Event - Web APIs 编辑

The Event interface represents an event which takes place in the DOM.

An event can be triggered by the user action e.g. clicking the mouse button or tapping keyboard, or generated by APIs to represent the progress of an asynchronous task. It can also be triggered programmatically, such as by calling the HTMLElement.click() method of an element, or by defining the event, then sending it to a specified target using EventTarget.dispatchEvent().

There are many types of events, some of which use other interfaces based on the main Event interface. Event itself contains the properties and methods which are common to all events.

Many DOM elements can be set up to accept (or "listen" for) these events, and execute code in response to process (or "handle") them. Event-handlers are usually connected (or "attached") to various HTML elements (such as <button>, <div>, <span>, etc.) using EventTarget.addEventListener(), and this generally replaces using the old HTML event handler attributes. Further, when properly added, such handlers can also be disconnected if needed using removeEventListener().

Note: One element can have several such handlers, even for the exact same event—particularly if separate, independent code modules attach them, each for its own independent purposes. (For example, a webpage with an advertising-module and statistics-module both monitoring video-watching.)

When there are many nested elements, each with its own handler(s), event processing can become very complicated—especially where a parent element receives the very same event as its child elements because "spatially" they overlap so the event technically occurs in both, and the processing order of such events depends on the Event bubbling and capture settings of each handler triggered.

Interfaces based on Event

Below is a list of interfaces which are based on the main Event interface, with links to their respective documentation in the MDN API reference.

Note that all event interfaces have names which end in "Event".

• AnimationEvent
• AudioProcessingEvent
• BeforeInputEvent
• BeforeUnloadEvent
• BlobEvent
• ClipboardEvent
• CloseEvent
• CompositionEvent
• CSSFontFaceLoadEvent
• CustomEvent
• DeviceLightEvent
• DeviceMotionEvent
• DeviceOrientationEvent
• DeviceProximityEvent
• DOMTransactionEvent
• DragEvent
• EditingBeforeInputEvent
• ErrorEvent
• FetchEvent
• FocusEvent
• GamepadEvent
• HashChangeEvent
• IDBVersionChangeEvent
• InputEvent
• KeyboardEvent
• MediaStreamEvent
• MessageEvent
• MouseEvent
• MutationEvent
• OfflineAudioCompletionEvent
• OverconstrainedError
• PageTransitionEvent
• PaymentRequestUpdateEvent
• PointerEvent
• PopStateEvent
• ProgressEvent
• RelatedEvent
• RTCDataChannelEvent
• RTCIdentityErrorEvent
• RTCIdentityEvent
• RTCPeerConnectionIceEvent
• SensorEvent
• StorageEvent
• SVGEvent
• SVGZoomEvent
• TimeEvent
• TouchEvent
• TrackEvent
• TransitionEvent
• UIEvent
• UserProximityEvent
• WebGLContextEvent
• WheelEvent

Constructor

Event()

Creates an Event object, returning it to the caller.

Properties

Event.bubbles Read only

A boolean indicating whether or not the event bubbles up through the DOM.

Event.cancelBubble

A historical alias to Event.stopPropagation(). Setting its value to true before returning from an event handler prevents propagation of the event.

Event.cancelable Read only

A boolean indicating whether the event is cancelable.

Event.composed Read only

A boolean indicating whether or not the event can bubble across the boundary between the shadow DOM and the regular DOM.

Event.currentTarget Read only

A reference to the currently registered target for the event. This is the object to which the event is currently slated to be sent. It's possible this has been changed along the way through retargeting.

Event.deepPath

An Array of DOM Nodes through which the event has bubbled.

Event.defaultPrevented Read only

Indicates whether or not the call to event.preventDefault() canceled the event.

Event.eventPhase Read only

Indicates which phase of the event flow is being processed.

Event.explicitOriginalTarget Read only

The explicit original target of the event (Mozilla-specific.)

Event.originalTarget Read only

The original target of the event, before any retargetings. (Mozilla-specific.)

Event.returnValue

A historical property introduced by Internet Explorer and eventually adopted into the DOM specification in order to ensure existing sites continue to work. Ideally, you should try to use Event.preventDefault() and Event.defaultPrevented instead, but you can use returnValue if you choose to do so.

Event.srcElement

A non-standard alias (from old versions of Microsoft Internet Explorer) for Event.target. Some other browsers are starting to support it for web compatibility purposes.

Event.target Read only

A reference to the target to which the event was originally dispatched.

Event.timeStamp Read only

The time at which the event was created (in milliseconds). By specification, this value is time since epoch—but in reality, browsers' definitions vary. In addition, work is underway to change this to be a DOMHighResTimeStamp instead.

Event.type Read only

The name of the event. Case-insensitive.

Event.isTrusted Read only

Indicates whether or not the event was initiated by the browser (after a user click, for instance) or by a script (using an event creation method, like Event.initEvent).

Deprecated properties

Event.scoped Read only

A Boolean indicating whether the given event will bubble across through the shadow root into the standard DOM. Use composed instead.

Methods

Event.composedPath()

Returns the event’s path (objects on which listeners will be invoked). This does not include nodes in shadow trees if the shadow root was created with its ShadowRoot.mode closed.

Event.preventDefault()

Cancels the event (if it is cancelable).

Event.stopImmediatePropagation()

For this particular event, prevent all other listeners from being called. This includes listeners attached to the same element as well as those attached to elements that will be traversed later (during the capture phase, for instance).

Event.stopPropagation()

Stops the propagation of events further along in the DOM.

Deprecated methods

Event.createEvent()

Creates a new event, which must then be initialized by calling its initEvent() method.

Event.initEvent()

Initializes the value of an Event created. If the event has already been dispatched, this method does nothing.

Event.getPreventDefault()

Returns the value of Event.defaultPrevented.

Event.preventBubble()

Prevents the event from bubbling. Use event.stopPropagation instead.

Event.preventCapture()

Prevents the event from bubbling. Use event.stopPropagation instead.

Specifications

Browser compatibility

BCD tables only load in the browser

See also

• Types of events available: Event reference
• Comparison of Event Targets (target vs currentTarget vs relatedTarget vs originalTarget)
• Creating and triggering custom events