p-limit是一个用于限制异步任务并发数量的javascript库，可以有效地管理资源并避免系统负载过高

简单使用如下：

import pLimit from "p-limit"// 限制并发数量为2
const limit = pLimit(2)const delay = (ms) => {return new Promise(resolve => setTimeout(resolve, ms));
};const asyncTask1 = async () => {await delay(5000)console.log("输出任务1")
}const asyncTask2 = async () => {await delay(5000)console.log("输出任务2")
}const asyncTask3 = async () => {await delay(5000)console.log("输出任务3")
}const input = [limit(() => asyncTask1()), limit(() => asyncTask2()), limit(() => asyncTask3())]const result = await Promise.all(input)// 输出如下
输出任务1 // 5s后出现
输出任务2 // 5s后出现
输出任务3 // 10s后出现

源码实现其实也不复杂，主要使用了队列 + 变量来控制

import Queue from "yocto-queue";
import { AsyncResource } from "#async_hooks";export default function pLimit(concurrency) {// 参数判断if (!((Number.isInteger(concurrency) ||concurrency === Number.POSITIVE_INFINITY) &&concurrency > 0)) {throw new TypeError("Expected `concurrency` to be a number from 1 and up");}// 初始化队列const queue = new Queue();let activeCount = 0;const next = () => {// 当前执行数量减一activeCount--;// 如果队列里有的话继续取出执行if (queue.size > 0) {queue.dequeue()();}};const run = async (function_, resolve, arguments_) => {// 当前执行数量加一activeCount++;// 定义一个函数来异步执行用户传入的方法，并返回一个promiseconst result = (async () => function_(...arguments_))();// 回传result// 如果用户在外面定义了then方法或者Promise.all方法，都可以接收到resultresolve(result);// 等待 result Promise 完成// try...catch 是用于捕获异常// 这样的话即使出现异常程序也能继续执行下去try {await result;} catch {}next();};const enqueue = (function_, resolve, arguments_) => {// 把run函数加入队列// 并传入用户定义方法和参数queue.enqueue(AsyncResource.bind(run.bind(undefined, function_, resolve, arguments_)));// 异步(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();// 判断当前执行的函数数量是否小于并发数// 是的话出列执行if (activeCount < concurrency && queue.size > 0) {queue.dequeue()();}})();};// 暴露给用户的函数，返回一个Promise// 传入了resolve用于执行用户传递的函数之后回传给用户函数结果const generator = (function_, ...arguments_) =>new Promise((resolve) => {enqueue(function_, resolve, arguments_);});Object.defineProperties(generator, {// 正在运行的数量activeCount: {get: () => activeCount,},// 正在等待的数量pendingCount: {get: () => queue.size,},// 清除队列clearQueue: {value() {queue.clear();},},});return generator;
}