CODEWITHSUNDEEP
typescript
Johannes Schacht
Johannes Schacht

Reputation: 1334

Promise.all does not behave as expected

I wonder why in the below code the first console log prints 0. The second does print 1oo as expected. Can someone explain?

let count = 0;

async function incr(): Promise<number> {
    setTimeout(() => count++, 1);
    return count;
}

const promises = [];
for (let i = 0; i < 100; i++) promises.push(incr());
Promise.all(promises).then(async () => console.log('count = ' + count));

setTimeout(() => {
    console.log('count = ' + count);
}, 100);

Upvotes: 0

Views: 40

Answers (1)

CertainPerformance
CertainPerformance

Reputation: 370659

Your incr function doesn't return anything explicitly, so it returns a Promise that resolves immediately.

Promise.all runs in the microtask queue as soon as all Promises in the passed array resolve. So, that's what runs first - the callback passed to Promise.all(promises).then(.

The setTimeout queues a macrotask, and such tasks run much later than microtasks.

If you wanted it to log 100, do:

let count = 0;

function incr() {
  return new Promise((resolve) => {
    setTimeout(() => {
      count++;
      resolve();
    }, 1);
  });
}

const promises = [];
for (let i = 0; i < 100; i++) promises.push(incr());
Promise.all(promises).then(async() => console.log('count = ' + count));

setTimeout(() => {
  console.log('count = ' + count);
}, 100);

Upvotes: 1

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