用Promise实现前端并发请求

/**
* 构造假请求
*/
async function request(url) {return new Promise((resolve) => {setTimeout(() => {resolve(url);},// Math.random() * 500 + 800,1000,);});
}

请求一次，查看耗时，预计应该是1s：

async function requestOnce() {const t1 = Date.now();const res = await request(1);console.log('一次请求耗时=', Date.now() - t1, 'ms,请求结果=', res);
}
requestOnce();

结果：

12个数据发起并行请求，预计耗时1s：

async function requestAsync() {const t2 = Date.now();const requests = new Array(12).fill(2).map(request);const res2 = await Promise.all(requests);console.log('批量请求耗时=', Date.now() - t2, 'ms,请求结果=', res2, ',预计是 1000 ms');
}requestAsync();

结果：

12个请求并发进行，但限制最多3个请求可并发，预计耗时4s：

async function concurrencyRequest(urls, maxNum) {const buffer = [];//并发请求池，最多有maxNum个const t3 = Date.now();const res = [];for (let i = 0; i < urls.length && buffer.length < maxNum; ++i) {// console.log('next=', i);const promise = new Promise((resolve) => request(urls[i]).then(resolve));buffer.push(promise);promise.then((response) => (res[i] = response)).finally(() => {//每个请求完成后都从buffer中删除并保存响应const index = buffer.findIndex((item) => item === promise);buffer.splice(index, 1);// console.log('删除一个promise', buffer.length);});if (buffer.length === maxNum) {//必须等待buffer size小于maxNum才能继续请求await Promise.race(buffer);}}await Promise.all(buffer);//需要等待最后剩下的几个请求完成才算完成console.log('并发请求耗时=', Date.now() - t3, '请求结果=', res, '预计时间是 4000 ms');
}concurrencyRequest(new Array(12).fill(1).map((_, i) => i + 1),3,
);