promise-pool.js

let count = 0;
const f = (value, t = Math.floor(100 + Math.random() * 100)) => () => new Promise(resolve => {
    console.log(`start: ${value} count: ${++count}`);
    setTimeout(() => {
        console.log(`end: ${value} count: ${--count}`);
        resolve(value);
    }, t);
});
const pool = [ f('A'), f('B'), f('C'), f('D'), f('E'), f('F'), f('G') ];

const promise_pool = (list, size = 10) => new Promise(resolve => {
    const result = Array(list.length).fill({status: 0});
    const callback = async cell => {
        const next = result.findIndex(e => e.status === 0);
        if (next >= 0) {
            result[next] = {status: 1};
            const value = await list[next]();
            result[next] = {status: 2, value};
            callback(cell);
        } else if (result.findIndex(e => e.status === 1) < 0) {
            resolve(result.map(({ value }) => value));
        }
    }
    for (let cell = 0, length = Math.min(list.length, size); cell < length; cell++) {
        callback(cell);
    }
});

promise_pool(pool, 3).then(a => {console.log('done:', a)});