The CustomEvent interface represents events initialized by an application for any purpose.

Note: This feature is available in Web Workers


Creates a CustomEvent.


CustomEvent.detail Read only
Any data passed when initializing the event.

This interface inherits properties from its parent, Event:

Event.bubbles Read only
A boolean indicating whether or not the event bubbles up through the DOM.
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.
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.)
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.
A non-standard alias (from old versions of Microsoft Internet Explorer) for Some other browsers are starting to support it for web compatibility purposes. 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 value indicating whether the given event will bubble across through the shadow root into the standard DOM. Use composed instead.



Initializes a CustomEvent object. If the event has already being dispatched, this method does nothing.

This interface inherits methods from its parent, Event:

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.
Cancels the event (if it is cancelable).
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).
Stops the propagation of events further along in the DOM.

Deprecated methods

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


DOM Standard (DOM)
# interface-customevent

Browser compatibility

BCD tables only load in the browser

Firing from privileged code to non-privileged code

When firing a CustomEvent from privileged code (i.e. an extension) to non-privileged code (i.e. a webpage), security issues should be considered. Firefox and other Gecko applications restrict an object created in one context from being directly used for another, which will automatically prevent security holes, but these restrictions may also prevent your code from running as expected.

While creating a CustomEvent object, you must create the object from the same window. The detail attribute of your CustomEvent will be subjected to the same restrictions. String and Array values will be readable by the content without restrictions, but custom Object will not. While using a custom object, you will need to define the attributes of that object that are readable from the content script using Components.utils.cloneInto().

// doc is a reference to the content document
function dispatchCustomEvent(doc) {
  var eventDetail = Components.utils.cloneInto({foo: 'bar'}, doc.defaultView);
  var myEvent = doc.defaultView.CustomEvent("mytype", eventDetail);

But one needs to keep in mind that exposing a function will allow the content script to run it with chrome privileges, which can open a security vulnerability.

See also