Events & cancellation

Node's EventTarget (written in TypeScript, like Node's own JavaScript): addEventListener with once/passive/capture/signal and handleEvent objects, duplicate suppression, removeEventListener, dispatchEvent returning !defaultPrevented. A listener's throw is reported as an uncaught exception on the next tick while dispatch continues. Subclassable: class X extends EventTarget.

typestring

Event type name to register or fire.

listener(e: Event) => void

A function (called with this = the target) or an object with handleEvent.

opts{ once?, capture?, passive?, signal? }

once, passive (preventDefault is ignored), capture (part of the listener's identity), signal (aborting removes the listener).

boolean — dispatchEvent returns false when a listener called preventDefault().

const bus = new EventTarget()
bus.addEventListener("ping", (e: Event) => console.log("ping"))
bus.dispatchEvent(new Event("ping"))

Event and CustomEvent with the full property set: type, bubbles, cancelable, composed, defaultPrevented, timeStamp, isTrusted, target/currentTarget, eventPhase, returnValue, cancelBubble, composedPath(), preventDefault(), stopPropagation(), stopImmediatePropagation(); CustomEvent.detail (null by default). Both subclassable.

typestring

The event type name.

init{ detail?, cancelable?, bubbles? }

CustomEvent.detail and cancelable are read; bubbles is accepted but ignored.

Event | CustomEvent — The constructed event object.

const clicked = new Event("click", { cancelable: true })
clicked.preventDefault()
console.log(clicked.defaultPrevented)     // true
const greeting = new CustomEvent("greet", { detail: "hello" })
console.log(greeting.detail)              // hello

The WHATWG cancellation token. An AbortSignal is an EventTarget that fires an "abort" event, plus an aborted flag and a reason. Listen with signal.addEventListener("abort", cb) or the signal.onabort = cb event-handler property (fired together on controller.abort(); = null clears it). fetch(url, { signal }) cancellation works — pre-flight, mid-flight, and via AbortSignal.timeout(ms) — on both the top-level await path and the fiber path inside an http.listen handler: the transfer is torn down and the fetch rejects with signal.reason itself, Node parity — by default a DOMException named AbortError for a manual abort and TimeoutError for a timeout, and a custom abort(reason) value rethrown as-is. DOMException inherits Error, so both instanceof DOMException and instanceof Error hold; new DOMException(message?, name?) is constructible directly.

reasonany

Optional abort reason stored on signal.reason — any value, type preserved; defaults to an AbortError DOMException. A never-aborted signal's reason is undefined.

msnumber

AbortSignal.timeout(ms) deadline in milliseconds.

AbortController | AbortSignal — The controller, or a signal from AbortSignal.timeout.

const controller = new AbortController()
controller.abort()
try {
	await fetch("http://example.com", { signal: controller.signal })
} catch (e) {
	console.log(e.name)   // AbortError
}