greene, James Robinson, Jeffrey Yasskin, Jens Lindström, Jesse McCarthy, Jinho Bang, João Eiras, Joe Kesselman, John Atkins, John Dai, Jonas Sicking, Jonathan Robie, Joris van der Wel, Joshua Bell, Jungkee Song, Justin Summerlin, 呂康豪 (Kang-Hao Lu), Kevin Sweeney, Kirill Topolyan, Koji Ishii, Lachlan Hunt, Lauren Wood, Magne Andersson, Majid Valipour, Malte Ubl, Manish Goregaokar, Manish Tripathi, Marcos Caceres, Mark Miller, Martijn van der Ven, Mats Palmgren, Mounir Lamouri, Michael™ Smith, Mike Champion, Mike Taylor, Mike West, Ojan Vafai, Oliver Nightingale, Olli Pettay, Ondřej Žára, Peter Sharpe, Philip Jägenstedt, Philippe Le Hégaret, Ra’Shaun Stovall (Snuggs), Rafael Weinstein, Richard Bradshaw, Rick Byers, Rick Waldron, Robbert Broersma, Robin Berjon, Roland Steiner, Rune F. Halvorsen, Russell Bicknell, Ruud Steltenpool, Ryosuke Niwa, Sam Dutton, Samuel Giles, Sebastian Mayr, Seo Sanghyeon, Sergey G. Grekhov, Shiki Okasaka, Shinya Kawanaka, Simon Pieters, Stef Busking, Steve Byrne, Stig Halvorsen, Tab Atkins, Takashi Sakamoto, Takayoshi Kochi, Theresa O’Connor, timeless, Timo Tijhof, Tobie Langel, Tom Pixley, Travis Leithead, triple-underscore, Veli Şenol, Vidur Apparao, Warren He, Xidorn Quan, Yehuda Katz, Yoav Weiss, Yoichi Osato, Yoshinori Sano, and Zack Weinberg for being awesome!

This standard is written by Anne van Kesteren (Mozilla, annevk@annevk.nl) with substantial contributions from Aryeh Gregor (ayg@aryeh.name) and Ms2ger (ms2ger@gmail.com).

Part of the revision history of the integration points related to custom elements can be found in the w3c/webcomponents repository, which is available under the W3C Permissive Document License.

Copyright © 2018 WHATWG (Apple, Google, Mozilla, Microsoft) and W3C^® (MIT, ERCIM, Keio, Beihang). This work is licensed under a Creative Commons Attribution 4.0 International License. W3C liability and trademark rules apply.

W3C Recommendation Status

This Living Standard is also endorsed as a W3C (Candidate Recommendation|Proposed Recommendation|Recommendation) (choose one and update as it proceeds through the process) under the terms of the Memorandum of Understanding. For those purposes, it contains the following metadata:

Canonical version / latest Living Standard:

https://dom.spec.whatwg.org/

This version:

https://dom.spec.whatwg.org/review-drafts/2018-03-21

W3C Version Mnenomic:

DOM 2018-03

Status of This Document

This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at https://www.w3.org/TR/.

[if Recommendation] Please check the errata for any critical errors or issues reported since publication.

[if Candidate Recommendation] The following features are at-risk for the purpose of the W3C Candidate Recommendation:

• [list of features and their associated sections/characterization]

[if [Proposed] Recommendation] The following are non-normative for the purpose of the W3C [Proposed] Recommendation:

• [list of sections/characterization]

The W3C HTML Working Group co-produced this document under the W3C Patent Policy and the 1 February 2018 W3C Process Document. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.