UI Events

1. Introduction

1.1. Overview

UI Events is designed with two main goals. The first goal is the design of an event system which allows registration of event listeners and describes event flow through a tree structure. Additionally, the specification will provide standard modules of events for user interface control and document mutation notifications, including defined contextual information for each of these event modules.

The second goal of UI Events is to provide a common subset of the current event systems used in existing browsers. This is intended to foster interoperability of existing scripts and content. It is not expected that this goal will be met with full backwards compatibility. However, the specification attempts to achieve this when possible.

1.2. Conformance

This section is normative.

Within this specification, the key words MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD, SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL are to be interpreted as described in [RFC2119].

This specification is to be understood in the context of the DOM Level 3 Core specification [DOM-Level-3-Core] and the general considerations for DOM implementations apply. For example, handling of namespace URIs is discussed in XML Namespaces. For additional information about conformance, please see the DOM Level 3 Core specification [DOM-Level-3-Core]. A user agent is not required to conform to the entirety of another specification in order to conform to this specification, but it MUST conform to the specific parts of any other specification which are called out in this specification (e.g., a conforming UI Events user agent MUST support the DOMString data type as defined in [WebIDL], but need not support every method or data type defined in [WebIDL] in order to conform to UI Events).

This specification defines several classes of conformance for different user agents, specifications, and content authors:

1.2.1. Web browsers and other dynamic or interactive user agents

A dynamic or interactive user agent, referred to here as a browser (be it a Web browser, AT (Accessibility Technology) application, or other similar program), conforms to UI Events if it supports:

• the Core module defined in [DOM-Level-3-Core]

• the § 3.1 Event dispatch and DOM event flow mechanism

• all the interfaces and events with their associated methods, attributes, and semantics defined in this specification with the exception of those marked as deprecated (a conforming user agent MAY implement the deprecated interfaces, events, or APIs for backwards compatibility, but is not required to do so in order to be conforming)

• the complete set of key and code values defined in [UIEvents-Key] and [UIEvents-Code] (subject to platform availability), and

• all other normative requirements defined in this specification.

A conforming browser MUST dispatch events appropriate to the given EventTarget when the conditions defined for that event type have been met.

A browser conforms specifically to UI Events if it implements the interfaces and related event types specified in § 5 Event Types.

A conforming browser MUST support scripting, declarative interactivity, or some other means of detecting and dispatching events in the manner described by this specification, and MUST support the APIs specified for that event type.

In addition to meeting all other conformance criteria, a conforming browser MAY implement features of this specification marked as deprecated, for backwards compatibility with existing content, but such implementation is discouraged.

A conforming browser MAY also support features not found in this specification, but which use the § 3.1 Event dispatch and DOM event flow mechanism, interfaces, events, or other features defined in this specification, and MAY implement additional interfaces and event types appropriate to that implementation. Such features can be later standardized in future specifications.

A browser which does not conform to all required portions of this specification MUST NOT claim conformance to UI Events. Such an implementation which does conform to portions of this specification MAY claim conformance to those specific portions.

A conforming browser MUST also be a conforming implementation of the IDL fragments in this specification, as described in the Web IDL specification [WebIDL].

1.2.2. Authoring tools

A content authoring tool conforms to UI Events if it produces content which uses the event types and § 3.1 Event dispatch and DOM event flow model, consistent in a manner as defined in this specification.

A content authoring tool MUST NOT claim conformance to UI Events for content it produces which uses features of this specification marked as deprecated in this specification.

A conforming content authoring tool SHOULD provide to the content author a means to use all event types and interfaces appropriate to all host languages in the content document being produced.

1.2.3. Content authors and content

A content author creates conforming UI Events content if that content uses the event types and § 3.1 Event dispatch and DOM event flow model, consistent in a manner as defined in this specification.

A content author SHOULD NOT use features of this specification marked as deprecated, but SHOULD rely instead upon replacement mechanisms defined in this specification and elsewhere.

Conforming content MUST use the semantics of the interfaces and event types as described in this specification.

Content authors are advised to follow best practices as described in accessibility and internationalization guideline specifications.

1.2.4. Specifications and host languages

A specification or host language conforms to UI Events if it references and uses the § 3.1 Event dispatch and DOM event flow mechanism, interfaces, events, or other features defined in [DOM], and does not extend these features in incompatible ways.

A specification or host language conforms specifically to UI Events if it references and uses the interfaces and related event types specified in § 5 Event Types. A conforming specification MAY define additional interfaces and event types appropriate to that specification, or MAY extend the UI Events interfaces and event types in a manner that does not contradict or conflict with the definitions of those interfaces and event types in this specification.

Specifications or host languages which reference UI Events SHOULD NOT use or recommend features of this specification marked as deprecated, but SHOULD use or recommend the indicated replacement for that the feature (if available).

2. Stylistic Conventions

This specification follows the Proposed W3C Specification Conventions, with the following supplemental additions:

• The key cap printed on a key is shown as ↓, = or Q. This is used to refer to a key from the user’s perspective without regard for the key and code values in the generated KeyboardEvent.

• Glyphs representing character are shown as: "𣧂".

• Unicode character encodings are shown as: U+003d.

• Names of key values generated by a key press (i.e., the value of KeyboardEvent.key) are shown as: "ArrowDown", "=", "q" or "Q".

• Names of key codes associated with the physical keys (i.e., the value of KeyboardEvent.code) are shown as: "ArrowDown", "Equal" or "KeyQ".

In addition, certain terms are used in this specification with particular meanings. The term implementation applies to a browser, content authoring tool, or other user agent that implements this specification, while a content author is a person who writes script or code that takes advantage of the interfaces, methods, attributes, events, and other features described in this specification in order to make Web applications, and a user is the person who uses those Web applications in an implementation.

And finally:

This is a note.

This is an open issue.

This is a warning.

interface Example {
    // This is an IDL definition.
};

3. DOM Event Architecture

This section is non-normative. Refer to [DOM] for a normative description of the DOM event architecture

3.1. Event dispatch and DOM event flow

This section gives a brief overview of the event dispatch mechanism and describes how events propagate through the DOM tree. Applications can dispatch event objects using the dispatchEvent() method, and the event object will propagate through the DOM tree as determined by the DOM event flow.

Event objects are dispatched to an event target. But before dispatch can begin, the event object’s propagation path must first be determined.

The propagation path is an ordered list of current event targets through which the event passes. This propagation path reflects the hierarchical tree structure of the document. The last item in the list is the event target, and the preceding items in the list are referred to as the target’s ancestors, with the immediately preceding item as the target’s parent.

Once the propagation path has been determined, the event object passes through one or more event phases. There are three event phases: capture phase, target phase and bubble phase. Event objects complete these phases as described below. A phase will be skipped if it is not supported, or if the event object’s propagation has been stopped. For example, if the bubbles attribute is set to false, the bubble phase will be skipped, and if stopPropagation() has been called prior to the dispatch, all phases will be skipped.

• The capture phase: The event object propagates through the target’s ancestors from the Window to the target’s parent. This phase is also known as the capturing phase.

• The target phase: The event object arrives at the event object’s event target. This phase is also known as the at-target phase. If the event type indicates that the event doesn’t bubble, then the event object will halt after completion of this phase.

• The bubble phase: The event object propagates through the target’s ancestors in reverse order, starting with the target’s parent and ending with the Window. This phase is also known as the bubbling phase.

3.2. Default actions and cancelable events

Events are typically dispatched by the implementation as a result of a user action, in response to the completion of a task, or to signal progress during asynchronous activity (such as a network request). Some events can be used to control the behavior that the implementation may take next (or undo an action that the implementation already took). Events in this category are said to be cancelable and the behavior they cancel is called their default action. Cancelable event objects can be associated with one or more 'default actions'. To cancel an event, call the preventDefault() method.

A mousedown event is dispatched immediately after the user presses down a button on a pointing device (typically a mouse). One possible default action taken by the implementation is to set up a state machine that allows the user to drag images or select text. The default action depends on what happens next — for example, if the user’s pointing device is over text, a text selection might begin. If the user’s pointing device is over an image, then an image-drag action could begin. Preventing the default action of a mousedown event prevents these actions from occurring.

Default actions are usually performed after the event dispatch has been completed, but in exceptional cases they may also be performed immediately before the event is dispatched.

The default action associated with the click event on <input type="checkbox"> elements toggles the checked IDL attribute value of that element. If the click event’s default action is cancelled, then the value is restored to its former state.

When an event is canceled, then the conditional default actions associated with the event is skipped (or as mentioned above, if the default actions are carried out before the dispatch, their effect is undone). Whether an event object is cancelable is indicated by the cancelable attribute. Calling preventDefault() stops all related default actions of an event object. The defaultPrevented attribute indicates whether an event has already been canceled (e.g., by a prior event listener). If the DOM application itself initiated the dispatch, then the return value of the dispatchEvent() method indicates whether the event object was cancelled.

Many implementations additionally interpret an event listener’s return value, such as the value false, to mean that the default action of cancelable events will be cancelled (though window.onerror handlers are cancelled by returning true).

3.3. Synchronous and asynchronous events

Events may be dispatched either synchronously or asynchronously.

Events which are synchronous (sync events) are treated as if they are in a virtual queue in a first-in-first-out model, ordered by sequence of temporal occurrence with respect to other events, to changes in the DOM, and to user interaction. Each event in this virtual queue is delayed until the previous event has completed its propagation behavior, or been canceled. Some sync events are driven by a specific device or process, such as mouse button events. These events are governed by the event order algorithms defined for that set of events, and user agents will dispatch these events in the defined order.

Events which are asynchronous (async events) may be dispatched as the results of the action are completed, with no relation to other events, to other changes in the DOM, nor to user interaction.

During loading of a document, an inline script element is parsed and executed. The load event is queued to be fired asynchronously at the script element. However, because it is an async event, its order with relation to other synchronous events fired during document load (such as the DOMContentLoaded event from [HTML5]) is not guaranteed.

3.4. Trusted events

Events that are generated by the user agent, either as a result of user interaction, or as a direct result of changes to the DOM, are trusted by the user agent with privileges that are not afforded to events generated by script through the createEvent() method, modified using the initEvent() method, or dispatched via the dispatchEvent() method. The isTrusted attribute of trusted events has a value of true, while untrusted events have a isTrusted attribute value of false.

Most untrusted events will not trigger default actions, with the exception of the click event. This event always triggers the default action, even if the isTrusted attribute is false (this behavior is retained for backward-compatibility). All other untrusted events behave as if the preventDefault() method had been called on that event.

3.5. Activation triggers and behavior

Certain event targets (such as a link or button element) may have associated activation behavior (such as following a link) that implementations perform in response to an activation trigger (such as clicking a link).

Both HTML and SVG have an <a> element which indicates a link. Relevant activation triggers for an <a> element are a click event on the text or image content of the <a> element, or a keydown event with a key attribute value of "Enter" key when the <a> element has focus. The activation behavior for an <a> element is normally to change the content of the window to the content of the new document, in the case of external links, or to reposition the current document relative to the new anchor, in the case of internal links.

An activation trigger is a user action or an event which indicates to the implementation that an activation behavior should be initiated. User-initiated activation triggers include clicking a mouse button on an activatable element, pressing the Enter key when an activatable element has focus, or pressing a key that is somehow linked to an activatable element (a hotkey or access key) even when that element does not have focus. Event-based activation triggers may include timer-based events that activate an element at a certain clock time or after a certain time period has elapsed, progress events after a certain action has been completed, or many other condition-based or state-based events.

3.6. Constructing Mouse and Keyboard Events

Generally, when a constructor of an Event interface, or of an interface inherited from the Event interface, is invoked, the steps described in [DOM] should be followed. However the KeyboardEvent and MouseEvent interfaces provide additional dictionary members for initializing the internal state of the Event object’s key modifiers: specifically, the internal state queried for using the getModifierState() and getModifierState() methods. This section supplements the DOM4 steps for intializing a new Event object with these optional modifier states.

For the purposes of constructing a KeyboardEvent, MouseEvent, or object derived from these objects using the algorithm below, all KeyboardEvent, MouseEvent, and derived objects have internal key modifier state which can be set and retrieved using the key modifier names described in the Modifier Keys table in [UIEvents-Key].

The following steps supplement the algorithm defined for constructing events in DOM4:

• If the Event being constructed is a KeyboardEvent or MouseEvent object or an object that derives from either of these, and a EventModifierInit argument was provided to the constructor, then run the following sub-steps:

  • For each EventModifierInit argument, if the dictionary member begins with the string "modifier", then let the key modifier name be the dictionary member’s name excluding the prefix "modifier", and set the Event object’s internal key modifier state that matches the key modifier name to the corresponding value.

4. Basic Event Interfaces

The basic event interfaces defined in [DOM] are fundamental to UI Events. These basic event interfaces MUST always be supported by the implementation:

• The Event interface and its following constants, methods and attributes:

  • NONE constant

  • CAPTURING_PHASE constant

  • AT_TARGET constant

  • BUBBLING_PHASE constant

  • type attribute

  • target attribute

  • currentTarget attribute

  • eventPhase attribute

  • bubbles attribute

  • cancelable attribute

  • composed attribute

  • timeStamp attribute

  • defaultPrevented attribute

  • isTrusted attribute

  • stopPropagation() method

  • stopImmediatePropagation() method

  • preventDefault() method

  • initEvent() method

• The CustomEvent interface and its following method and attribute:

  • initCustomEvent() method

  • detail attribute

• The EventTarget interface and its following methods:

  • addEventListener() method

  • removeEventListener() method

  • dispatchEvent() method

• The EventListener interface and its handleEvent method

• The Document interface’s createEvent() method

The event types defined in this specification derive from these basic interfaces, and MUST inherit all of the attributes, methods, and constants of the interfaces they derive from.

The following chart describes the inheritance structure of the interfaces described in this specification.

4.1. List of Event Types

Each event MUST be associated with a type, called event type and available as the type attribute on the event object. The event type MUST be of type DOMString.

Depending on the level of DOM support, or the devices used for display (e.g., screen) or interaction (e.g., mouse, keyboard, touch screen, or voice), these event types can be generated by the implementation. When used with an [XML] or [HTML5] application, the specifications of those languages MAY restrict the semantics and scope (in particular the possible event targets) associated with an event type. Refer to the specification defining the language used in order to find those restrictions or to find event types that are not defined in this document.

The following table provides an informative summary of the event types described in this specification.

For a list of events which are deprecated in this specification, see the Legacy Event Types appendix at the end of this document.

The following is one way to interpret the above tables: the load event will trigger event listeners attached on Element nodes for that event and on the capture and target phases. This event is not cancelable. If an event listener for the load event is attached to a node other than Window, Document, or Element nodes, or if it is attached to the bubbling phase only, this event listener would not be triggered.

Don’t interpret the above tables as definitive for the listed event types. For example, the load event is used in other specifications, for example, in XMLHttpRequest. Similarly, dispatchEvent() can be used to dispatch untrusted events to listeners on any object that also implements EventTarget.

The event objects associated with the event types described above contain additional context information--refer to the description of the DOM interfaces for further information.

5. Event Types

The DOM Event Model allows a DOM implementation to support multiple modules of events. The model has been designed to allow addition of new event modules in the future. This document does not attempt to define all possible events. For purposes of interoperability, the DOM defines a module of user interface events including lower level device dependent events and a module of document mutation events.

5.1. User Interface Events

The User Interface event module contains basic event types associated with user interfaces and document manipulation.

5.1.1. Interface UIEvent

Introduced in DOM Level 2

The UIEvent interface provides specific contextual information associated with User Interface events.

To create an instance of the UIEvent interface, use the UIEvent constructor, passing an optional UIEventInit dictionary.

For newly defined events, you don’t have to inherit UIEvent interface just because they are related to user interface. Inherit only when members of UIEventInit make sense to those events.

5.1.1.1. UIEvent

[Exposed=Window]
interface UIEvent : Event {
  constructor(DOMString type, optional UIEventInit eventInitDict);
  readonly attribute Window? view;
  readonly attribute long detail;
};

UIEvent . view

The view attribute identifies the Window from which the event was generated.

The un-initialized value of this attribute MUST be null.

UIEvent . detail

Specifies some detail information about the Event, depending on the type of event.

The un-initialized value of this attribute MUST be 0.

5.1.1.2. UIEventInit

dictionary UIEventInit : EventInit {
  Window? view = null;
  long detail = 0;
};

UIEventInit . view

Should be initialized to the Window object of the global environment in which this event will be dispatched. If this event will be dispatched to an element, the view property should be set to the Window object containing the element’s ownerDocument.

UIEventInit . detail

This value is initialized to a number that is application-specific.

5.1.2. UI Event Types

The User Interface event types are listed below. Some of these events use the UIEvent interface if generated from a user interface, but the Event interface otherwise, as detailed in each event.

5.1.2.1. load

A user agent MUST dispatch this event when the DOM implementation finishes loading the resource (such as the document) and any dependent resources (such as images, style sheets, or scripts). Dependent resources that fail to load MUST NOT prevent this event from firing if the resource that loaded them is still accessible via the DOM. If this event type is dispatched, implementations are REQUIRED to dispatch this event at least on the Document node.

For legacy reasons, load events for resources inside the document (e.g., images) do not include the Window in the propagation path in HTML implementations. See [HTML5] for more information.

5.1.2.2. unload

A user agent MUST dispatch this event when the DOM Implementation removes from the environment the resource (such as the document) or any dependent resources (such as images, style sheets, scripts). The document MUST be unloaded after the dispatch of this event type. If this event type is dispatched, implementations are REQUIRED to dispatch this event at least on the Document node.

5.1.2.3. abort

A user agent MUST dispatch this event when the loading of a resource has been aborted, such as by a user canceling the load while it is still in progress.

5.1.2.4. error

A user agent MUST dispatch this event when a resource failed to load, or has been loaded but cannot be interpreted according to its semantics, such as an invalid image, a script execution error, or non-well-formed XML.

5.1.2.5. select

A user agent MUST dispatch this event when a user selects some text. This event is dispatched after the selection has occurred.

This specification does not provide contextual information to access the selected text. Where applicable, a host language SHOULD define rules for how a user MAY select content (with consideration for international language conventions), at what point the select event is dispatched, and how a content author MAY access the user-selected content.

In order to access to user-selected content, content authors will use native capabilities of the host languages, such as the Document.getSelection() method of the HTML Editing APIs [Editing].

The select event might not be available for all elements in all languages. For example, in [HTML5], select events can be dispatched only on form input and textarea elements. Implementations can dispatch select events in any context deemed appropriate, including text selections outside of form controls, or image or markup selections such as in SVG.

5.2. Focus Events

This interface and its associated event types and § 5.2.2 Focus Event Order were designed in accordance to the concepts and guidelines defined in User Agent Accessibility Guidelines 2.0 [UAAG20], with particular attention on the focus mechanism and the terms defined in the glossary entry for focus.

5.2.1. Interface FocusEvent

Introduced in this specification

The FocusEvent interface provides specific contextual information associated with Focus events.

To create an instance of the FocusEvent interface, use the FocusEvent constructor, passing an optional FocusEventInit dictionary.

5.2.1.1. FocusEvent

[Exposed=Window]
interface FocusEvent : UIEvent {
  constructor(DOMString type, optional FocusEventInit eventInitDict);
  readonly attribute EventTarget? relatedTarget;
};

FocusEvent . relatedTarget

Used to identify a secondary EventTarget related to a Focus event, depending on the type of event.

For security reasons with nested browsing contexts, when tabbing into or out of a nested context, the relevant EventTarget SHOULD be null.

The un-initialized value of this attribute MUST be null.

5.2.1.2. FocusEventInit

dictionary FocusEventInit : UIEventInit {
  EventTarget? relatedTarget = null;
};

FocusEventInit . relatedTarget

The relatedTarget should be initialized to the element losing focus (in the case of a focus or focusin event) or the element gaining focus (in the case of a blur or focusout event).

5.2.2. Focus Event Order

The focus events defined in this specification occur in a set order relative to one another. The following is the typical sequence of events when a focus is shifted between elements (this order assumes that no element is initially focused):

This specification does not define the behavior of focus events when interacting with methods such as focus() or blur(). See the relevant specifications where those methods are defined for such behavior.

5.2.3. Document Focus and Focus Context

This event module includes event types for notification of changes in document focus. There are three distinct focus contexts that are relevant to this discussion:

• The operating system focus context which MAY be on one of many different applications currently running on the computer. One of these applications with focus can be a browser.

• When the browser has focus, the user can switch (such as with the tab key) the application focus context among the different browser user interface fields (e.g., the Web site location bar, a search field, etc.). One of these user interface fields can be the document being shown in a tab.

• When the document itself has focus, the document focus context can be set to any of the focusable elements in the document.

The event types defined in this specification deal exclusively with document focus, and the event target identified in the event details MUST only be part of the document or documents in the window, never a part of the browser or operating system, even when switching from one focus context to another.

Normally, a document always has a focused element (even if it is the document element itself) and a persistent focus ring. When switching between focus contexts, the document’s currently focused element and focus ring normally remain in their current state. For example, if a document has three focusable elements, with the second element focused, when a user changes operating system focus to another application and then back to the browser, the second element will still be focused within the document, and tabbing will change the focus to the third element. A host language MAY define specific elements which might receive focus, the conditions under which an element MAY receive focus, the means by which focus MAY be changed, and the order in which the focus changes. For example, in some cases an element might be given focus by moving a pointer over it, while other circumstances might require a mouse click. Some elements might not be focusable at all, and some might be focusable only by special means (clicking on the element), but not by tabbing to it. Documents MAY contain multiple focus rings. Other specifications MAY define a more complex focus model than is described in this specification, including allowing multiple elements to have the current focus.

5.2.4. Focus Event Types

The Focus event types are listed below.

5.2.4.1. blur

A user agent MUST dispatch this event when an event target loses focus. The focus MUST be taken from the element before the dispatch of this event type. This event type is similar to focusout, but is dispatched after focus is shifted, and does not bubble.

5.2.4.2. focus

A user agent MUST dispatch this event when an event target receives focus. The focus MUST be given to the element before the dispatch of this event type. This event type is similar to focusin, but is dispatched after focus is shifted, and does not bubble.

5.2.4.3. focusin

A user agent MUST dispatch this event when an event target is about to receive focus. This event type MUST be dispatched before the element is given focus. The event target MUST be the element which is about to receive focus. This event type is similar to focus, but is dispatched before focus is shifted, and does bubble.

When using this event type, the content author can use the event’s relatedTarget attribute (or a host-language-specific method or means) to get the currently focused element before the focus shifts to the next focus event target, thus having access to both the element losing focus and the element gaining focus without the use of the blur or focusout event types.

5.2.4.4. focusout

A user agent MUST dispatch this event when an event target is about to lose focus. This event type MUST be dispatched before the element loses focus. The event target MUST be the element which is about to lose focus. This event type is similar to blur, but is dispatched before focus is shifted, and does bubble.

5.3. Mouse Events

The mouse event module originates from the [HTML401] onclick, ondblclick, onmousedown, onmouseup, onmouseover, onmousemove, and onmouseout attributes. This event module is specifically designed for use with pointing input devices, such as a mouse or a trackball.

5.3.1. Interface MouseEvent

Introduced in DOM Level 2, modified in this specification

The MouseEvent interface provides specific contextual information associated with Mouse events.

In the case of nested elements, mouse events are always targeted at the most deeply nested element.

Ancestors of the targeted element can use event bubbling to obtain notifications of mouse events which occur within their descendent elements.

To create an instance of the MouseEvent interface, use the MouseEvent constructor, passing an optional MouseEventInit dictionary.

When initializing MouseEvent objects using initMouseEvent, implementations can use the client coordinates clientX and clientY for calculation of other coordinates (such as target coordinates exposed by DOM Level 0 implementations or other proprietary attributes, e.g., pageX).

5.3.1.1. MouseEvent

[Exposed=Window]
interface MouseEvent : UIEvent {
  constructor(DOMString type, optional MouseEventInit eventInitDict);
  readonly attribute long screenX;
  readonly attribute long screenY;
  readonly attribute long clientX;
  readonly attribute long clientY;

  readonly attribute boolean ctrlKey;
  readonly attribute boolean shiftKey;
  readonly attribute boolean altKey;
  readonly attribute boolean metaKey;

  readonly attribute short button;
  readonly attribute unsigned short buttons;

  readonly attribute EventTarget? relatedTarget;

  boolean getModifierState(DOMString keyArg);
};

screenX, of type long, readonly

The horizontal coordinate at which the event occurred relative to the origin of the screen coordinate system.

The un-initialized value of this attribute MUST be 0.

screenY, of type long, readonly

The vertical coordinate at which the event occurred relative to the origin of the screen coordinate system.

The un-initialized value of this attribute MUST be 0.

clientX, of type long, readonly

The horizontal coordinate at which the event occurred relative to the viewport associated with the event.

The un-initialized value of this attribute MUST be 0.

clientY, of type long, readonly

The vertical coordinate at which the event occurred relative to the viewport associated with the event.

The un-initialized value of this attribute MUST be 0.

ctrlKey, of type boolean, readonly

Refer to the KeyboardEvent's ctrlKey attribute.

The un-initialized value of this attribute MUST be false.

shiftKey, of type boolean, readonly

Refer to the KeyboardEvent's shiftKey attribute.

The un-initialized value of this attribute MUST be false.

altKey, of type boolean, readonly

Refer to the KeyboardEvent's altKey attribute.

The un-initialized value of this attribute MUST be false.

metaKey, of type boolean, readonly

Refer to the KeyboardEvent's metaKey attribute.

The un-initialized value of this attribute MUST be false.

button, of type short, readonly

During mouse events caused by the depression or release of a mouse button, button MUST be used to indicate which pointer device button changed state.

The value of the button attribute MUST be as follows:

• 0 MUST indicate the primary button of the device (in general, the left button or the only button on single-button devices, used to activate a user interface control or select text) or the un-initialized value.

• 1 MUST indicate the auxiliary button (in general, the middle button, often combined with a mouse wheel).

• 2 MUST indicate the secondary button (in general, the right button, often used to display a context menu).

• 3 MUST indicate the X1 (back) button.

• 4 MUST indicate the X2 (forward) button.

Some pointing devices provide or simulate more button states, and values higher than 2 or lower than 0 MAY be used to represent such buttons.

The value of button is not updated for events not caused by the depression/release of a mouse button. In these scenarios, take care not to interpret the value 0 as the left button, but rather as the un-initialized value.

Some default actions related to events such as mousedown and mouseup depend on the specific mouse button in use.

The un-initialized value of this attribute MUST be 0.

✔MDN

MouseEvent/buttons

In all current engines.

FirefoxYesSafari11.1+Chrome43+

---

OperaYesEdge79+

---

Edge (Legacy)12+IE9+

---

Firefox for AndroidYesiOS Safari11.3+Chrome for AndroidNoneAndroid WebViewNoneSamsung InternetNoneOpera MobileYes

buttons, of type unsigned short, readonly

During any mouse events, buttons MUST be used to indicate which combination of mouse buttons are currently being pressed, expressed as a bitmask.

Though similarly named, the values for the buttons attribute and the button attribute are very different. The value of button is assumed to be valid during mousedown / mouseup event handlers, whereas the buttons attribute reflects the state of the mouse’s buttons for any trusted MouseEvent object (while it is being dispatched), because it can represent the "no button currently active" state (0).

The value of the buttons attribute MUST be as follows:

• 0 MUST indicate no button is currently active.

• 1 MUST indicate the primary button of the device (in general, the left button or the only button on single-button devices, used to activate a user interface control or select text).

• 2 MUST indicate the secondary button (in general, the right button, often used to display a context menu), if present.

• 4 MUST indicate the auxiliary button (in general, the middle button, often combined with a mouse wheel).

Some pointing devices provide or simulate more buttons. To represent such buttons, the value MUST be doubled for each successive button (in the binary series 8, 16, 32, ... ).

Because the sum of any set of button values is a unique number, a content author can use a bitwise operation to determine how many buttons are currently being pressed and which buttons they are, for an arbitrary number of mouse buttons on a device. For example, the value 3 indicates that the left and right button are currently both pressed, while the value 5 indicates that the left and middle button are currently both pressed.

Some default actions related to events such as mousedown and mouseup depend on the specific mouse button in use.

The un-initialized value of this attribute MUST be 0.

✔MDN

MouseEvent/relatedTarget

In all current engines.

Firefox48+SafariYesChromeYes

---

OperaYesEdgeYes

---

Edge (Legacy)12+IEYes

---

Firefox for AndroidYesiOS SafariYesChrome for AndroidYesAndroid WebViewYesSamsung InternetYesOpera MobileYes

relatedTarget, of type EventTarget, readonly, nullable

Used to identify a secondary EventTarget related to a UI event, depending on the type of event.

The un-initialized value of this attribute MUST be null.

getModifierState(keyArg)

Introduced in this specification

Queries the state of a modifier using a key value.

Returns true if it is a modifier key and the modifier is activated, false otherwise.

DOMString keyArg

Refer to the KeyboardEvent's getModifierState() method for a description of this parameter.

5.3.1.2. MouseEventInit

dictionary MouseEventInit : EventModifierInit {
  long screenX = 0;
  long screenY = 0;
  long clientX = 0;
  long clientY = 0;

  short button = 0;
  unsigned short buttons = 0;
  EventTarget? relatedTarget = null;
};

screenX, of type long, defaulting to 0

Initializes the screenX attribute of the MouseEvent object to the desired horizontal relative position of the mouse pointer on the user’s screen.

Initializing the event object to the given mouse position must not move the user’s mouse pointer to the initialized position.

screenY, of type long, defaulting to 0

Initializes the screenY attribute of the MouseEvent object to the desired vertical relative position of the mouse pointer on the user’s screen.

Initializing the event object to the given mouse position must not move the user’s mouse pointer to the initialized position.

clientX, of type long, defaulting to 0

Initializes the clientX attribute of the MouseEvent object to the desired horizontal position of the mouse pointer relative to the client window of the user’s browser.

Initializing the event object to the given mouse position must not move the user’s mouse pointer to the initialized position.

clientY, of type long, defaulting to 0

Initializes the clientY attribute of the MouseEvent object to the desired vertical position of the mouse pointer relative to the client window of the user’s browser.

Initializing the event object to the given mouse position must not move the user’s mouse pointer to the initialized position.

button, of type short, defaulting to 0

Initializes the button attribute of the MouseEvent object to a number representing the desired state of the button(s) of the mouse.

The value 0 is used to represent the primary mouse button, 1 is used to represent the auxiliary/middle mouse button, and 2 to represent the right mouse button. Numbers greater than 2 are also possible, but are not specified in this document.

buttons, of type unsigned short, defaulting to 0

Initializes the buttons attribute of the MouseEvent object to a number representing one or more of the button(s) of the mouse that are to be considered active.

The buttons attribute is a bit-field. If a mask value of 1 is true when applied to the value of the bit field, then the primary mouse button is down. If a mask value of 2 is true when applied to the value of the bit field, then the right mouse button is down. If a mask value of 4 is true when applied to the value of the bit field, then the auxiliary/middle button is down.

In JavaScript, to initialize the buttons attribute as if the right (2) and middle button (4) were being pressed simultaneously, the buttons value can be assigned as either:
  { buttons: 2 | 4 }
or:
  { buttons: 6 }

relatedTarget, of type EventTarget, nullable, defaulting to null

The relatedTarget should be initialized to the element whose bounds the mouse pointer just left (in the case of a mouseover or mouseenter event) or the element whose bounds the mouse pointer is entering (in the case of a mouseout or mouseleave or focusout event). For other events, this value need not be assigned (and will default to null).

Implementations MUST maintain the current click count when generating mouse events. This MUST be a non-negative integer indicating the number of consecutive clicks of a pointing device button within a specific time. The delay after which the count resets is specific to the environment configuration.

5.3.2. Event Modifier Initializers

The MouseEvent and KeyboardEvent interfaces share a set of keyboard modifier attributes and support a mechanism for retrieving additional modifier states. The following dictionary enables authors to initialize keyboard modifier attributes of the MouseEvent and KeyboardEvent interfaces, as well as the additional modifier states queried via getModifierState(). The steps for constructing events using this dictionary are defined in the event constructors section.

dictionary EventModifierInit : UIEventInit {
  boolean ctrlKey = false;
  boolean shiftKey = false;
  boolean altKey = false;
  boolean metaKey = false;

  boolean modifierAltGraph = false;
  boolean modifierCapsLock = false;
  boolean modifierFn = false;
  boolean modifierFnLock = false;
  boolean modifierHyper = false;
  boolean modifierNumLock = false;
  boolean modifierScrollLock = false;
  boolean modifierSuper = false;
  boolean modifierSymbol = false;
  boolean modifierSymbolLock = false;
};

ctrlKey, of type boolean, defaulting to false

Initializes the ctrlKey attribute of the MouseEvent or KeyboardEvent objects to true if the Control key modifier is to be considered active, false otherwise.

When true, implementations must also initialize the event object’s key modifier state such that calls to the getModifierState() or getModifierState() when provided with the parameter Control must return true.

shiftKey, of type boolean, defaulting to false

Initializes the shiftKey attribute of the MouseEvent or KeyboardEvent objects to true if the Shift key modifier is to be considered active, false otherwise.

When true, implementations must also initialize the event object’s key modifier state such that calls to the getModifierState() or getModifierState() when provided with the parameter Shift must return true.

altKey, of type boolean, defaulting to false

Initializes the altKey attribute of the MouseEvent or KeyboardEvent objects to true if the Alt (alternative) (or Option) key modifier is to be considered active, false otherwise.

When true, implementations must also initialize the event object’s key modifier state such that calls to the getModifierState() or getModifierState() when provided with the parameter Alt must return true.

metaKey, of type boolean, defaulting to false

Initializes the metaKey attribute of the MouseEvent or KeyboardEvent objects to true if the Meta key modifier is to be considered active, false otherwise.

When true, implementations must also initialize the event object’s key modifier state such that calls to the getModifierState() or getModifierState() when provided with either the parameter Meta must return true.

modifierAltGraph, of type boolean, defaulting to false

Initializes the event object’s key modifier state such that calls to the getModifierState() or getModifierState() when provided with the parameter AltGraph must return true.

modifierCapsLock, of type boolean, defaulting to false

Initializes the event object’s key modifier state such that calls to the getModifierState() or getModifierState() when provided with the parameter CapsLock must return true.

modifierFn, of type boolean, defaulting to false

Initializes the event object’s key modifier state such that calls to the getModifierState() or getModifierState() when provided with the parameter Fn must return true.

modifierFnLock, of type boolean, defaulting to false

Initializes the event object’s key modifier state such that calls to the getModifierState() or getModifierState() when provided with the parameter FnLock must return true.

modifierHyper, of type boolean, defaulting to false

Initializes the event object’s key modifier state such that calls to the getModifierState() or getModifierState() when provided with the parameter Hyper must return true.

modifierNumLock, of type boolean, defaulting to false

Initializes the event object’s key modifier state such that calls to the getModifierState() or getModifierState() when provided with the parameter NumLock must return true.

modifierScrollLock, of type boolean, defaulting to false

Initializes the event object’s key modifier state such that calls to the getModifierState() or getModifierState() when provided with the parameter ScrollLock must return true.

modifierSuper, of type boolean, defaulting to false

Initializes the event object’s key modifier state such that calls to the getModifierState() or getModifierState() when provided with the parameter Super must return true.

modifierSymbol, of type boolean, defaulting to false

Initializes the event object’s key modifier state such that calls to the getModifierState() or getModifierState() when provided with the parameter Symbol must return true.

modifierSymbolLock, of type boolean, defaulting to false

Initializes the event object’s key modifier state such that calls to the getModifierState() or getModifierState() when provided with the parameter SymbolLock must return true.

5.3.3. Mouse Event Order

Certain mouse events defined in this specification MUST occur in a set order relative to one another. The following shows the event sequence that MUST occur when a pointing device’s cursor is moved over an element:

When a pointing device is moved into an element A, and then into a nested element B and then back out again, the following sequence of events MUST occur:

Sometimes elements can be visually overlapped using CSS. In the following example, three elements labeled A, B, and C all have the same dimensions and absolute position on a web page. Element C is a child of B, and B is a child of A in the DOM:

When the pointing device is moved from outside the element stack to the element labeled C and then moved out again, the following series of events MUST occur:

The mouseover/mouseout events are only fired once, while mouseenter/mouseleave events are fired three times (once to each element).

The following is the typical sequence of events when a button associated with a pointing device (e.g., a mouse button or trackpad) is pressed and released over an element:

The lag time, degree, distance, and number of mousemove events allowed between the mousedown and mouseup events while still firing a click or dblclick event will be implementation-, device-, and platform-specific. This tolerance can aid users that have physical disabilities like unsteady hands when these users interact with a pointing device.

Each implementation will determine the appropriate hysteresis tolerance, but in general SHOULD fire click and dblclick events when the event target of the associated mousedown and mouseup events is the same element with no mouseout or mouseleave events intervening, and SHOULD fire click and dblclick events on the nearest common inclusive ancestor when the associated mousedown and mouseup event targets are different.

If a mousedown event was targeted at an HTML document’s body element, and the corresponding mouseup event was targeted at the root element, then the click event will be dispatched to the root element, since it is the nearest common inclusive ancestor.

If the event target (e.g. the target element) is removed from the DOM during the mouse events sequence, the remaining events of the sequence MUST NOT be fired on that element.

If the target element is removed from the DOM as the result of a mousedown event, no events for that element will be dispatched for mouseup, click, or dblclick, nor any default activation events. However, the mouseup event will still be dispatched on the element that is exposed to the mouse after the removal of the initial target element. Similarly, if the target element is removed from the DOM during the dispatch of a mouseup event, the click and subsequent events will not be dispatched.

5.3.4. Mouse Event Types

The Mouse event types are listed below. In the case of nested elements, mouse event types are always targeted at the most deeply nested element. Ancestors of the targeted element MAY use bubbling to obtain notification of mouse events which occur within its descendent elements.

5.3.4.1. auxclick

The auxclick event type MUST be dispatched on the topmost event target indicated by the pointer, when the user presses down and releases the non-primary pointer button, or otherwise activates the pointer in a manner that simulates such an action. The actuation method of the mouse button depends upon the pointer device and the environment configuration, e.g., it MAY depend on the screen location or the delay between the press and release of the pointing device button.

The auxclick event should only be fired for the non-primary pointer buttons (i.e., when button value is not 0, buttons value is greater than 1). The primary button (like the left button on a standard mouse) MUST NOT fire auxclick events. See click for a corresponding event that is associated with the primary button.

The auxclick event MAY be preceded by the mousedown and mouseup events on the same element, disregarding changes between other node types (e.g., text nodes). Depending upon the environment configuration, the auxclick event MAY be dispatched if one or more of the event types mouseover, mousemove, and mouseout occur between the press and release of the pointing device button.

The default action of the auxclick event type varies based on the event target of the event and the value of the button or buttons attributes. Typical default actions of the auxclick event type are as follows:

• If the event target has associated activation behavior, the default action MUST be to execute that activation behavior (see § 3.5 Activation triggers and behavior).

Receiving and handling auxclick for the middle button.
myLink.addEventListener("auxclick", function(e) { if (e.button === 1) { // This would prevent the default behavior which is for example // opening a new tab when middle clicking on a link. e.preventDefault(); // Do something else to handle middle button click like taking // care of opening link or non-link buttons in new tabs in a way // that fits the app. Other actions like closing a tab in a tab-strip // which should be done on the click action can be done here too. } });

In the case of right button, the auxclick event is dispatched after any contextmenu event. Note that some user agents swallow all input events while a context menu is being displayed, so auxclick may not be available to applications in such scenarios. See this example for more clarification.

Receiving and handling auxlick for the right button
myDiv.addEventListener("contextmenu", function(e) { // This call makes sure no context menu is shown // to interfere with page receiving the events. e.preventDefault(); }); myDiv.addEventListener("auxclick", function(e) { if (e.button === 2) { // Do something else to handle right button click like opening a // customized context menu inside the app. } });

5.3.4.2. click

The click event type MUST be dispatched on the topmost event target indicated by the pointer, when the user presses down and releases the primary pointer button, or otherwise activates the pointer in a manner that simulates such an action. The actuation method of the mouse button depends upon the pointer device and the environment configuration, e.g., it MAY depend on the screen location or the delay between the press and release of the pointing device button.

The click event should only be fired for the primary pointer button (i.e., when button value is 0, buttons value is 1). Secondary buttons (like the middle or right button on a standard mouse) MUST NOT fire click events. See auxclick for a corresponding event that is associated with the non-primary buttons.

The click event MAY be preceded by the mousedown and mouseup events on the same element, disregarding changes between other node types (e.g., text nodes). Depending upon the environment configuration, the click event MAY be dispatched if one or more of the event types mouseover, mousemove, and mouseout occur between the press and release of the pointing device button. The click event MAY also be followed by the dblclick event.

If a user mouses down on a text node child of a <p> element which has been styled with a large line-height, shifts the mouse slightly such that it is no longer over an area containing text but is still within the containing block of that <p> element (i.e., the pointer is between lines of the same text block, but not over the text node per se), then subsequently mouses up, this will likely still trigger a click event (if it falls within the normal temporal hysteresis for a click), since the user has stayed within the scope of the same element. Note that user-agent-generated mouse events are not dispatched on text nodes.

In addition to being associated with pointer devices, the click event type MUST be dispatched as part of an element activation, as described in § 3.5 Activation triggers and behavior.

For maximum accessibility, content authors are encouraged to use the click event type when defining activation behavior for custom controls, rather than other pointing-device event types such as mousedown or mouseup, which are more device-specific. Though the click event type has its origins in pointer devices (e.g., a mouse), subsequent implementation enhancements have extended it beyond that association, and it can be considered a device-independent event type for element activation.

The default action of the click event type varies based on the event target of the event and the value of the button or buttons attributes. Typical default actions of the click event type are as follows:

• If the event target has associated activation behavior, the default action MUST be to execute that activation behavior (see § 3.5 Activation triggers and behavior).

• If the event target is focusable, the default action MUST be to give that element document focus.

5.3.4.3. dblclick

A user agent MUST dispatch this event when the primary button of a pointing device is clicked twice over an element. The definition of a double click depends on the environment configuration, except that the event target MUST be the same between mousedown, mouseup, and dblclick. This event type MUST be dispatched after the event type click if a click and double click occur simultaneously, and after the event type mouseup otherwise.

As with the click event, the dblclick event should only be fired for the primary pointer button. Secondary buttons MUST NOT fire dblclick events.

Canceling the click event does not affect the firing of a dblclick event.

As with the click event type, the default action of the dblclick event type varies based on the event target of the event and the value of the button or buttons attributes. Normally, the typical default actions of the dblclick event type match those of the click event type, with the following additional behavior:

• If the event target is selectable, the default action MUST be to select part or all of the selectable content. Subsequent clicks MAY select additional selectable portions of that content.

5.3.4.4. mousedown

A user agent MUST dispatch this event when a pointing device button is pressed over an element.

Many implementations use the mousedown event to begin a variety of contextually dependent default actions. These default actions can be prevented if this event is canceled. Some of these default actions could include: beginning a drag/drop interaction with an image or link, starting text selection, etc. Additionally, some implementations provide a mouse-driven panning feature that is activated when the middle mouse button is pressed at the time the mousedown event is dispatched.

5.3.4.5. mouseenter

A user agent MUST dispatch this event when a pointing device is moved onto the boundaries of an element or one of its descendent elements. A user agent MUST also dispatch this event when the element or one of its descendants moves to be underneath the primary pointing device. This event type is similar to mouseover, but differs in that it does not bubble, and MUST NOT be dispatched when the pointer device moves from an element onto the boundaries of one of its descendent elements.

There are similarities between this event type and the CSS :hover pseudo-class [CSS2]. See also the mouseleave event type.

5.3.4.6. mouseleave

A user agent MUST dispatch this event when a pointing device is moved off of the boundaries of an element and all of its descendent elements. A user agent MUST also dispatch this event when the element or one of its descendants moves to be no longer underneath the primary pointing device. This event type is similar to mouseout, but differs in that does not bubble, and that it MUST NOT be dispatched until the pointing device has left the boundaries of the element and the boundaries of all of its children.

There are similarities between this event type and the CSS :hover pseudo-class [CSS2]. See also the mouseenter event type.

5.3.4.7. mousemove

A user agent MUST dispatch this event when a pointing device is moved while it is over an element. The frequency rate of events while the pointing device is moved is implementation-, device-, and platform-specific, but multiple consecutive mousemove events SHOULD be fired for sustained pointer-device movement, rather than a single event for each instance of mouse movement. Implementations are encouraged to determine the optimal frequency rate to balance responsiveness with performance.

In some implementation environments, such as a browser, mousemove events can continue to fire if the user began a drag operation (e.g., a mouse button is pressed) and the pointing device has left the boundary of the user agent.

This event was formerly specified to be non-cancelable in DOM Level 2 Events, but was changed to reflect existing interoperability between user agents.

5.3.4.8. mouseout

A user agent MUST dispatch this event when a pointing device is moved off of the boundaries of an element or when the element is moved to be no longer underneath the primary pointing device. This event type is similar to mouseleave, but differs in that does bubble, and that it MUST be dispatched when the pointer device moves from an element onto the boundaries of one of its descendent elements.

See also the mouseover event type.

5.3.4.9. mouseover

A user agent MUST dispatch this event when a pointing device is moved onto the boundaries of an element or when the element is moved to be underneath the primary pointing device. This event type is similar to mouseenter, but differs in that it bubbles, and that it MUST be dispatched when the pointer device moves onto the boundaries of an element whose ancestor element is the event target for the same event listener instance.

See also the mouseout event type.

5.3.4.10. mouseup

A user agent MUST dispatch this event when a pointing device button is released over an element.

Many implementations will invoke a context menu as the default action of this event if the right mouse button is being released.

In some implementation environments, such as a browser, a mouseup event can be dispatched even if the pointing device has left the boundary of the user agent, e.g., if the user began a drag operation with a mouse button pressed.

5.4. Wheel Events

Wheels are devices that can be rotated in one or more spatial dimensions, and which can be associated with a pointer device. The coordinate system depends on the environment configuration.

The user’s environment might be configured to associate vertical scrolling with rotation along the y-axis, horizontal scrolling with rotation along the x-axis, and zooming with rotation along the z-axis.

The deltaX, deltaY, and deltaZ attributes of WheelEvent objects indicate a measurement along their respective axes in units of pixels, lines, or pages. The reported measurements are provided after an environment-specific algorithm translates the actual rotation/movement of the wheel device into the appropriate values and units.

A user’s environment settings can be customized to interpret actual rotation/movement of a wheel device in different ways. One movement of a common dented mouse wheel can produce a measurement of 162 pixels (162 is just an example value, actual values can depend on the current screen dimensions of the user-agent). But a user can change their default environment settings to speed-up their mouse wheel, increasing this number. Furthermore, some mouse wheel software can support acceleration (the faster the wheel is rotated/moved, the greater the delta of each measurement) or even sub-pixel rotation measurements. Because of this, authors can not assume a given rotation amount in one user agent will produce the same delta value in all user agents.

The sign (positive or negative) of the values of the deltaX, deltaY, and deltaZ attributes MUST be consistent between multiple dispatches of the wheel event while the motion of the actual wheel device is rotating/moving in the same direction. If a user agent scrolls as the default action of the wheel event then the sign of the delta SHOULD be given by a right-hand coordinate system where positive X, Y, and Z axes are directed towards the right-most edge, bottom-most edge, and farthest depth (away from the user) of the document, respectively.

Individual user agents can (depending on their environment and hardware configuration) interpret the same physical user interaction on the wheel differently. For example, a vertical swipe on the edge of a trackpad from top to bottom can be interpreted as a wheel action intended to either scroll the page down or to pan the page up (i.e., resulting in either a positive or negative deltaY value respectively).

5.4.1. Interface WheelEvent

Introduced in this specification

The WheelEvent interface provides specific contextual information associated with wheel events.

To create an instance of the WheelEvent interface, use the WheelEvent constructor, passing an optional WheelEventInit dictionary.

5.4.1.1. WheelEvent

[Exposed=Window]
interface WheelEvent : MouseEvent {
  constructor(DOMString type, optional WheelEventInit eventInitDict);
  // DeltaModeCode
  const unsigned long DOM_DELTA_PIXEL = 0x00;
  const unsigned long DOM_DELTA_LINE  = 0x01;
  const unsigned long DOM_DELTA_PAGE  = 0x02;

  readonly attribute double deltaX;
  readonly attribute double deltaY;
  readonly attribute double deltaZ;
  readonly attribute unsigned long deltaMode;
};

DOM_DELTA_PIXEL

The units of measurement for the delta MUST be pixels. This is the most typical case in most operating system and implementation configurations.

DOM_DELTA_LINE

The units of measurement for the delta MUST be individual lines of text. This is the case for many form controls.

DOM_DELTA_PAGE

The units of measurement for the delta MUST be pages, either defined as a single screen or as a demarcated page.

deltaX, of type double, readonly

In user agents where the default action of the wheel event is to scroll, the value MUST be the measurement along the x-axis (in pixels, lines, or pages) to be scrolled in the case where the event is not cancelled. Otherwise, this is an implementation-specific measurement (in pixels, lines, or pages) of the movement of a wheel device around the x-axis.

The un-initialized value of this attribute MUST be 0.0.

deltaY, of type double, readonly

In user agents where the default action of the wheel event is to scroll, the value MUST be the measurement along the y-axis (in pixels, lines, or pages) to be scrolled in the case where the event is not cancelled. Otherwise, this is an implementation-specific measurement (in pixels, lines, or pages) of the movement of a wheel device around the y-axis.

The un-initialized value of this attribute MUST be 0.0.

deltaZ, of type double, readonly

In user agents where the default action of the wheel event is to scroll, the value MUST be the measurement along the z-axis (in pixels, lines, or pages) to be scrolled in the case where the event is not cancelled. Otherwise, this is an implementation-specific measurement (in pixels, lines, or pages) of the movement of a wheel device around the z-axis.

The un-initialized value of this attribute MUST be 0.0.

deltaMode, of type unsigned long, readonly

The deltaMode attribute contains an indication of the units of measurement for the delta values. The default value is DOM_DELTA_PIXEL (pixels).

This attribute MUST be set to one of the DOM_DELTA constants to indicate the units of measurement for the delta values. The precise measurement is specific to device, operating system, and application configurations.

The un-initialized value of this attribute MUST be 0.

5.4.1.2. WheelEventInit

dictionary WheelEventInit : MouseEventInit {
  double deltaX = 0.0;
  double deltaY = 0.0;
  double deltaZ = 0.0;
  unsigned long deltaMode = 0;
};

deltaX, of type double, defaulting to 0.0

See deltaZ attribute.

deltaY, of type double, defaulting to 0.0

See deltaZ attribute.

deltaZ, of type double, defaulting to 0.0

Initializes the deltaZ attribute of the WheelEvent object. Relative positive values for this attribute (as well as the deltaX and deltaY attributes) are given by a right-hand coordinate system where the X, Y, and Z axes are directed towards the right-most edge, bottom-most edge, and farthest depth (away from the user) of the document, respectively. Negative relative values are in the respective opposite directions.

deltaMode, of type unsigned long, defaulting to 0

Initializes the deltaMode attribute on the WheelEvent object to the enumerated values 0, 1, or 2, which represent the amount of pixels scrolled (DOM_DELTA_PIXEL), lines scrolled (DOM_DELTA_LINE), or pages scrolled (DOM_DELTA_PAGE) if the rotation of the wheel would have resulted in scrolling.

5.4.2. Wheel Event Types

5.4.2.1. wheel

A user agent MUST dispatch this event when a mouse wheel has been rotated around any axis, or when an equivalent input device (such as a mouse-ball, certain tablets or touchpads, etc.) has emulated such an action. Depending on the platform and input device, diagonal wheel deltas MAY be delivered either as a single wheel event with multiple non-zero axes or as separate wheel events for each non-zero axis.

The typical default action of the wheel event type is to scroll (or in some cases, zoom) the document by the indicated amount. If this event is canceled, the implementation MUST NOT scroll or zoom the document (or perform whatever other implementation-specific default action is associated with this event type).

In some user agents, or with some input devices, the speed that the wheel has been turned can affect the delta values, with a faster speed producing a higher delta value.

5.4.2.2. cancelability of wheel events

Calling preventDefault on a wheel event can prevent or otherwise interrupt scrolling. For maximum scroll performance, a user agent may not wait for each wheel event associated with the scroll to be processed to see if it will be canceled. In such cases it is possible that only the first wheel event of a scrolling sequence is cancelable. For the rest of the wheel events the user agent should set their cancelable property to false.

5.5. Input Events

Input events are sent as notifications whenever the DOM is being updated (or about to be updated) as a direct result of a user action (e.g., keyboard input in an editable region, deleting or formatting text, ...).

5.5.1. Interface InputEvent

5.5.1.1. InputEvent

Introduced in DOM Level 3

[Exposed=Window]
interface InputEvent : UIEvent {
  constructor(DOMString type, optional InputEventInit eventInitDict);
  readonly attribute DOMString? data;
  readonly attribute boolean isComposing;
  readonly attribute DOMString inputType;
};

data, of type DOMString, readonly, nullable

data holds the value of the characters generated by an input method. This MAY be a single Unicode character or a non-empty sequence of Unicode characters [Unicode]. Characters SHOULD be normalized as defined by the Unicode normalization form NFC, defined in [UAX15]. This attribute MAY contain the empty string.

The un-initialized value of this attribute MUST be null.

isComposing, of type boolean, readonly

true if the input event occurs as part of a composition session, i.e., after a compositionstart event and before the corresponding compositionend event.

The un-initialized value of this attribute MUST be false.

✔MDN

InputEvent/inputType

In all current engines.

Firefox66+Safari10.1+Chrome60+

---

Opera47+Edge79+

---

Edge (Legacy)NoneIENone

---

Firefox for Android66+iOS Safari10.3+Chrome for Android60+Android WebView60+Samsung Internet8.0+Opera Mobile44+

inputType, of type DOMString, readonly

inputType contains a string that identifies the type of input associated with the event.

For a list of valid values for this attribute, refer to the [Input-Events] specification.

The un-initialized value of this attribute MUST be the empty string "".

5.5.1.2. InputEventInit

dictionary InputEventInit : UIEventInit {
  DOMString? data = "";
  boolean isComposing = false;
  DOMString inputType = "";
};

data, of type DOMString, nullable, defaulting to ""

Initializes the data attribute of the InputEvent object.

isComposing, of type boolean, defaulting to false

Initializes the isComposing attribute of the InputEvent object.

inputType, of type DOMString, defaulting to ""

Initializes the inputType attribute of the InputEvent object.

5.5.2. Input Event Order

The input events defined in this specification MUST occur in a set order relative to one another.

5.5.3. Input Event Types

5.5.3.1. beforeinput

A user agent MUST dispatch this event when the DOM is about to be updated.

5.5.3.2. input

A user agent MUST dispatch this event immediately after the DOM has been updated.

5.6. Keyboard Events

Keyboard events are device dependent, i.e., they rely on the capabilities of the input devices and how they are mapped in the operating systems. Refer to Keyboard events and key values for more details, including examples on how Keyboard Events are used in combination with Composition Events. Depending on the character generation device, keyboard events might not be generated.

Keyboard events are only one modality of providing textual input. For editing scenarios, consider also using the InputEvent as an alternate to (or in addition to) keyboard events.

5.6.1. Interface KeyboardEvent

Introduced in this specification

The KeyboardEvent interface provides specific contextual information associated with keyboard devices. Each keyboard event references a key using a value. Keyboard events are commonly directed at the element that has the focus.

The KeyboardEvent interface provides convenient attributes for some common modifiers keys: ctrlKey, shiftKey, altKey, metaKey. These attributes are equivalent to using the method getModifierState() with Control, Shift, Alt, or Meta respectively.

To create an instance of the KeyboardEvent interface, use the KeyboardEvent constructor, passing an optional KeyboardEventInit dictionary.

5.6.1.1. KeyboardEvent

[Exposed=Window]
interface KeyboardEvent : UIEvent {
  constructor(DOMString type, optional KeyboardEventInit eventInitDict);
  // KeyLocationCode
  const unsigned long DOM_KEY_LOCATION_STANDARD = 0x00;
  const unsigned long DOM_KEY_LOCATION_LEFT = 0x01;
  const unsigned long DOM_KEY_LOCATION_RIGHT = 0x02;
  const unsigned long DOM_KEY_LOCATION_NUMPAD = 0x03;

  readonly attribute DOMString key;
  readonly attribute DOMString code;
  readonly attribute unsigned long location;

  readonly attribute boolean ctrlKey;
  readonly attribute boolean shiftKey;
  readonly attribute boolean altKey;
  readonly attribute boolean metaKey;

  readonly attribute boolean repeat;
  readonly attribute boolean isComposing;

  boolean getModifierState(DOMString keyArg);
};

DOM_KEY_LOCATION_STANDARD

The key activation MUST NOT be distinguished as the left or right version of the key, and (other than the NumLock key) did not originate from the numeric keypad (or did not originate with a virtual key corresponding to the numeric keypad).

The Q key on a PC 101 Key US keyboard.
The NumLock key on a PC 101 Key US keyboard.
The 1 key on a PC 101 Key US keyboard located in the main section of the keyboard.

DOM_KEY_LOCATION_LEFT

The key activated originated from the left key location (when there is more than one possible location for this key).

The left Control key on a PC 101 Key US keyboard.

DOM_KEY_LOCATION_RIGHT

The key activation originated from the right key location (when there is more than one possible location for this key).

The right Shift key on a PC 101 Key US keyboard.

DOM_KEY_LOCATION_NUMPAD

The key activation originated on the numeric keypad or with a virtual key corresponding to the numeric keypad (when there is more than one possible location for this key). Note that the NumLock key should always be encoded with a location of DOM_KEY_LOCATION_STANDARD.

The 1 key on a PC 101 Key US keyboard located on the numeric pad.

✔MDN

KeyboardEvent/key

In all current engines.

Firefox23+Safari10+Chrome51+

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Opera38+Edge79+

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Edge (Legacy)12+IE9+

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Firefox for Android23+iOS Safari10+Chrome for Android51+Android WebView51+Samsung Internet5.0+Opera Mobile41+

key, of type DOMString, readonly

key holds a key attribute value corresponding to the key pressed.

The key attribute is not related to the legacy keyCode attribute and does not have the same set of values.

The un-initialized value of this attribute MUST be "" (the empty string).

✔MDN

KeyboardEvent/code

In all current engines.

Firefox38+Safari10+Chrome48+

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Opera35+Edge79+

---

Edge (Legacy)NoneIENone

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Firefox for Android38+iOS Safari10+Chrome for Android48+Android WebView48+Samsung Internet5.0+Opera Mobile35+

code, of type DOMString, readonly

code holds a string that identifies the physical key being pressed. The value is not affected by the current keyboard layout or modifier state, so a particular key will always return the same value.

The un-initialized value of this attribute MUST be "" (the empty string).

location, of type unsigned long, readonly

The location attribute contains an indication of the logical location of the key on the device.

This attribute MUST be set to one of the DOM_KEY_LOCATION constants to indicate the location of a key on the device.

If a user agent allows keys to be remapped, then the location value for a remapped key MUST be set to a value which is appropriate for the new key. For example, if the "ControlLeft" key is mapped to the "KeyQ" key, then the location attribute MUST be set to DOM_KEY_LOCATION_STANDARD. Conversely, if the "KeyQ" key is remapped to one of the Control keys, then the location attribute MUST be set to either DOM_KEY_LOCATION_LEFT or DOM_KEY_LOCATION_RIGHT.

The un-initialized value of this attribute MUST be 0.

ctrlKey, of type boolean, readonly

true if the Control (control) key modifier was active.

The un-initialized value of this attribute MUST be false.

shiftKey, of type boolean, readonly

true if the shift (Shift) key modifier was active.

The un-initialized value of this attribute MUST be false.

altKey, of type boolean, readonly

true if the Alt (alternative) (or "Option") key modifier was active.

The un-initialized value of this attribute MUST be false.

metaKey, of type boolean, readonly

true if the meta (Meta) key modifier was active.

The "Command" ("⌘") key modifier on Macintosh systems is represented using this key modifier.

The un-initialized value of this attribute MUST be false.

repeat, of type boolean, readonly

true if the key has been pressed in a sustained manner. Holding down a key MUST result in the repeating the events keydown, beforeinput, input in this order, at a rate determined by the system configuration. For mobile devices which have long-key-press behavior, the first key event with a repeat attribute value of true MUST serve as an indication of a long-key-press. The length of time that the key MUST be pressed in order to begin repeating is configuration-dependent.

The un-initialized value of this attribute MUST be false.

✔MDN

KeyboardEvent/isComposing

In all current engines.

Firefox31+Safari10.1+Chrome56+

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Opera43+Edge79+

---

Edge (Legacy)NoneIENone

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Firefox for Android31+iOS Safari10.3+Chrome for Android56+Android WebView56+Samsung Internet6.0+Opera Mobile43+

isComposing, of type boolean, readonly

true if the key event occurs as part of a composition session, i.e., after a compositionstart event and before the corresponding compositionend event.

The un-initialized value of this attribute MUST be false.

getModifierState(keyArg)

Queries the state of a modifier using a key value.

Returns true if it is a modifier key and the modifier is activated, false otherwise.

DOMString keyArg

A modifier key value. Valid modifier keys are defined in the Modifier Keys table in [UIEvents-Key].

If an application wishes to distinguish between right and left modifiers, this information could be deduced using keyboard events and location.

5.6.1.2. KeyboardEventInit

dictionary KeyboardEventInit : EventModifierInit {
  DOMString key = "";
  DOMString code = "";
  unsigned long location = 0;
  boolean repeat = false;
  boolean isComposing = false;
};

key, of type DOMString, defaulting to ""

Initializes the key attribute of the KeyboardEvent object to the unicode character string representing the meaning of a key after taking into account all keyboard modifiers (such as shift-state). This value is the final effective value of the key. If the key is not a printable character, then it should be one of the key values defined in [UIEvents-Key].

code, of type DOMString, defaulting to ""

Initializes the code attribute of the KeyboardEvent object to the unicode character string representing the key that was pressed, ignoring any keyboard modifications such as keyboard layout. This value should be one of the code values defined in [UIEvents-Code].

location, of type unsigned long, defaulting to 0

Initializes the location attribute of the KeyboardEvent object to one of the following location numerical constants:

• DOM_KEY_LOCATION_STANDARD (numerical value 0)

• DOM_KEY_LOCATION_LEFT (numerical value 1)

• DOM_KEY_LOCATION_RIGHT (numerical value 2)

• DOM_KEY_LOCATION_NUMPAD (numerical value 3)

repeat, of type boolean, defaulting to false

Initializes the repeat attribute of the KeyboardEvent object. This attribute should be set to true if the the current KeyboardEvent is considered part of a repeating sequence of similar events caused by the long depression of any single key, false otherwise.

isComposing, of type boolean, defaulting to false

Initializes the isComposing attribute of the KeyboardEvent object. This attribute should be set to true if the event being constructed occurs as part of a composition sequence, false otherwise.

For compatibility with existing content, virtual keyboards, such as software keyboards on screen-based input devices, are expected to produce the normal range of keyboard events, even though they do not possess physical keys.

In some implementations or system configurations, some key events, or their values, might be suppressed by the IME in use.

5.6.2. Keyboard Event Key Location

The location attribute can be used to disambiguate between key values that can be generated by different physical keys on the keyboard, for example, the left and right Shift key or the physical arrow keys vs. the numpad arrow keys (when NumLock is off).

The following table defines the valid location values for the special keys that have more than one location on the keyboard:

For all other keys not listed in this table, the location attribute MUST always be set to DOM_KEY_LOCATION_STANDARD.

5.6.3. Keyboard Event Order

The keyboard events defined in this specification occur in a set order relative to one another, for any given key: