queue.js

module.exports = (func, concurrency) => {

    let numRunning = 0;
    const queue = [];

    const invoke = ({args, resolve, reject}) => {
        func(...args)
            .then(resolve)
            .catch(reject)
            .finally(() => {
                if(queue.length > 0) {
                    invoke(queue.shift());
                } else {
                    numRunning--;
                }
            });
    };

    return (...args) => new Promise((resolve, reject) => {
        if(numRunning < concurrency) {
            numRunning++;
            invoke({args, resolve, reject});
        } else {
            queue.push({args, resolve, reject});
        }
    });

};