Async iterators, generators, and try-finally

description.md

Turns out JS can throw an error on a line like return 42;

generator - loops forever sleeping 1s at a time, and when you try to stop it, it waits 1s and then throws an error.

iteration - calls generator via a for-await and attempts to return within the iteration. This forces iteration to wait for generators finally section to run which includes a 1s await!

for-await-try-finally.js

async function * generator() {
    try {
      let i = 0;
      while(true) {
        await sleep(1000);
        yield i++;
      }
    } catch(e) {
        console.log('caught', e);
    } finally {
        console.log('finally');
        await sleep(1000);
        throw new Error('finally-await');
    }
}

async function iteration() {
  try {
    for await (const g of generator()) {
      return 42;
    }
  } catch (e) {
    console.log('caught', e);
    return 43;
  }
}

function main() {
  iteration().then(
    a => console.log('res', a),
    b => console.log('rej', b)
  );
}

const sleep = n => new Promise(res => setTimeout(res, n));

main();
/*
Output:

finally
caught Error: finally-await
    at generator (<anonymous>:13:15)
    at async iteration (<anonymous>:20:7)
res 43
*/