Event

The Event interface represents any event which takes place in the DOM; some are user-generated (such as mouse or keyboard events), while others are generated by APIs (such as events that indicate an animation has finished running, a video has been paused, and so forth). While events are usually triggered by such "external" sources, they 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 that 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.

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".

Event() (en-US)

Creates an Event object, returning it to the caller.

Event.bubbles (en-US) Read only

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

Event.cancelBubble (en-US)

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

Event.cancelable (en-US) Read only

A Boolean indicating whether the event is cancelable.

Event.composed (en-US) Read only

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

Event.currentTarget (en-US) 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 (en-US) 

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

Event.defaultPrevented (en-US) Read only

Indicates whether or not event.preventDefault() (en-US) has been called on the event.

Event.eventPhase (en-US) Read only

Indicates which phase of the event flow is being processed.

Event.explicitOriginalTarget (en-US) Read only

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

Event.originalTarget (en-US) Read only

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

Event.returnValue (en-US)

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() (en-US) and Event.defaultPrevented (en-US) instead, but you can use returnValue if you choose to do so.

Event.srcElement (en-US)

A non-standard alias (from old versions of Microsoft Internet Explorer) for Event.target (en-US), which is starting to be supported in some other browsers for web compatibility purposes.

Event.target (en-US) Read only

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

Event.timeStamp (en-US) 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 (en-US) instead.

Event.type (en-US) Read only

The name of the event (case-insensitive).

Event.isTrusted (en-US) 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).

Event.scoped (en-US) Read only

A Boolean indicating whether the given event will bubble across through the shadow root into the standard DOM. This property has been renamed to composed (en-US).

Event.createEvent() (en-US) 

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

Event.composedPath() (en-US)

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 (en-US) closed.

Event.initEvent() (en-US)

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

Event.preventDefault() (en-US)

Cancels the event (if it is cancelable).

Event.stopImmediatePropagation() (en-US)

For this particular event, no other listener will be called. Neither those attached on the same element, nor those attached on elements which will be traversed later (in capture phase, for instance)

Event.stopPropagation() (en-US)

Stops the propagation of events further along in the DOM.

Event.getPreventDefault()

Non-standard. Returns the value of Event.defaultPrevented (en-US). Use Event.defaultPrevented (en-US) instead.

Event.preventBubble() Obsolete since Gecko 24

Prevents the event from bubbling. Obsolete, use event.stopPropagation (en-US) instead.

Event.preventCapture() Obsolete since Gecko 24

Obsolete, use event.stopPropagation (en-US) instead.

• Types of events available: Event reference
• Comparison of Event Targets (target vs currentTarget vs relatedTarget vs originalTarget)
• Creating and triggering custom events
• For Firefox add-on developers:
  • Listening to events in Firefox extensions
  • Listening to events on all tabs