SOURCE

class promise_pool {
    constructor(max) {
        this.max_val = max;
    }
    max_val = 5;
    running_pool = [];
    wait_queue = [];
    stop = false;

    push(fn) {
        this.wait_queue.push(fn);
        for(let i = 0; i < Math.min(this.wait_queue.length, this.max_val-this.running_pool.length); i++) {
            this.running_pool.push(this.wait_queue.shift());
        }
    }

    run() {
        let flag = 1;
        while(flag || running_pool.length > 0) {
            // 退出条件
            if(stop) {
                flag = 0;
            }

            let finish =  Promise.race(this.running_pool);
            getTask(finish);
        }
    }

    getTask(fn) {
        this.running_pool.slice(this.running_pool.indexOf(fn), 1);
        if(this.wait_queue.length > 0) {
            this.running_pool.push(this.wait_queue.shift());
        }
    }
}

const URLS = [
    '地址1',
    '地址2',
    '地址3',
]
//请求函数
let requestFn = url => {
    return new Promise(resolve => {
        setTimeout(() => {
            resolve(`${url}任务完成`)
        }, 10000 * Math.random())
    }).then(res => {
        console.log(res);
        console.log('________________________________');
    })
}

let pool = new promise_pool(2);
URLS.forEach(item => {
    pool.push(requestFn(item))
})