The CustomEvent() constructor creates a new CustomEvent.

Note: This feature is available in Web Workers.


 event = new CustomEvent(typeArg, customEventInit);


A DOMString representing the name of the event.
customEventInit Optional
A CustomEventInit dictionary, having the following fields:
  • "detail", optional and defaulting to null, of type any, that is an event-dependent value associated with the event.

The CustomEventInit dictionary also accepts fields from the EventInit dictionary.

Return value

A new CustomEvent object of the specified type, with any other properties configured according to the CustomEventInit dictionary (if one was provided).


// add an appropriate event listener
obj.addEventListener("cat", function(e) { process(e.detail) });

// create and dispatch the event
var event = new CustomEvent("cat", {
  detail: {
    hazcheeseburger: true

Additional examples can be found at Creating and triggering events.


Specification Status Comment
The definition of 'CustomEvent()' in that specification.
Living Standard Initial definition.

Browser compatibility

BCD tables only load in the browser


You can polyfill the CustomEvent() constructor functionality in Internet Explorer 9 and higher with the following code:

(function () {

  if ( typeof window.CustomEvent === "function" ) return false;

  function CustomEvent ( event, params ) {
    params = params || { bubbles: false, cancelable: false, detail: null };
    var evt = document.createEvent( 'CustomEvent' );
    evt.initCustomEvent( event, params.bubbles, params.cancelable, params.detail );
    return evt;

  window.CustomEvent = CustomEvent;

Internet Explorer >= 9 adds a CustomEvent object to the window, but with correct implementations, this is a function.

See also