ES6 Promise replacement of async.eachLimit / async.mapLimit

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If you don’t care about the results, then it’s quick to whip one up:

Promise.eachLimit = async (funcs, limit) => {
  let rest = funcs.slice(limit);
  await Promise.all(funcs.slice(0, limit).map(async func => {
    await func();
    while (rest.length) {
      await rest.shift()();
    }
  }));
};

// Demo:

var wait = ms => new Promise(resolve => setTimeout(resolve, ms));

async function foo(s) {
  await wait(Math.random() * 2000);
  console.log(s);
}

(async () => {
  let funcs = "ABCDEFGHIJKLMNOPQRSTUVWXYZ".split("").map(s => () => foo(s));
  await Promise.eachLimit(funcs, 5);
})();

A key performance property is running the next available function as soon as any function finishes.

Preserving results

Preserving the results in order makes it a little less elegant perhaps, but not too bad:

Promise.mapLimit = async (funcs, limit) => {
  let results = [];
  await Promise.all(funcs.slice(0, limit).map(async (func, i) => {
    results[i] = await func();
    while ((i = limit++) < funcs.length) {
      results[i] = await funcs[i]();
    }
  }));
  return results;
};

// Demo:

var wait = ms => new Promise(resolve => setTimeout(resolve, ms));

async function foo(s) {
  await wait(Math.random() * 2000);
  console.log(s);
  return s.toLowerCase();
}

(async () => {
  let funcs = "ABCDEFGHIJKLMNOPQRSTUVWXYZ".split("").map(s => () => foo(s));
  console.log((await Promise.mapLimit(funcs, 5)).join(""));
})();

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