Add an off method to the promise returned from .once() (#100)

Co-authored-by: Sindre Sorhus <sindresorhus@gmail.com>

Author: futpib

index.d.ts

@@ -132,6 +132,13 @@ interface Options<EventData> {
 	debug?: DebugOptions<EventData>;
 }
 
+/**
+A promise returned from `emittery.once` with an extra `off` method to cancel your subscription.
+*/
+interface EmitteryOncePromise<T> extends Promise<T> {
+	off(): void;
+}
+
 /**
 Emittery is a strictly typed, fully async EventEmitter implementation. Event listeners can be registered with `on` or `once`, and events can be emitted with `emit`.
 
@@ -436,7 +443,7 @@ declare class Emittery<
 	Subscribe to one or more events only once. It will be unsubscribed after the first
 	event.
 
-	@returns The event data when `eventName` is emitted.
+	@returns The promise of event data when `eventName` is emitted. This promise is extended with an `off` method.
 
 	@example
 	```
@@ -457,7 +464,7 @@ declare class Emittery<
 	emitter.emit('🐶', '🍖'); // Nothing happens
 	```
 	*/
-	once<Name extends keyof AllEventData>(eventName: Name | Name[]): Promise<AllEventData[Name]>;
+	once<Name extends keyof AllEventData>(eventName: Name | Name[]): EmitteryOncePromise<AllEventData[Name]>;
 
 	/**
 	Trigger an event asynchronously, optionally with some data. Listeners are called in the order they were added, but executed concurrently.

index.js

@@ -283,12 +283,17 @@ class Emittery {
 	}
 
 	once(eventNames) {
-		return new Promise(resolve => {
-			const off = this.on(eventNames, data => {
-				off();
+		let off_;
+
+		const promise = new Promise(resolve => {
+			off_ = this.on(eventNames, data => {
+				off_();
 				resolve(data);
 			});
 		});
+
+		promise.off = off_;
+		return promise;
 	}
 
 	events(eventNames) {

index.test-d.ts

@@ -54,6 +54,9 @@ type AnyListener = (eventData?: unknown) => void | Promise<void>;
 			expectType<PropertyKey | undefined>(eventName);
 			expectType<AnyListener>(listener);
 		});
+		const oncePromise = ee.once('anotherEvent');
+		oncePromise.off();
+		await oncePromise;
 	};
 }
 

readme.md

@@ -284,7 +284,7 @@ const listener = data => console.log(data);
 
 Subscribe to one or more events only once. It will be unsubscribed after the first event.
 
-Returns a promise for the event data when `eventName` is emitted.
+Returns a promise for the event data when `eventName` is emitted. This promise is extended with an `off` method.
 
 ```js
 const Emittery = require('emittery');

test/index.js

@@ -374,6 +374,30 @@ test('once() - eventName must be a string, symbol, or number', async t => {
 	await t.throwsAsync(emitter.once(true), TypeError);
 });
 
+test('once() - returns a promise with an unsubscribe method', async t => {
+	const fixture = '🌈';
+	const emitter = new Emittery();
+	const oncePromise = emitter.once('🦄');
+
+	const testFailurePromise = Promise.race([
+		(async () => {
+			await oncePromise;
+			t.fail();
+		})(),
+		new Promise(resolve => {
+			setTimeout(() => {
+				resolve(false);
+			}, 100);
+		})
+	]);
+
+	oncePromise.off();
+	emitter.emit('🦄', fixture);
+
+	await testFailurePromise;
+	t.pass();
+});
+
 test.cb('emit() - one event', t => {
 	t.plan(1);