Add error & completed events (#130)

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

readme.md

@@ -112,6 +112,8 @@ If `true`, specifies that any [pending](https://developer.mozilla.org/en-US/docs
 
 Adds a sync or async task to the queue. Always returns a promise.
 
+Note: If your items can potentially throw an exception, you must handle those errors from the returned Promise or they may be reported as an unhandled Promise rejection and potentially cause your process to exit immediately.
+
 ##### fn
 
 Type: `Function`
@@ -229,6 +231,40 @@ queue.add(() => Promise.resolve());
 queue.add(() => delay(500));
 ```
 
+#### completed
+
+Emitted when an item completes without error.
+
+```js
+import delay from 'delay';
+import PQueue from 'p-queue';
+
+const queue = new PQueue({concurrency: 2});
+
+queue.on('completed', result => {
+	console.log(result);
+});
+
+queue.add(() => Promise.resolve('hello, world!'));
+```
+
+#### error
+
+Emitted if an item throws an error.
+
+```js
+import delay from 'delay';
+import PQueue from 'p-queue';
+
+const queue = new PQueue({concurrency: 2});
+
+queue.on('error', error => {
+	console.error(error);
+});
+
+queue.add(() => Promise.reject(new Error('error')));
+```
+
 #### idle
 
 Emitted every time the queue becomes empty and all promises have completed; `queue.size === 0 && queue.pending === 0`.
@@ -262,7 +298,7 @@ Emitted every time the add method is called and the number of pending or queued
 
 #### next
 
-Emitted every time a task is completed and the number of pending or queued tasks is decreased.
+Emitted every time a task is completed and the number of pending or queued tasks is decreased. This is emitted regardless of whether the task completed normally or with an error.
 
 ```js
 import delay from 'delay';

source/index.ts

@@ -18,7 +18,7 @@ const timeoutError = new TimeoutError();
 /**
 Promise queue with concurrency control.
 */
-export default class PQueue<QueueType extends Queue<RunFunction, EnqueueOptionsType> = PriorityQueue, EnqueueOptionsType extends QueueAddOptions = DefaultAddOptions> extends EventEmitter<'active' | 'idle' | 'add' | 'next'> {
+export default class PQueue<QueueType extends Queue<RunFunction, EnqueueOptionsType> = PriorityQueue, EnqueueOptionsType extends QueueAddOptions = DefaultAddOptions> extends EventEmitter<'active' | 'idle' | 'add' | 'next' | 'completed' | 'error'> {
 	private readonly _carryoverConcurrencyCount: boolean;
 
 	private readonly _isIntervalIgnored: boolean;
@@ -252,11 +252,12 @@ export default class PQueue<QueueType extends Queue<RunFunction, EnqueueOptionsT
 						}
 					);
 
-					// TODO: Fix this ignore.
-					// @ts-expect-error
-					resolve(await operation);
+					const result = await operation;
+					resolve(result!);
+					this.emit('completed', result);
 				} catch (error: unknown) {
 					reject(error);
+					this.emit('error', error);
 				}
 
 				this._next();

test/test.ts

@@ -980,6 +980,63 @@ test('should emit next event when completing task', async t => {
 	t.is(timesCalled, 3);
 });
 
+test('should emit completed / error events', async t => {
+	const queue = new PQueue({concurrency: 1});
+
+	let errorEvents = 0;
+	let completedEvents = 0;
+	queue.on('error', () => {
+		errorEvents++;
+	});
+	queue.on('completed', () => {
+		completedEvents++;
+	});
+
+	const job1 = queue.add(async () => delay(100));
+
+	t.is(queue.pending, 1);
+	t.is(queue.size, 0);
+	t.is(errorEvents, 0);
+	t.is(completedEvents, 0);
+
+	const job2 = queue.add(async () => {
+		await delay(1);
+		throw new Error('failure');
+	});
+
+	t.is(queue.pending, 1);
+	t.is(queue.size, 1);
+	t.is(errorEvents, 0);
+	t.is(completedEvents, 0);
+
+	await job1;
+
+	t.is(queue.pending, 1);
+	t.is(queue.size, 0);
+	t.is(errorEvents, 0);
+	t.is(completedEvents, 1);
+
+	await t.throwsAsync(job2);
+
+	t.is(queue.pending, 0);
+	t.is(queue.size, 0);
+	t.is(errorEvents, 1);
+	t.is(completedEvents, 1);
+
+	const job3 = queue.add(async () => delay(100));
+
+	t.is(queue.pending, 1);
+	t.is(queue.size, 0);
+	t.is(errorEvents, 1);
+	t.is(completedEvents, 1);
+
+	await job3;
+	t.is(queue.pending, 0);
+	t.is(queue.size, 0);
+	t.is(errorEvents, 1);
+	t.is(completedEvents, 2);
+});
+
 test('should verify timeout overrides passed to add', async t => {
 	const queue = new PQueue({timeout: 200, throwOnTimeout: true});