A simple promise queue in JavaScript to serialize the execution of a bunch of promises

pq.js

// A promise queue

const q = []

const add = (task) => {
  let handlers;
  const p = new Promise((...args) => (handlers = args));
  q.push({task, handlers});
  if (q.length == 1) next();
  return p;
}

const next = () => {
  const item = q[0];
  if (!item) return;
  const {task, handlers} = item;
  task().then(...handlers).finally(() => {
    q.shift();
    next();
  });
}

// Some code to test it.

const wait = () => new Promise((res) => setTimeout(res, Math.random() * 2000));

makePromise = async (p) => {
  console.log(p, "start");
  await wait();
  if (p % 2) throw new Error(`${p} rejecting`)
  console.log(p, "end");
  return p ** 2;
}

const watch = (p, n) => {
  p.then((z) => console.log(n, "fullfilled with", z), (e) => console.log(n, "rejected with", e.message));
  console.log(n, "add");
}

main = async () => {
  console.log("go", "a");
  for (let i = 0; i < 10; i++) {
    watch(add(() => makePromise(i)), i);
    await wait();
  }
  console.log("go", "b");
}

main()