p-limit/index.js

Run multiple promise-returning & async functions with limited concurrency

import Queue from 'yocto-queue';

export default function pLimit(concurrency) {
	validateConcurrency(concurrency);

	const queue = new Queue();
	let activeCount = 0;

	const resumeNext = () => {
		// Process the next queued function if we're under the concurrency limit
		if (activeCount < concurrency && queue.size > 0) {
			activeCount++;
			queue.dequeue()();
		}
	};

	const next = () => {
		activeCount--;
		resumeNext();
	};

	const run = async (function_, resolve, arguments_) => {
		// Execute the function and capture the result promise
		const result = (async () => function_(...arguments_))();

		// Resolve immediately with the promise (don't wait for completion)
		resolve(result);

		// Wait for the function to complete (success or failure)
		// We catch errors here to prevent unhandled rejections,
		// but the original promise rejection is preserved for the caller
		try {
			await result;
		} catch {}

		// Decrement active count and process next queued function
		next();
	};

	const enqueue = (function_, resolve, arguments_) => {
		// Queue the internal resolve function instead of the run function
		// to preserve the asynchronous execution context.
		new Promise(internalResolve => { // eslint-disable-line promise/param-names
			queue.enqueue(internalResolve);
		}).then(run.bind(undefined, function_, resolve, arguments_)); // eslint-disable-line promise/prefer-await-to-then

		// Start processing immediately if we haven't reached the concurrency limit
		if (activeCount < concurrency) {
			resumeNext();
		}
	};

	const generator = (function_, ...arguments_) => new Promise(resolve => {
		enqueue(function_, resolve, arguments_);
	});

	Object.defineProperties(generator, {
		activeCount: {
			get: () => activeCount,
		},
		pendingCount: {
			get: () => queue.size,
		},
		clearQueue: {
			value() {
				queue.clear();
			},
		},
		concurrency: {
			get: () => concurrency,

			set(newConcurrency) {
				validateConcurrency(newConcurrency);
				concurrency = newConcurrency;

				queueMicrotask(() => {
					// eslint-disable-next-line no-unmodified-loop-condition
					while (activeCount < concurrency && queue.size > 0) {
						resumeNext();
					}
				});
			},
		},
		map: {
			async value(array, function_) {
				const promises = array.map((value, index) => this(function_, value, index));
				return Promise.all(promises);
			},
		},
	});

	return generator;
}

export function limitFunction(function_, options) {
	const {concurrency} = options;
	const limit = pLimit(concurrency);

	return (...arguments_) => limit(() => function_(...arguments_));
}

function validateConcurrency(concurrency) {
	if (!((Number.isInteger(concurrency) || concurrency === Number.POSITIVE_INFINITY) && concurrency > 0)) {
		throw new TypeError('Expected `concurrency` to be a number from 1 and up');
	}
}