"Sleep" execution inside asynchronous iteration "for await" (ES2018)

Question

I am using asynchronous iteration for getting items from DynamoDB. For every iteration (item) I execute some http requests. I need to "sleep" 1 second in every iteration in order to limit the request flow. I tried with promisify(setTimeout) but the execution stops.

const sleep = require('util').promisify(setTimeout)

for await (const item of mapper.scan(MyDomainObject)) {

    await sleep(1000);   //This doesn't work
    // do some http requests
}

What it's the proper way to "sleep" inside a "for await" interation?

Answer 1

I figured it out. The problem was that I was executing the code with Jest. When I executed the code normally the above code works perfectly. Thanks to Noseratio answer I looked elsewhere. I was too focused in the for await.

Answer 2

Your code seems to be correct, I've simulated your asynchronous generator like this:

const sleep = require('util').promisify(setTimeout);

async function* scan(arg) {
  for (let i = 0; i < 4; i++) {
    console.log(`yielding ${i}`);
    await sleep(500);
    yield i;
  }
}

async function test()
{
  for await (let item of scan({})) {
    console.log(`got ${await item}`);
    await sleep(1000);
  }
}

test();

The execution doesn't stop for me, no matter how many iterations I provision for inside scan.

Thus, it's not the sleep that's causing the problem, you should be looking elsewhere. Perhaps, the issue is with mapper.scan, or with those http requests, or maybe you just need to upgrade your node.js (I'm on v10.11.0).