makeRequests.js

const assert = require('assert');

const www = {
    'url1': 'result1',
    'url2': 'result2',
    'url3': 'result3',
    'url4': 'result4'
};

const fetch = (url, ms = 1000) => {
    return new Promise((resolve) => {
        setTimeout(() => {
            resolve(www[url]);
        }, ms)
    });
};

const createDeferred = () => {
    let resolve;
    let reject;

    const promise = new Promise((res, rej) => {
        resolve = res;
        reject = rej;
    });

    return {
        promise,
        resolve,
        reject
    };
};

/**
 * @param {Array<string>} urls
 * @param {number} concurrent
 * @returns Promise<Array<any>>
 */
function makeRequests(urls, concurrent) {
    const deferredRequests = urls.map(createDeferred);
    const processedRequests = new Map();
    let completed = 0;
    let pending = 0;
    let current = 0;

    const run = () => {
        while (pending < concurrent && completed < urls.length && current < urls.length) {
            const deferred = deferredRequests[current];
            const url = urls[current];
            current++;

            if (processedRequests.has(url)) {
                processedRequests.get(url).promise.then((result) => {
                    deferred.resolve(result);
                    completed++;
                    run();
                });
            } else {
                processedRequests.set(url, deferred);

                pending++;

                console.log(`fetch ${url} start`);
                fetch(url).then((result) => {
                    console.log(`fetch ${url} end`);
                    deferred.resolve(result);
                    pending--;
                    completed++;
                    run();
                });
            }
        }
    }

    run();

    return Promise.all(deferredRequests.map((req) => req.promise));
}

const test = () => {
    (() => {
        makeRequests(
            [
                'url1',
                'url2',
                'url1',
                'url3',
                'url4',
                'url2',
                'url2',
                'url2'
            ],
            2
        ).then((results) => {
            assert.deepEqual(
                results,
                [
                    'result1',
                    'result2',
                    'result1',
                    'result3',
                    'result4',
                    'result2',
                    'result2',
                    'result2'
                ]
            );
        });
    })();
};

test();