Document Object Model Events

13 November, 2000

# 1. Document Object Model Events

Editors
  Tom Pixley, Netscape Communications Corp.

## Table of contents

## 1.1. Overview of the DOM Level 2 Event Model

The DOM Level 2 Event Model is designed with two main goals. The first goal is the design of a generic event system which allows registration of event handlers, describes event flow through a tree structure, and provides basic contextual information for each event. 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 the event model is to provide a common subset of the current event systems used in DOM Level 0 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.

The following sections of the Event Model specification define both the specification for the DOM Event Model and a number of conformant event modules designed for use within the model. The Event Model consists of the two sections on event propagation and event listener registration and the Event interface.

A DOM application may use the `hasFeature(feature, version)` method of the `DOMImplementation` interface with parameter values "Events" and "2.0" (respectively) to determine whether or not the event module is supported by the implementation. In order to fully support this module, an implementation must also support the "Core" feature defined in the DOM Level 2 Core specification [DOM Level 2 Core]. Please, refer to additional information about conformance in the DOM Level 2 Core specification [DOM Level 2 Core].

Each event module describes its own feature string in the event module listing.

### 1.1.1. Terminology

UI events
  User interface events. These events are generated by user interaction through an external device (mouse, keyboard, etc.)
UI Logical events
  Device independent user interface events such as focus change messages or element triggering notifications.
Mutation events
  Events caused by any action which modifies the structure of the document.
Capturing
  The process by which an event can be handled by one of the event's target's ancestors before being handled by the event's target.
Bubbling
  The process by which an event propagates upward through its ancestors after being handled by the event's target.
Cancelable
  A designation for events which indicates that upon handling the event the client may choose to prevent the DOM implementation from processing any default action associated with the event.

## 1.2. Description of event flow

Event flow is the process through which the an event originates from the DOM implementation and is passed into the Document Object Model. The methods of event capture and event bubbling, along with various event listener registration techniques, allow the event to then be handled in a number of ways. It can be handled locally at the `EventTarget` level or centrally from an `EventTarget` higher in the document tree.

### 1.2.1. Basic event flow

Each event has an `EventTarget` toward which the event is directed by the DOM implementation. This `EventTarget` is specified in the `Event`'s `target` attribute. When the event reaches the target, any event listeners registered on the `EventTarget` are triggered. Although all `EventListeners` on the `EventTarget` are guaranteed to be triggered by any event which is received by that `EventTarget`, no specification is made as to the order in which they will receive the event with regards to the other `EventListeners` on the `EventTarget`. If neither event capture or event bubbling are in use for that particular event, the event flow process will complete after all listeners have been triggered. If event capture or event bubbling is in use, the event flow will be modified as described in the sections below.

Any exceptions thrown inside an `EventListener` will not stop propagation of the event. It will continue processing any additional `EventListener` in the described manner.

It is expected that actions taken by `EventListener`s may cause additional events to fire. Additional events should be handled in a synchronous manner and may cause reentrancy into the event model.

### 1.2.2. Event capture

Event capture is the process by which an EventListener registered on an ancestor of the event's target can intercept events of a given type before they are received by the event's target. Capture operates from the top of the tree, generally the `Document`, downward, making it the symmetrical opposite of bubbling which is described below. The chain of `EventTarget`s from the top of the tree to the event's target is determined before the initial dispatch of the event. If modifications occur to the tree during event processing, event flow will proceed based on the initial state of the tree.

An `EventListener` being registered on an `EventTarget` may choose to have that `EventListener` capture events by specifying the `useCapture` parameter of the `addEventListener` method to be `true`. Thereafter, when an event of the given type is dispatched toward a descendant of the capturing object, the event will trigger any capturing event listeners of the appropriate type which exist in the direct line between the top of the document and the event's target. This downward propagation continues until the event's target is reached. A capturing `EventListener` will not be triggered by events dispatched directly to the `EventTarget` upon which it is registered.

If the capturing `EventListener` wishes to prevent further processing of the event from occurring it may call the `stopProgagation` method of the `Event` interface. This will prevent further dispatch of the event, although additional `EventListeners` registered at the same hierarchy level will still receive the event. Once an event's `stopPropagation` method has been called, further calls to that method have no additional effect. If no additional capturers exist and `stopPropagation` has not been called, the event triggers the appropriate `EventListeners` on the target itself.

Although event capture is similar to the delegation based event model in which all interested parties register their listeners directly on the target about which they wish to receive notifications, it is different in two important respects. First, event capture only allows interception of events which are targeted at descendants of the capturing `EventTarget`. It does not allow interception of events targeted to the capturer's ancestors, its siblings, or its sibling's descendants. Secondly, event capture is not specified for a single `EventTarget`, it is specified for a specific type of event. Once specified, event capture intercepts all events of the specified type targeted toward any of the capturer's descendants.

### 1.2.3. Event bubbling

Events which are designated as bubbling will initially proceed with the same event flow as non-bubbling events. The event is dispatched to its target `EventTarget` and any event listeners found there are triggered. Bubbling events will then trigger any additional event listeners found by following the `EventTarget`'s parent chain upward, checking for any event listeners registered on each successive `EventTarget`. This upward propagation will continue up to and including the `Document`. `EventListener`s registered as capturers will not be triggered during this phase. The chain of `EventTarget`s from the event target to the top of the tree is determined before the initial dispatch of the event. If modifications occur to the tree during event processing, event flow will proceed based on the initial state of the tree.

Any event handler may choose to prevent further event propagation by calling the `stopPropagation` method of the `Event` interface. If any `EventListener` calls this method, all additional `EventListeners` on the current `EventTarget` will be triggered but bubbling will cease at that level. Only one call to `stopPropagation` is required to prevent further bubbling.

### 1.2.4. Event cancelation

Some events are specified as cancelable. For these events, the DOM implementation generally has a default action associated with the event. An example of this is a hyperlink in a web browser. When the user clicks on the hyperlink the default action is generally to active that hyperlink. Before processing these events, the implementation must check for event listeners registered to receive the event and dispatch the event to those listeners. These listeners then have the option of canceling the implementation's default action or allowing the default action to proceed. In the case of the hyperlink in the browser, canceling the action would have the result of not activating the hyperlink.

Cancelation is accomplished by calling the `Event`'s `preventDefault` method. If one or more `EventListeners` call `preventDefault` during any phase of event flow the default action will be canceled.

Different implementations will specify their own default actions, if any, associated with each event. The DOM does not attempt to specify these actions.

## 1.3. Event listener registration

### 1.3.1. Event registration interfaces

Interface EventTarget (introduced in DOM Level 2)

  The `EventTarget` interface is implemented by all `Nodes` in an implementation which supports the DOM Event Model. Therefore, this interface can be obtained by using binding-specific casting methods on an instance of the `Node` interface. The interface allows registration and removal of `EventListeners` on an `EventTarget` and dispatch of events to that `EventTarget`.

  IDL Definition

        // Introduced in DOM Level 2:
        interface EventTarget {
          void               addEventListener(in DOMString type,
                                              in EventListener listener,
                                              in boolean useCapture);
          void               removeEventListener(in DOMString type,
                                                 in EventListener listener,
                                                 in boolean useCapture);
          boolean            dispatchEvent(in Event evt)
                                                raises(EventException);
        };

  Methods
    `addEventListener`
      This method allows the registration of event listeners on the event target. If an `EventListener` is added to an `EventTarget` while it is processing an event, it will not be triggered by the current actions but may be triggered during a later stage of event flow, such as the bubbling phase.
      If multiple identical `EventListener`s are registered on the same `EventTarget` with the same parameters the duplicate instances are discarded. They do not cause the `EventListener` to be called twice and since they are discarded they do not need to be removed with the `removeEventListener` method.
      Parameters
      `type` of type `DOMString`
        The event type for which the user is registering
      `listener` of type `EventListener`
        The `listener` parameter takes an interface implemented by the user which contains the methods to be called when the event occurs.
      `useCapture` of type `boolean`
        If true, `useCapture` indicates that the user wishes to initiate capture. After initiating capture, all events of the specified type will be dispatched to the registered `EventListener` before being dispatched to any `EventTargets` beneath them in the tree. Events which are bubbling upward through the tree will not trigger an `EventListener` designated to use capture.
      No Return Value
      No Exceptions
    `dispatchEvent`
      This method allows the dispatch of events into the implementations event model. Events dispatched in this manner will have the same capturing and bubbling behavior as events dispatched directly by the implementation. The target of the event is the `EventTarget` on which `dispatchEvent` is called.
      Parameters
      `evt` of type `Event`
        Specifies the event type, behavior, and contextual information to be used in processing the event.
      Return Value

      `boolean` | The return value of `dispatchEvent` indicates whether any of the listeners which handled the event called `preventDefault`. If `preventDefault` was called the value is false, else the value is true.

      Exceptions

      `EventException` | UNSPECIFIED_EVENT_TYPE_ERR: Raised if the `Event`'s type was not specified by initializing the event before `dispatchEvent` was called. Specification of the `Event`'s type as `null` or an empty string will also trigger this exception.

    `removeEventListener`
      This method allows the removal of event listeners from the event target. If an `EventListener` is removed from an `EventTarget` while it is processing an event, it will not be triggered by the current actions. `EventListener`s can never be invoked after being removed.
      Calling `removeEventListener` with arguments which do not identify any currently registered `EventListener` on the `EventTarget` has no effect.
      Parameters
      `type` of type `DOMString`
        Specifies the event type of the `EventListener` being removed.
      `listener` of type `EventListener`
        The `EventListener` parameter indicates the `EventListener` to be removed.
      `useCapture` of type `boolean`
        Specifies whether the `EventListener` being removed was registered as a capturing listener or not. If a listener was registered twice, one with capture and one without, each must be removed separately. Removal of a capturing listener does not affect a non-capturing version of the same listener, and vice versa.
      No Return Value
      No Exceptions
Interface EventListener (introduced in DOM Level 2)

  The `EventListener` interface is the primary method for handling events. Users implement the `EventListener` interface and register their listener on an `EventTarget` using the `AddEventListener` method. The users should also remove their `EventListener` from its `EventTarget` after they have completed using the listener.

  When a `Node` is copied using the `cloneNode` method the `EventListener`s attached to the source `Node` are not attached to the copied `Node`. If the user wishes the same `EventListener`s to be added to the newly created copy the user must add them manually.

  IDL Definition

        // Introduced in DOM Level 2:
        interface EventListener {
          void               handleEvent(in Event evt);
        };

  Methods
    `handleEvent`
      This method is called whenever an event occurs of the type for which the `EventListener` interface was registered.
      Parameters
      `evt` of type `Event`
        The `Event` contains contextual information about the event. It also contains the `stopPropagation` and `preventDefault` methods which are used in determining the event's flow and default action.
      No Return Value
      No Exceptions

### 1.3.2. Interaction with HTML 4.0 event listeners

In HTML 4.0, event listeners were specified as attributes of an element. As such, registration of a second event listener of the same type would replace the first listener. The DOM Event Model allows registration of multiple event listeners on a single `EventTarget`. To achieve this, event listeners are no longer stored as attribute values.

In order to achieve compatibility with HTML 4.0, implementors may view the setting of attributes which represent event handlers as the creation and registration of an `EventListener` on the `EventTarget`. The value of `useCapture` defaults to `false`. This `EventListener` behaves in the same manner as any other `EventListeners` which may be registered on the `EventTarget`. If the attribute representing the event listener is changed, this may be viewed as the removal of the previously registered `EventListener` and the registration of a new one. No technique is provided to allow HTML 4.0 event listeners access to the context information defined for each event.

## 1.4. Event interface

Interface Event (introduced in DOM Level 2)

  The `Event` interface is used to provide contextual information about an event to the handler processing the event. An object which implements the `Event` interface is generally passed as the first parameter to an event handler. More specific context information is passed to event handlers by deriving additional interfaces from `Event` which contain information directly relating to the type of event they accompany. These derived interfaces are also implemented by the object passed to the event listener.

  IDL Definition

        // Introduced in DOM Level 2:
        interface Event {

          // PhaseType
          const unsigned short      CAPTURING_PHASE                = 1;
          const unsigned short      AT_TARGET                      = 2;
          const unsigned short      BUBBLING_PHASE                 = 3;

          readonly attribute DOMString        type;
          readonly attribute EventTarget      target;
          readonly attribute EventTarget      currentTarget;
          readonly attribute unsigned short   eventPhase;
          readonly attribute boolean          bubbles;
          readonly attribute boolean          cancelable;
          readonly attribute DOMTimeStamp     timeStamp;
          void               stopPropagation();
          void               preventDefault();
          void               initEvent(in DOMString eventTypeArg,
                                       in boolean canBubbleArg,
                                       in boolean cancelableArg);
        };

  Definition group PhaseType

    An integer indicating which phase of event flow is being processed.

    Defined Constants
      `AT_TARGET`
        The event is currently being evaluated at the target `EventTarget`.
      `BUBBLING_PHASE`
        The current event phase is the bubbling phase.
      `CAPTURING_PHASE`
        The current event phase is the capturing phase.
  Attributes
    `bubbles` of type `boolean`, readonly
      Used to indicate whether or not an event is a bubbling event. If the event can bubble the value is true, else the value is false.
    `cancelable` of type `boolean`, readonly
      Used to indicate whether or not an event can have its default action prevented. If the default action can be prevented the value is true, else the value is false.
    `currentTarget` of type `EventTarget`, readonly
      Used to indicate the `EventTarget` whose `EventListeners` are currently being processed. This is particularly useful during capturing and bubbling.
    `eventPhase` of type `unsigned short`, readonly
      Used to indicate which phase of event flow is currently being evaluated.
    `target` of type `EventTarget`, readonly
      Used to indicate the `EventTarget` to which the event was originally dispatched.
    `timeStamp` of type `DOMTimeStamp`, readonly
      Used to specify the time (in milliseconds relative to the epoch) at which the event was created. Due to the fact that some systems may not provide this information the value of `timeStamp` may be not available for all events. When not available, a value of 0 will be returned. Examples of epoch time are the time of the system start or 0:0:0 UTC 1st January 1970.
    `type` of type `DOMString`, readonly
      The name of the event (case-insensitive). The name must be an XML name.
  Methods
    `initEvent`
      The `initEvent` method is used to initialize the value of an `Event` created through the `DocumentEvent` interface. This method may only be called before the `Event` has been dispatched via the `dispatchEvent` method, though it may be called multiple times during that phase if necessary. If called multiple times the final invocation takes precedence. If called from a subclass of `Event` interface only the values specified in the `initEvent` method are modified, all other attributes are left unchanged.
      Parameters
      `eventTypeArg` of type `DOMString`
        Specifies the event type. This type may be any event type currently defined in this specification or a new event type.. The string must be an XML name.
        Any new event type must not begin with any upper, lower, or mixed case version of the string "DOM". This prefix is reserved for future DOM event sets. It is also strongly recommended that third parties adding their own events use their own prefix to avoid confusion and lessen the probability of conflicts with other new events.
      `canBubbleArg` of type `boolean`
        Specifies whether or not the event can bubble.
      `cancelableArg` of type `boolean`
        Specifies whether or not the event's default action can be prevented.
      No Return Value
      No Exceptions
    `preventDefault`
      If an event is cancelable, the `preventDefault` method is used to signify that the event is to be canceled, meaning any default action normally taken by the implementation as a result of the event will not occur. If, during any stage of event flow, the `preventDefault` method is called the event is canceled. Any default action associated with the event will not occur. Calling this method for a non-cancelable event has no effect. Once `preventDefault` has been called it will remain in effect throughout the remainder of the event's propagation. This method may be used during any stage of event flow.
      No Parameters
      No Return Value
      No Exceptions
    `stopPropagation`
      The `stopPropagation` method is used prevent further propagation of an event during event flow. If this method is called by any `EventListener` the event will cease propagating through the tree. The event will complete dispatch to all listeners on the current `EventTarget` before event flow stops. This method may be used during any stage of event flow.
      No Parameters
      No Return Value
      No Exceptions
Exception EventException introduced in DOM Level 2

  Event operations may throw an `EventException` as specified in their method descriptions.

  IDL Definition

        // Introduced in DOM Level 2:
        exception EventException {
          unsigned short   code;
        };
        // EventExceptionCode
        const unsigned short      UNSPECIFIED_EVENT_TYPE_ERR     = 0;

  Definition group EventExceptionCode

    An integer indicating the type of error generated.

    Defined Constants
      `UNSPECIFIED_EVENT_TYPE_ERR`
        If the `Event`'s type was not specified by initializing the event before the method was called. Specification of the Event's type as `null` or an empty string will also trigger this exception.

## 1.5. DocumentEvent interface

Interface DocumentEvent (introduced in DOM Level 2)

  The `DocumentEvent` interface provides a mechanism by which the user can create an Event of a type supported by the implementation. It is expected that the `DocumentEvent` interface will be implemented on the same object which implements the `Document` interface in an implementation which supports the Event model.

  IDL Definition

        // Introduced in DOM Level 2:
        interface DocumentEvent {
          Event              createEvent(in DOMString eventType)
                                                raises(DOMException);
        };

  Methods
    `createEvent`
      Parameters
      `eventType` of type `DOMString`
        The `eventType` parameter specifies the type of `Event` interface to be created. If the `Event` interface specified is supported by the implementation this method will return a new `Event` of the interface type requested. If the `Event` is to be dispatched via the `dispatchEvent` method the appropriate event init method must be called after creation in order to initialize the `Event`'s values. As an example, a user wishing to synthesize some kind of `UIEvent` would call `createEvent` with the parameter "UIEvents". The `initUIEvent` method could then be called on the newly created `UIEvent` to set the specific type of UIEvent to be dispatched and set its context information.
        The `createEvent` method is used in creating `Event`s when it is either inconvenient or unnecessary for the user to create an `Event` themselves. In cases where the implementation provided `Event` is insufficient, users may supply their own `Event` implementations for use with the `dispatchEvent` method.
      Return Value

      `Event` | The newly created `Event`

      Exceptions

      `DOMException` | NOT_SUPPORTED_ERR: Raised if the implementation does not support the type of `Event` interface requested

## 1.6. Event module definitions

The DOM Level 2 Event Model allows a DOM implementation to support multiple modules of events. The model has been designed to allow addition of new event modules as is required. The DOM will not attempt to define all possible events. For purposes of interoperability, the DOM will define a module of user interface events including lower level device dependent events, a module of UI logical events, and a module of document mutation events. Any new event types defined by third parties must not begin with any upper, lower, or mixed case version of the string "DOM". This prefix is reserved for future DOM event modules. It is also strongly recommended that third parties adding their own events use their own prefix to avoid confusion and lessen the probability of conflicts with other new events.

### 1.6.1. User Interface event types

The User Interface event module is composed of events listed in HTML 4.0 and additional events which are supported in DOM Level 0 browsers.

A DOM application may use the `hasFeature(feature, version)` method of the `DOMImplementation` interface with parameter values "UIEvents" and "2.0" (respectively) to determine whether or not the User Interface event module is supported by the implementation. In order to fully support this module, an implementation must also support the "Events" feature defined in this specification and the "Views" feature defined in the DOM Level 2 Views specification [DOM Level 2 Views]. Please, refer to additional information about conformance in the DOM Level 2 Core specification [DOM Level 2 Core].

Note: To create an instance of the `UIEvent` interface, use the feature string "UIEvents" as the value of the input parameter used with the `createEvent` method of the `DocumentEvent` interface.

Interface UIEvent (introduced in DOM Level 2)

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

  IDL Definition

        // Introduced in DOM Level 2:
        interface UIEvent : Event {
          readonly attribute views::AbstractView  view;
          readonly attribute long             detail;
          void               initUIEvent(in DOMString typeArg,
                                         in boolean canBubbleArg,
                                         in boolean cancelableArg,
                                         in views::AbstractView viewArg,
                                         in long detailArg);
        };

  Attributes
    `detail` of type `long`, readonly
      Specifies some detail information about the `Event`, depending on the type of event.
    `view` of type `views::AbstractView`, readonly
      The `view` attribute identifies the `AbstractView` from which the event was generated.
  Methods
    `initUIEvent`
      The `initUIEvent` method is used to initialize the value of a `UIEvent` created through the `DocumentEvent` interface. This method may only be called before the `UIEvent` has been dispatched via the `dispatchEvent` method, though it may be called multiple times during that phase if necessary. If called multiple times, the final invocation takes precedence.
      Parameters
      `typeArg` of type `DOMString`
        Specifies the event type.
      `canBubbleArg` of type `boolean`
        Specifies whether or not the event can bubble.
      `cancelableArg` of type `boolean`
        Specifies whether or not the event's default action can be prevented.
      `viewArg` of type `views::AbstractView`
        Specifies the `Event`'s `AbstractView`.
      `detailArg` of type `long`
        Specifies the `Event`'s detail.
      No Return Value
      No Exceptions

The different types of such events that can occur are:

DOMFocusIn
  The DOMFocusIn event occurs when an `EventTarget` receives focus, for instance via a pointing device being moved onto an element or by tabbing navigation to the element. Unlike the HTML event focus, DOMFocusIn can be applied to any focusable `EventTarget`, not just FORM controls.
  - Bubbles: Yes
  - Cancelable: No
  - Context Info: None
DOMFocusOut
  The DOMFocusOut event occurs when a `EventTarget` loses focus, for instance via a pointing device being moved out of an element or by tabbing navigation out of the element. Unlike the HTML event blur, DOMFocusOut can be applied to any focusable `EventTarget`, not just FORM controls.
  - Bubbles: Yes
  - Cancelable: No
  - Context Info: None
DOMActivate
  The activate event occurs when an element is activated, for instance, thru a mouse click or a keypress. A numerical argument is provided to give an indication of the type of activation that occurs: 1 for a simple activation (e.g. a simple click or Enter), 2 for hyperactivation (for instance a double click or Shift Enter).
  - Bubbles: Yes
  - Cancelable: Yes
  - Context Info: detail (the numerical value)

### 1.6.2. Mouse event types

The Mouse event module is composed of events listed in HTML 4.0 and additional events which are supported in DOM Level 0 browsers. This event module is specifically designed for use with mouse input devices.

A DOM application may use the `hasFeature(feature, version)` method of the `DOMImplementation` interface with parameter values "MouseEvents" and "2.0" (respectively) to determine whether or not the Mouse event module is supported by the implementation. In order to fully support this module, an implementation must also support the "UIEvents" feature defined in this specification. Please, refer to additional information about conformance in the DOM Level 2 Core specification [DOM Level 2 Core].

Note: To create an instance of the `MouseEvent` interface, use the feature string "MouseEvents" as the value of the input parameter used with the `createEvent` method of the `DocumentEvent` interface.

Interface MouseEvent (introduced in DOM Level 2)

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

  The `detail` attribute inherited from `UIEvent` indicates the number of times a mouse button has been pressed and released over the same screen location during a user action. The attribute value is 1 when the user begins this action and increments by 1 for each full sequence of pressing and releasing. If the user moves the mouse between the mousedown and mouseup the value will be set to 0, indicating that no click is occurring.

  In the case of nested elements mouse events 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.

  IDL Definition

        // Introduced in DOM Level 2:
        interface MouseEvent : UIEvent {
          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 unsigned short   button;
          readonly attribute EventTarget      relatedTarget;
          void               initMouseEvent(in DOMString typeArg,
                                            in boolean canBubbleArg,
                                            in boolean cancelableArg,
                                            in views::AbstractView viewArg,
                                            in long detailArg,
                                            in long screenXArg,
                                            in long screenYArg,
                                            in long clientXArg,
                                            in long clientYArg,
                                            in boolean ctrlKeyArg,
                                            in boolean altKeyArg,
                                            in boolean shiftKeyArg,
                                            in boolean metaKeyArg,
                                            in unsigned short buttonArg,
                                            in EventTarget relatedTargetArg);
        };

  Attributes
    `altKey` of type `boolean`, readonly
      Used to indicate whether the 'alt' key was depressed during the firing of the event. On some platforms this key may map to an alternative key name.
    `button` of type `unsigned short`, readonly
      During mouse events caused by the depression or release of a mouse button, `button` is used to indicate which mouse button changed state. The values for `button` range from zero to indicate the left button of the mouse, one to indicate the middle button if present, and two to indicate the right button. For mice configured for left handed use in which the button actions are reversed the values are instead read from right to left.
    `clientX` of type `long`, readonly
      The horizontal coordinate at which the event occurred relative to the DOM implementation's client area.
    `clientY` of type `long`, readonly
      The vertical coordinate at which the event occurred relative to the DOM implementation's client area.
    `ctrlKey` of type `boolean`, readonly
      Used to indicate whether the 'ctrl' key was depressed during the firing of the event.
    `metaKey` of type `boolean`, readonly
      Used to indicate whether the 'meta' key was depressed during the firing of the event. On some platforms this key may map to an alternative key name.
    `relatedTarget` of type `EventTarget`, readonly
      Used to identify a secondary `EventTarget` related to a UI event. Currently this attribute is used with the mouseover event to indicate the `EventTarget` which the pointing device exited and with the mouseout event to indicate the `EventTarget` which the pointing device entered.
    `screenX` of type `long`, readonly
      The horizontal coordinate at which the event occurred relative to the origin of the screen coordinate system.
    `screenY` of type `long`, readonly
      The vertical coordinate at which the event occurred relative to the origin of the screen coordinate system.
    `shiftKey` of type `boolean`, readonly
      Used to indicate whether the 'shift' key was depressed during the firing of the event.
  Methods
    `initMouseEvent`
      The `initMouseEvent` method is used to initialize the value of a `MouseEvent` created through the `DocumentEvent` interface. This method may only be called before the `MouseEvent` has been dispatched via the `dispatchEvent` method, though it may be called multiple times during that phase if necessary. If called multiple times, the final invocation takes precedence.
      Parameters
      `typeArg` of type `DOMString`
        Specifies the event type.
      `canBubbleArg` of type `boolean`
        Specifies whether or not the event can bubble.
      `cancelableArg` of type `boolean`
        Specifies whether or not the event's default action can be prevented.
      `viewArg` of type `views::AbstractView`
        Specifies the `Event`'s `AbstractView`.
      `detailArg` of type `long`
        Specifies the `Event`'s mouse click count.
      `screenXArg` of type `long`
        Specifies the `Event`'s screen x coordinate
      `screenYArg` of type `long`
        Specifies the `Event`'s screen y coordinate
      `clientXArg` of type `long`
        Specifies the `Event`'s client x coordinate
      `clientYArg` of type `long`
        Specifies the `Event`'s client y coordinate
      `ctrlKeyArg` of type `boolean`
        Specifies whether or not control key was depressed during the `Event`.
      `altKeyArg` of type `boolean`
        Specifies whether or not alt key was depressed during the `Event`.
      `shiftKeyArg` of type `boolean`
        Specifies whether or not shift key was depressed during the `Event`.
      `metaKeyArg` of type `boolean`
        Specifies whether or not meta key was depressed during the `Event`.
      `buttonArg` of type `unsigned short`
        Specifies the `Event`'s mouse button.
      `relatedTargetArg` of type `EventTarget`
        Specifies the `Event`'s related `EventTarget`.
      No Return Value
      No Exceptions

The different types of Mouse events that can occur are:

click
  The click event occurs when the pointing device button is clicked over an element. A click is defined as a mousedown and mouseup over the same screen location. The sequence of these events is:

          mousedown
          mouseup
          click

  If multiple clicks occur at the same screen location, the sequence repeats with the `detail` attribute incrementing with each repetition. This event is valid for most elements.
  - Bubbles: Yes
  - Cancelable: Yes
  - Context Info: screenX, screenY, clientX, clientY, altKey, ctrlKey, shiftKey, metaKey, button, detail
mousedown
  The mousedown event occurs when the pointing device button is pressed over an element. This event is valid for most elements.
  - Bubbles: Yes
  - Cancelable: Yes
  - Context Info: screenX, screenY, clientX, clientY, altKey, ctrlKey, shiftKey, metaKey, button, detail
mouseup
  The mouseup event occurs when the pointing device button is released over an element. This event is valid for most elements.
  - Bubbles: Yes
  - Cancelable: Yes
  - Context Info: screenX, screenY, clientX, clientY, altKey, ctrlKey, shiftKey, metaKey, button, detail
mouseover
  The mouseover event occurs when the pointing device is moved onto an element. This event is valid for most elements.
  - Bubbles: Yes
  - Cancelable: Yes
  - Context Info: screenX, screenY, clientX, clientY, altKey, ctrlKey, shiftKey, metaKey, relatedTarget indicates the `EventTarget` the pointing device is exiting.
mousemove
  The mousemove event occurs when the pointing device is moved while it is over an element. This event is valid for most elements.
  - Bubbles: Yes
  - Cancelable: No
  - Context Info: screenX, screenY, clientX, clientY, altKey, ctrlKey, shiftKey, metaKey
mouseout
  The mouseout event occurs when the pointing device is moved away from an element. This event is valid for most elements..
  - Bubbles: Yes
  - Cancelable: Yes
  - Context Info: screenX, screenY, clientX, clientY, altKey, ctrlKey, shiftKey, metaKey, relatedTarget indicates the `EventTarget` the pointing device is entering.

### 1.6.3. Key events

The DOM Level 2 Event specification does not provide a key event module. An event module designed for use with keyboard input devices will be included in a later version of the DOM specification.

### 1.6.4. Mutation event types

The mutation event module is designed to allow notification of any changes to the structure of a document, including attr and text modifications. It may be noted that none of the mutation events listed are designated as cancelable. This stems from the fact that it is very difficult to make use of existing DOM interfaces which cause document modifications if any change to the document might or might not take place due to cancelation of the related event. Although this is still a desired capability, it was decided that it would be better left until the addition of transactions into the DOM.

Many single modifications of the tree can cause multiple mutation events to be fired. Rather than attempt to specify the ordering of mutation events due to every possible modification of the tree, the ordering of these events is left to the implementation.

A DOM application may use the `hasFeature(feature, version)` method of the `DOMImplementation` interface with parameter values "MutationEvents" and "2.0" (respectively) to determine whether or not the Mutation event module is supported by the implementation. In order to fully support this module, an implementation must also support the "Events" feature defined in this specification. Please, refer to additional information about conformance in the DOM Level 2 Core specification [DOM Level 2 Core].

Note: To create an instance of the `MutationEvent` interface, use the feature string "MutationEvents" as the value of the input parameter used with the `createEvent` method of the `DocumentEvent` interface.

Interface MutationEvent (introduced in DOM Level 2)

  The `MutationEvent` interface provides specific contextual information associated with Mutation events.

  IDL Definition

        // Introduced in DOM Level 2:
        interface MutationEvent : Event {

          // attrChangeType
          const unsigned short      MODIFICATION                   = 1;
          const unsigned short      ADDITION                       = 2;
          const unsigned short      REMOVAL                        = 3;

          readonly attribute Node             relatedNode;
          readonly attribute DOMString        prevValue;
          readonly attribute DOMString        newValue;
          readonly attribute DOMString        attrName;
          readonly attribute unsigned short   attrChange;
          void               initMutationEvent(in DOMString typeArg,
                                               in boolean canBubbleArg,
                                               in boolean cancelableArg,
                                               in Node relatedNodeArg,
                                               in DOMString prevValueArg,
                                               in DOMString newValueArg,
                                               in DOMString attrNameArg,
                                               in unsigned short attrChangeArg);
        };

  Definition group attrChangeType

    An integer indicating in which way the `Attr` was changed.

    Defined Constants
      `ADDITION`
        The `Attr` was just added.
      `MODIFICATION`
        The `Attr` was modified in place.
      `REMOVAL`
        The `Attr` was just removed.
  Attributes
    `attrChange` of type `unsigned short`, readonly
      `attrChange` indicates the type of change which triggered the DOMAttrModified event. The values can be `MODIFICATION`, `ADDITION`, or `REMOVAL`.
    `attrName` of type `DOMString`, readonly
      `attrName` indicates the name of the changed `Attr` node in a DOMAttrModified event.
    `newValue` of type `DOMString`, readonly
      `newValue` indicates the new value of the `Attr` node in DOMAttrModified events, and of the `CharacterData` node in DOMCharDataModified events.
    `prevValue` of type `DOMString`, readonly
      `prevValue` indicates the previous value of the `Attr` node in DOMAttrModified events, and of the `CharacterData` node in DOMCharDataModified events.
    `relatedNode` of type `Node`, readonly
      `relatedNode` is used to identify a secondary node related to a mutation event. For example, if a mutation event is dispatched to a node indicating that its parent has changed, the `relatedNode` is the changed parent. If an event is instead dispatched to a subtree indicating a node was changed within it, the `relatedNode` is the changed node. In the case of the DOMAttrModified event it indicates the `Attr` node which was modified, added, or removed.
  Methods
    `initMutationEvent`
      The `initMutationEvent` method is used to initialize the value of a `MutationEvent` created through the `DocumentEvent` interface. This method may only be called before the `MutationEvent` has been dispatched via the `dispatchEvent` method, though it may be called multiple times during that phase if necessary. If called multiple times, the final invocation takes precedence.
      Parameters
      `typeArg` of type `DOMString`
        Specifies the event type.
      `canBubbleArg` of type `boolean`
        Specifies whether or not the event can bubble.
      `cancelableArg` of type `boolean`
        Specifies whether or not the event's default action can be prevented.
      `relatedNodeArg` of type `Node`
        Specifies the `Event`'s related Node.
      `prevValueArg` of type `DOMString`
        Specifies the `Event`'s `prevValue` attribute. This value may be null.
      `newValueArg` of type `DOMString`
        Specifies the `Event`'s `newValue` attribute. This value may be null.
      `attrNameArg` of type `DOMString`
        Specifies the `Event`'s `attrName` attribute. This value may be null.
      `attrChangeArg` of type `unsigned short`
        Specifies the `Event`'s `attrChange` attribute
      No Return Value
      No Exceptions

The different types of Mutation events that can occur are:

DOMSubtreeModified
  This is a general event for notification of all changes to the document. It can be used instead of the more specific events listed below. It may be fired after a single modification to the document or, at the implementation's discretion, after multiple changes have occurred. The latter use should generally be used to accomodate multiple changes which occur either simultaneously or in rapid succession. The target of this event is the lowest common parent of the changes which have taken place. This event is dispatched after any other events caused by the mutation have fired.
  - Bubbles: Yes
  - Cancelable: No
  - Context Info: None
DOMNodeInserted
  Fired when a node has been added as a child of another node. This event is dispatched after the insertion has taken place. The target of this event is the node being inserted.
  - Bubbles: Yes
  - Cancelable: No
  - Context Info: relatedNode holds the parent node
DOMNodeRemoved
  Fired when a node is being removed from its parent node. This event is dispatched before the node is removed from the tree. The target of this event is the node being removed.
  - Bubbles: Yes
  - Cancelable: No
  - Context Info: relatedNode holds the parent node
DOMNodeRemovedFromDocument
  Fired when a node is being removed from a document, either through direct removal of the Node or removal of a subtree in which it is contained. This event is dispatched before the removal takes place. The target of this event is the Node being removed. If the Node is being directly removed the DOMNodeRemoved event will fire before the DOMNodeRemovedFromDocument event.
  - Bubbles: No
  - Cancelable: No
  - Context Info: None
DOMNodeInsertedIntoDocument
  Fired when a node is being inserted into a document, either through direct insertion of the Node or insertion of a subtree in which it is contained. This event is dispatched after the insertion has taken place. The target of this event is the node being inserted. If the Node is being directly inserted the DOMNodeInserted event will fire before the DOMNodeInsertedIntoDocument event.
  - Bubbles: No
  - Cancelable: No
  - Context Info: None
DOMAttrModified
  Fired after an `Attr` has been modified on a node. The target of this event is the `Node` whose `Attr` changed. The value of attrChange indicates whether the `Attr` was modified, added, or removed. The value of relatedNode indicates the `Attr` node whose value has been affected. It is expected that string based replacement of an `Attr` value will be viewed as a modification of the `Attr` since its identity does not change. Subsequently replacement of the `Attr` node with a different `Attr` node is viewed as the removal of the first `Attr` node and the addition of the second.
  - Bubbles: Yes
  - Cancelable: No
  - Context Info: attrName, attrChange, prevValue, newValue, relatedNode
DOMCharacterDataModified
  Fired after CharacterData within a node has been modified but the node itself has not been inserted or deleted. This event is also triggered by modifications to PI elements. The target of this event is the CharacterData node.
  - Bubbles: Yes
  - Cancelable: No
  - Context Info: prevValue, newValue

### 1.6.5. HTML event types

The HTML event module is composed of events listed in HTML 4.0 and additional events which are supported in DOM Level 0 browsers.

A DOM application may use the `hasFeature(feature, version)` method of the `DOMImplementation` interface with parameter values "HTMLEvents" and "2.0" (respectively) to determine whether or not the HTML event module is supported by the implementation. In order to fully support this module, an implementation must also support the "Events" feature defined in this specification. Please, refer to additional information about conformance in the DOM Level 2 Core specification [DOM Level 2 Core].

Note: To create an instance of the `Event` interface for the HTML event module, use the feature string "HTMLEvents" as the value of the input parameter used with the `createEvent` method of the `DocumentEvent` interface.

The HTML events use the base DOM Event interface to pass contextual information.

The different types of such events that can occur are:

load
  The load event occurs when the DOM implementation finishes loading all content within a document, all frames within a FRAMESET, or an OBJECT element.
  - Bubbles: No
  - Cancelable: No
  - Context Info: None
unload
  The unload event occurs when the DOM implementation removes a document from a window or frame. This event is valid for BODY and FRAMESET elements.
  - Bubbles: No
  - Cancelable: No
  - Context Info: None
abort
  The abort event occurs when page loading is stopped before an image has been allowed to completely load. This event applies to OBJECT elements.
  - Bubbles: Yes
  - Cancelable: No
  - Context Info: None
error
  The error event occurs when an image does not load properly or when an error occurs during script execution. This event is valid for OBJECT elements, BODY elements, and FRAMESET element.
  - Bubbles: Yes
  - Cancelable: No
  - Context Info: None
select
  The select event occurs when a user selects some text in a text field. This event is valid for INPUT and TEXTAREA elements.
  - Bubbles: Yes
  - Cancelable: No
  - Context Info: None
change
  The change event occurs when a control loses the input focus and its value has been modified since gaining focus. This event is valid for INPUT, SELECT, and TEXTAREA. element.
  - Bubbles: Yes
  - Cancelable: No
  - Context Info: None
submit
  The submit event occurs when a form is submitted. This event only applies to the FORM element.
  - Bubbles: Yes
  - Cancelable: Yes
  - Context Info: None
reset
  The reset event occurs when a form is reset. This event only applies to the FORM element.
  - Bubbles: Yes
  - Cancelable: No
  - Context Info: None
focus
  The focus event occurs when an element receives focus either via a pointing device or by tabbing navigation. This event is valid for the following elements: LABEL, INPUT, SELECT, TEXTAREA, and BUTTON.
  - Bubbles: No
  - Cancelable: No
  - Context Info: None
blur
  The blur event occurs when an element loses focus either via the pointing device or by tabbing navigation. This event is valid for the following elements: LABEL, INPUT, SELECT, TEXTAREA, and BUTTON.
  - Bubbles: No
  - Cancelable: No
  - Context Info: None
resize
  The resize event occurs when a document view is resized.
  - Bubbles: Yes
  - Cancelable: No
  - Context Info: None
scroll
  The scroll event occurs when a document view is scrolled.
  - Bubbles: Yes
  - Cancelable: No
  - Context Info: None