index.js

const queue = (concurrency = 4) => {
  let running = 0;
  const tasks = [];

  return {
    enqueue: async (task, ...params) => {
      tasks.push({ task, params }); // Add task to the list
      if (running >= concurrency) {
        return; // Do not run if we are above the concurrency limit
      }

      running += 1; // "Block" one concurrent task
      while (tasks.length > 0) {
        const { task, params } = tasks.shift(); // Take task from the list
        await task(...params); // Execute task with the provided params
      }
      running -= 1; // Release a spot
    },
  };
};

// Just a helper function, Javascript has no sleep function
const sleep = ms => new Promise(resolve => setTimeout(resolve, ms));

const printer = async num => {
  await sleep(2000);
  console.log(num, Date.now());
};

const q = queue();
// Add 8 tasks that will sleep and print a number
for (let num = 0; num < 8; num++) {
  q.enqueue(printer, num);
}