I am writing a function that should execute all async functions as fast as possible, however, only 5 of them can run at the same time.

I wanted to do it using Promise.race so the implementation is not the best. The problem is that the code execution doesn't stop at await. I would expect, that counter variable would be decremented only when the promise resolves but it doesn't wait for the promise to resolve. The code doesn't stop at Promise.all either which makes me thing that my understanding of promises is not proper. The code is below and the function in question is runPromises.

async function runPromises(promises, limit) {
  const LIMIT = limit || 10;
  let promsCopy = promises.slice();
  let counter = 0;
  let queue = [];
  while (promsCopy.length !== 0) {
    if (counter < LIMIT) {
      let prom = promsCopy.shift();
      if (prom) {
        queue.push(prom());
        counter  = 1;
        if (counter >= LIMIT) {
          let [completed] = await Promise.race(
            queue.map((p) => p.then((_) => [p]))
          );
          queue.splice(queue.indexOf(completed), 1);
          counter -= 1;
        }
      }
    }
  }
  await Promise.all(queue);
}

// the code below is just for testing

const asyncFuncLong = () => {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      resolve();
    }, 6000);
  });
};

const asyncFuncShort = () => {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      resolve();
    }, 1000);
  });
};

let fns = [
  asyncFuncLong,
  asyncFuncLong,
  asyncFuncLong,
  asyncFuncLong,
  asyncFuncLong,
  asyncFuncLong,
  asyncFuncLong,
  asyncFuncLong,
  asyncFuncLong,
  asyncFuncLong,
  asyncFuncLong,
  asyncFuncLong,
  asyncFuncLong,
  asyncFuncLong,
  asyncFuncLong,
  asyncFuncLong,
];

let start = Date.now();

runPromises(fns, 10).then(() => {
  console.log(Date.now() - start);
});

Edit: Fixed. Everything works now. Thank you both!

CodePudding user response：

The fns array is a list of functions. Those functions return Promise objects. You are then passing that list of functions to your runPromises function, which appears to want to take a list of Promise objects as an argument. But you are passing it a list of functions.

My suggestions are either

• change the runPromises function so that it actually calls prom() at some point (or just use a map, like const r = await Promise.all(queue.map((fn) => { return fn(); }));), or

• change the definition of fns to something like let fns = [ asyncFuncLong(), asyncFuncShort(), asyncFuncLong(), ... ];.