使用p-limit 限制并发数源码解析

import pLimit from 'p-limit';
const limit = pLimit(1);
const input = [
	limit(() => fetchSomething('foo')),
	limit(() => fetchSomething('bar')),
	limit(() => doSomething())
];
// Only one promise is run at once
const result = await Promise.all(input);
console.log(result);

import Queue from 'yocto-queue';
export default function pLimit(concurrency) {
	if (!((Number.isInteger(concurrency) || concurrency === Number.POSITIVE_INFINITY) && concurrency > 0)) {
		throw new TypeError('Expected `concurrency` to be a number from 1 and up');
	}
	const queue = new Queue();
	let activeCount = 0;
}

const next = () => {
		activeCount--;
		if (queue.size > 0) {
			queue.dequeue()();
		}
	};
	const run = async (fn, resolve, args) => {
		activeCount++;
		const result = (async () => fn(...args))();
		resolve(result);
		try {
			await result;
		} catch {}
		next();
	};

	const enqueue = (fn, resolve, args) => {
		queue.enqueue(run.bind(undefined, fn, resolve, args));
		(async () => {
			// This function needs to wait until the next microtask before comparing
			// `activeCount` to `concurrency`, because `activeCount` is updated asynchronously
			// when the run function is dequeued and called. The comparison in the if-statement
			// needs to happen asynchronously as well to get an up-to-date value for `activeCount`.
			await Promise.resolve();
			if (activeCount < concurrency && queue.size > 0) {
				queue.dequeue()();
			}
		})();
	};

	const generator = (fn, ...args) => new Promise(resolve => {
		enqueue(fn, resolve, args);
	});
	Object.defineProperties(generator, {
		activeCount: {
			get: () => activeCount,
		},
		pendinGCount: {
			get: () => queue.size,
		},
		clearQueue: {
			value: () => {
				queue.clear();
			},
		},
	});
	return generator;