Add onSizeLessThan helper method

readme.md

@@ -155,6 +155,16 @@ Returns a promise that settles when the queue becomes empty, and all promises ha
 
 The difference with `.onEmpty` is that `.onIdle` guarantees that all work from the queue has finished. `.onEmpty` merely signals that the queue is empty, but it could mean that some promises haven't completed yet.
 
+#### .onSizeLessThan(limit)
+
+Returns a promise that settles when the queue size is less than the given limit; `queue.size < limit`.
+
+If you want to avoid having the queue grow beyond a certain size you can `await queue.onSizeLessThan()` before
+adding a new item.
+
+Note that this only limits the number of items waiting to start; there could still be up to `concurrency` jobs
+already running that this call does not include in its calculation.
+
 #### .clear()
 
 Clear the queue.

source/index.ts

@@ -328,6 +328,28 @@ export default class PQueue<QueueType extends Queue<RunFunction, EnqueueOptionsT
 		});
 	}
 
+	/**
+	Wait for the queue size (the number of items not yet started) to fall below a certain threshold
+
+	@returns A promise that settles when the queue size is below the given limit
+	 */
+	async onSizeLessThan(limit: number): Promise<void> {
+		// Instantly resolve if the queue is empty
+		if (this._queue.size < limit) {
+			return;
+		}
+
+		return new Promise<void>(resolve => {
+			const listener = () => {
+				if(this._queue.size < limit) {
+					this.removeListener('next', listener);
+					resolve();
+				}
+			}
+			this.on('next', listener);
+		});
+	}
+
 	/**
 	The difference with `.onEmpty` is that `.onIdle` guarantees that all work from the queue has finished. `.onEmpty` merely signals that the queue is empty, but it could mean that some promises haven't completed yet.
 

test/test.ts

@@ -225,6 +225,31 @@ test('.onIdle()', async t => {
 	t.is(queue.pending, 0);
 });
 
+test('onSizeLessThan()', async t => {
+	const queue = new PQueue({concurrency: 1});
+
+	queue.add(async () => delay(100));
+	queue.add(async () => delay(100));
+	queue.add(async () => delay(100));
+	queue.add(async () => delay(100));
+	queue.add(async () => delay(100));
+
+	await queue.onSizeLessThan(4);
+	t.is(queue.size, 3);
+	t.is(queue.pending, 1);
+
+	await queue.onSizeLessThan(2);
+	t.is(queue.size, 1);
+	t.is(queue.pending, 1);
+
+	await queue.onSizeLessThan(10);
+	t.is(queue.size, 1);
+	t.is(queue.pending, 1);
+
+	await queue.onSizeLessThan(1);
+	t.is(queue.size, 0);
+	t.is(queue.pending, 1);
+})
 test('.onIdle() - no pending', async t => {
 	const queue = new PQueue();
 	t.is(queue.size, 0);