CustomEvent

Events events

Baseline: Widely available on the web, in every major browser since Jul 2015, and long enough to count on (since Jan 2018).

niso: Partial 66%. 669 of 1009 subtests pass, by system: Linux 66%, macOS 66%. Engines: QuickJS, the engine niso's WPT run uses; Node and Porffor have no recorded run. WPT results recorded at f3b4fa8, 2026-11-10; web-features 3.45.0. WPT tests · niso's support · CustomEvent, Event, EventTarget

CustomEvent is an interface in idl/dom.webidl, inheriting Event, exposed in Window, Worker. What it does is on MDN; this page says what niso has of it.

NisoHeld: detail is the caller’s value itself, not a copy.

Status

The web features its members belong to, each with its Baseline status on the web and how much of the feature’s WPT suite passes in niso (see Web platform support).

Feature	On the web	In niso	Members
Events	Widely available	Partial 66% 669 of 1009 subtests pass	3

Members

constructor

constructor

constructor(DOMString type, optional CustomEventInit eventInitDict = {});

int32_t niso_custom_event_new(niso_dom* dom, const char* type, size_t type_len, const niso_custom_event_init* event_init_dict, niso_handle* out);

detail

attribute

readonly attribute [NisoHeld] any detail;

int32_t niso_custom_event_get_detail(niso_dom* dom, niso_handle self, void** out);

initCustomEvent()

operation

undefined initCustomEvent(DOMString type, optional boolean bubbles = false, optional boolean cancelable = false, optional [NisoHeld] any detail = null);

int32_t niso_custom_event_init_custom_event(niso_dom* dom, niso_handle self, const char* type, size_t type_len, bool bubbles, bool cancelable, niso_held detail);

Not in niso yet

Members the web has, by the BCD keys web-features 3.45.0 lists, that idl/ doesn’t declare. The list is partial: web-features lists only the keys it maps to a feature.

• worker_support (Events)