parallel-promise-limit.js

function getRandomInt(min, max) {
  return Math.floor(Math.random() * (max - min)) + min;
}

function createFakeQueries(count, minTime, maxTime) {
  return Array(count)
    .fill()
    .map((_, i) => {
      const time = getRandomInt(minTime, maxTime);
      const queryCaller = () =>
        new Promise(resolve => {
          console.log('Starting query: %s, %d ms', i, time);

          setTimeout(() => {
            console.log('Finished query: %s, %d ms', i, time);
            resolve({ queryNumber: i, time });
          }, time);
        });

      return queryCaller;
    });
}

const queue = createFakeQueries(10, 3000, 5000);
const result = [];
const limit = 3;
let activeQueries = 0;

function shift(resolve) {
  if (queue.length > 0) {
    if (activeQueries < 3) {
      const remain = limit - activeQueries;
      const shifted = queue.splice(0, remain);

      console.log('taking:', remain);

      processQueue(shifted, resolve);
    }
  } else {
    Promise.all(result).then(resolve);
  }
}

function processQueue(shifted = [], resolve) {
  shifted.forEach(queryCaller => {
    activeQueries++;

    const query = queryCaller();

    query.then(() => {
      activeQueries--;
      shift(resolve);
    });
    result.push(query);
  });
}

function parallel(argument) {
  return new Promise((resolve, reject) => {
    shift(resolve, reject);
  });
}