LimitedParallelExecution.js

//We will now look at pattern that will execute set of given tasks in parallel with given concurrency.

function makeSimpleTask(name) {
  return (cb) => {
    console.log("started");
    setTimeout(() => {
      console.log(name, "completed");
      cb();
    }, Math.random() * 2000);
  };
}
const tasks = [
  makeSimpleTask("task 1"),
  makeSimpleTask("task 2"),
  makeSimpleTask("task 3"),
  makeSimpleTask("task 4"),
  makeSimpleTask("task 5"),
  makeSimpleTask("task 6"),
  makeSimpleTask("task 7"),
];

const concurrency = 2;
let running = 0;
let index = 0;
let completed = 0;

function next() {
  while (running < concurrency && index < tasks.length) {
    const task = tasks[index++];
    task(() => {
      if (++completed == tasks.length) {
        return finish();
      }
      running--;
      next();
    });
    running++;
  }
}

next();

function finish() {
  console.log("all tasks completed");
}